What To See Telescope(Jul-Sept) v1 - One

Transcription

WHAT TO SEE IN
A SMALL TELESCOPE
July-September
A Course By
Stargazer University
and
One-Minute Astronomer
www.oneminuteastronomer.com
Copyright © 2011-2012 Mintaka Publishing Inc.
www.oneminuteastronomer.com
2
Table of Contents
Introduction!............................................................................................................ 5
Tour 1 - Draco!........................................................................................................ 7
Nu Draconis
The “head and neck” of the constellation Draco
39 Draconis Sigma Draconis UX Draconis Kemble 2
NGC 6503 NGC 6543 7
8
8
9
9
9
10
10
Tour 2 - Hercules and Corona Borealis!................................................................ 12
M13, the Great Hercules Cluster
NGC 6207
M92
NGC 6210
Dolidze-Džimšeleljšvili 9
Alpha Herculis Kappa Herculis 95 Herculis
Zeta Corona Borealis Sigma and Nu Corona Borealis 12
13
14
15
15
15
16
16
16
17
Tour 3 - Lyra!......................................................................................................... 18
M57 - The Ring Nebula
M56 - A Dim Globular
Delta (1,2) Lyrae
Stephenson 1
Beta Lyrae
Zeta Lyrae
Epsilon Lyrae
Struve 2470 and 2474
18
19
19
20
20
21
21
22
Tour 4 - Ophiuchus and Serpens!......................................................................... 23
Taurus Poniatowski
70 Ophiuchi
67 Ophiuchi
IC 4665
IC 4756
NGC 6633 Struve 2375
Collinder 350
NGC 6572
M10
M12
M5
23
24
24
25
26
26
27
27
27
28
29
29
What To See With A Small Telescope (July-September) v1.2
3
5 Serpentis M19
NGC 6293 M9
Barnard 64
30
30
30
30
30
Tour 5 - Scorpius!..................................................................................................32
Beta Scorpii
The constellation Scorpius, including M4, M80, M6, M7, and the “False Comet”
Nu Scorpii
Antares M4
M80
The False Comet
M6, the “Butterfly Cluster”
M7
33
33
33
34
35
36
37
38
38
Tour 6 - Sagittarius (Part 1)!.................................................................................. 40
The M24 Star Cloud
Barnard 92 and 93 M18 M17 (The Swan Nebula)
M16 (The Eagle Nebula)
M22
M55
NGC 6822 (Barnard’s Galaxy)
NGC 6818 41
41
42
42
43
44
45
46
46
Tour 7 - Sagittarius (Part 2)!.................................................................................. 47
M8, The “Lagoon Nebula”
M20, The “Trifid Nebula”
M21
The Castaway Cluster
M28 Two Fine Doubles
47
48
49
50
50
51
Tour 8 - Scutum and Aquila!.................................................................................. 52
M11, the “Wild Duck” Cluster
Two Double Stars Near M11
The region around M11 and the Scutum Star Cloud
Scutum Star Cloud
Barnard 110, 111, and 113.
NGC 6664
M26
NGC 6712
NGC 6709
52
54
55
55
55
56
56
57
57
Tour 9 - Cygnus!................................................................................................... 58
M39
The “North America” Nebula
59
59
4
The “Northern Coalsack”
M29
NGC 6910 & IC 1318 61 Cygni
The “Blinking Planetary”
The Veil Nebula
61
61
61
62
62
63
Tour 10 - Vulpecula, Sagitta, and Delphinus!........................................................ 65
The “Coathanger”
The Dumbbell Nebula M71
Harvard 20
Gamma Delphinus NGC 7006
The “Toadstool”
65
66
67
68
69
69
69
Appendix - Easy Math for Astronomers!............................................................... 71
Appendix - Whole-Sky Star Maps!........................................................................ 73
5
Introduction
This guide helps you find and enjoy dozens of deep-sky objects with a small telescope in the midevening hours of July, August, and September. It contains all the maps and information you need
to find interesting stars, star clusters, nebulae, and galaxies using nothing more than a 3-inch or 4inch telescope, and to help you enjoy some of the most beautiful sights in nature.
The guide assumes you are at least somewhat familiar with the set-up and operation of your
telescope. If your telescope has a larger aperture than 4 inches, then you will see more detail in
most objects. Sights that benefit from larger-aperture telescopes, and descriptions of what you will
see with a 6-inch or 8-inch telescope are included where appropriate.
The guide also assumes you are somewhat familiar with the night sky, and that you know how to
locate specific major constellations. To help you get oriented, the last appendix includes three
whole-sky maps that show the constellations as seen from mid-northern latitudes in July, August,
and September.
Your telescope may have come with at least two eyepieces, and you may have acquired one or
two more. It’s important to know the magnification each of your eyepieces give you, and if
possible, the field of view of each eyepiece in degrees. If you do not know how to determine these
numbers, look to the first Appendix for information on how to do so.
A pair of binoculars is also handy to find your way to some objects. Indeed, a few objects are best
seen in binoculars. But you can get a good view in your telescope’s finder scope as well. So
binoculars are not critical.
One very useful accessory is a nebula filter. These optical elements thread into the back of your
eyepiece and filter out skyglow and light pollution, while passing very specific colors that are
common in emission and planetary nebulae. These filters go by the name of UHC (from Lumicon)
or Ultrablock (from Orion), for example, and are well worth the investment. Another type of filter-OIII (pronounced “oh-three”)-- is also useful with telescopes of 6-inch apertures or larger.
Each tour in this guide includes at least one map to help you follow the directions to find the
objects listed. In all maps, north is up and east is to the left.
Every tour includes a table of objects described. Each type of deep-sky object is assigned a shortform code according to this legend:
PN = planetary nebula
OC = open star cluster
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GC = globular star cluster
DS = double/multiple star
EN = emission nebula
RN = reflection nebula
DN = dark nebula
VS = variable star
GL = galaxy
AS = asterism
There is a lot of information in this guide, and nearly 100 deep-sky objects to explore over three
months. Don’t rush it, especially if you are a near-beginner. Take your time to find, enjoy, and
ponder each of these deep-sky sights. Some nights, you may see four or five sights. Some
nights, especially with hard-to-find objects, you may see just one or two. Don’t worry. There is no
rush. Be patient, take your time to enjoy the night sky, and to discover fascinating phenomena that
most people will never see.
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Tour 1 - Draco
Draco, the Dragon, is a long, winding constellation in the far northern sky. Its distinctive shape has
captured the imagination of stargazers for thousands of years. Ancient Egyptians identified the
constellation as the goddess Tawaret. Arabs called these stars the “Mother Camels”. And the
Greeks and Romans defined the constellation as a dragon who some say was killed by Minerva
(Athena) and hurled into the sky. The constellation was surely on Ptolemy’s list of 48 original
constellations of the classical world.
For much of the northern hemisphere, Draco is circumpolar, which means it never sets below the
horizon. The tail of the dragon begins between the bowls of the Big and Little Dippers, then heads
west towards the constellation Cepheus, then back east where its distinctive head ends not far
from the constellation Hercules.
The constellation is not visible from the deep-southern hemisphere.
Draco has dozens of faint galaxies throughout its long body, but most are too faint to see well in
small telescopes. But there are some excellent double stars here, a ruby-red carbon star, and a
famous planetary nebula which was the first to give up its spectroscopic secrets to 19th-century
astronomers.
Object
Type
Mag.
Dist. (ly)
RA
Dec
μ Dra
DS
4.9/4.9
99
17h 32.2m
+55o 11’
39 Dra
DS
5.1/8.0/8.1
190
18h 23.9m
+58o 48’
ο Dra
DS
4.8/8.3
320
18h 51.2m
+59o 23’
6.0 (var)
2000
19h 21.6m
+76o 34’
carbon star
18h 35.0m
+72o 23’
“mini Cassiopeia”
UX Dra
-
-
Comments
Kemble 2
AS
~8.0
NGC 6503
GL
10.2
17 million
17h 49.5m
+70o 09’
dwarf spiral
NGC 6543
PN
8.1
3000
17h 58.6m
+66o 38’
“Cat’s Eye” nebula
Table 1 - Objects to see in the constellation Draco
Nu Draconis
The head of Draco is made of a distinctive asterism of the four stars beta (β), gamma (γ), xi (ξ), and
mu (ν). The star xi, also called Grumium, joins the head of Draco to the rest of its body. Nu is an
excellent double star with a generous spacing of 62” (arcseconds). These two white stars, both of
magnitude 4.9, are easily visible in any small telescope at low magnification.
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The “head and neck” of the constellation Draco
39 Draconis
The star 39 Draconis is a little more interesting than nu. You can locate the star on a line from nu
(ν) through xi (ξ) at a distance from xi that’s just a little more than the distance of xi to nu. Again, at
low power in any telescope, you will see two stars here separated by 90”. The brighter primary is
magnitude 5.1 and the secondary will appear two magnitudes fainter. But crank up the
magnification to 125x or more, and you will see the fainter star itself resolve into two blue-white
components of magnitude 8.1. The trio appear like white gems set against a fairly sparse
background of space. A pleasant sight.
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Sigma Draconis
About 3.5o east of 39 Draconis, look for the unequal pair of sigma (σ) Draconis. Again, it is widely
spaced at 37”, but you have to look carefully for the 8.3-magnitude secondary in the presence of
the much brighter 4.8-magnitude primary. The primary appears orange to most observers. But
the secondary to some appears blue-green or even lilac! This may be an illusion of perception,
because the temperature and spectrum of the star suggest it is orange, just like the primary.
UX Draconis
Now to a most interesting star. UX Draconis is one of the brightest examples of a “carbon star”, an
evolved variable star that has an atmosphere rich in carbon compounds. These compounds
absorb most of the broad spectrum of light generated within the star except for red and infrared
wavelengths, which means these stars usually have a striking deep dull-red appearance. UX
Draconis is no exception.
This intriguing star is north of the first bend in the dragon’s body, and halfway to the north star
Polaris. First locate the 4th-magnitude stars epsilon (ε) and chi (χ). Just north of these two, locate
tau (τ), which makes an isoceles triangle with epsilon and chi. Now look about 3o north of tau for a
small patch of 6th and 7th magnitude stars. The reddest of this group is UX Draconis.
Carbon stars are usually variable and are in the late stages of their evolution. The carbon in the
atmosphere of UX Draconis was likely produced by nuclear fusion of helium in the star’s core. Deep zones of convection dredged up the carbon into the atmosphere.
UX varies in brightness from magnitude 5.9 to 7.1 over a period of approximately 170 days. So
you can return to this star and watch it slowly change over the course of weeks and months as it
slowly evolves into what will likely be a planetary nebula.
Kemble 2
Now look back south towards chi (χ) Draconis. Just 1o east of χ lies a beautiful little asterism
named “Kemble 2” after the Canadian stargazer and Franciscan monk Lucien Kemble. In a
telescope at lower power, this little W-shaped group, which is just 20’ (arc-minutes) across, looks
remarkably like a miniature version of the constellation Cassiopeia.
The stars of this group are just aligned by chance. The stars are unrelated and moving in different
directions through interstellar space. But they are a lovely sight. At magnitude 7 to 9, the stars
exhibit a range of colors. The brightest appears orange, others yellow, and one of the tips of the
“W” appears blue-white. Try a range eyepieces to see what magnification gives you the best view.
Because it looks like Cassiopeia, this little asterism is sometimes called “The Little Queen”.
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The asterism Kemble 2 in Draco
NGC 6503
Let’s leave our own galaxy for a moment and inspect the dwarf spiral NGC 6503 some 17 million
light years away. This small galaxy is just 30,000 light years across, about 1/3 the size of the Milky
Way. But it’s large enough to take on a lovely spiral shape seen in long-exposure images. The
galaxy has many pink emission nebulae where new stars form, and the spiral arms are chock full of
bright young blue stars.
The galaxy is a lonely one. It sits on the edge of the “Local Void”, a vast empty region of
intergalactic space some 100 million light years across.
NGC 6503 is magnitude 10.2, so you’ll need reasonably dark sky to spot it with a 3-4” telescope.
It is a tilted spiral, so it retains a high surface brightness. The galaxy sits on a line between the
stars chi (χ) and zeta (ζ) Draconis, about 1/3 of the way from the former to the latter. Unlike many
objects of comparable brightness, NGC 6503 was discovered quite late. The German university
student Arthur von Auwers first saw the galaxy with a 2.6-inch telescope in 1854.
NGC 6543
Now a visit to Draco’s most famous deep-sky sight, the “Cat’s Eye” nebula. Cataloged as NGC
6543, this planetary nebula is found about 3.6o south-southeast of NGC 6503 in a fairly star-poor
region of sky.
The Cat’s Eye is a tiny planetary. It appears star-like at 20x, though its greenish-blue appearance
may distinguish it from the surrounding stars. Increase magnification to 75x or more and the disk
of the nebula is resolved. The central star of NGC 6543 is fairly bright, and most telescopes reveal
it easily with modest magnification. At 125x or more, the nebula shows some texture, including a
somewhat darker inner region and a brighter ring around the outside. The shape is slightly oval,
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and the color is quite pleasing compared to the white stars in the background. Like most small
planetary nebulae, the Cat’s Eye responds well to high magnification if the sky is steady.
The Cat’s Eye nebula as imaged through a large telescope
The Cat’s Eye played an important role in astronomical history. It was the first to be examined in a
spectroscope by English amateur William Huggins in 1864. Huggins was shocked to find the
nebula had a totally different spectrum from any star. Its spectrum displayed a very small number
of emission lines at about 500 nm... the blue-green region of the visible spectrum. This led
Huggins to conclude the nebula was composed of a glowing gas, but he could not associate the
spectrum with any Earthly element. So the mysterious element was named “nebulium”. It turns
out the blue-green spectral lines are indeed from an Earthly element-- doubly ionized oxygen, or
OIII-- but it took some time for laboratory-based spectroscopists to figure this out because oxygen
must be in an extremely rarified state to emit light at this wavelength.
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Tour 2 - Hercules and Corona Borealis
Hercules is a fairly sprawling constellation to the east of Boötes and its bright star Arcturus. The
constellation takes its name from the classical mythological figure of the same name. The hero is
depicted in the sky kneeling, praying to Zeus for strength to finish his legendary great labors. The
constellation is home to an extremely distant supercluster of galaxies, most of which are beyond
the reach of telescopes less than 12” in aperture. But the constellation does have some fine
sights: two excellent globular clusters including the famous M13, a little-known open cluster, and a
good planetary nebula.
Corona Borealis, or the Northern Crown, is a small but lovely constellation just to the west of
Hercules. It too was listed in Ptolemy’s original 48 constellations. Its brightest star is Alphecca,
which marks the jewel in the crown. Like Hercules, Corona Borealis hosts a distant supercluster of
galaxies. The cluster Abell 2065 is a particularly rich cluster with more than 400 members. But
none is brighter than 16th magnitude, so a good 16” or 20” telescope is needed to glimpse these
extremely distant galaxies. Corona Borealis does have a few pleasing double stars that make for
excellent viewing in smaller telescopes.
Object
Type
Mag.
Dist. (ly)
RA
Dec
Comments
M13
GC
5.8
23500
16h 41.7m
+36o 28’
Great Hercules Cluster
NGC 6207
GL
11.6
46 million
16h 43.1m
+36o 50’
M92
GC
6.5
25400
17h 17.1m
+43o 08’
NGC 6210
PN
9.3
3600
16h 44.5m
+23o 49’
Do-Dz 9
OC
7.5
18h 08.8m
+32o 32’
α Herc
DS
2.9/5.4
380
17h 14.6m
+14o 23’
κ Herc
DS
5.1/6.2
390
16h 08.1m
+17o 03’
95 Herc
DS
4.8/5.2
470
18h 01.5m
+21o 36’
ζ CrB
DS
5.0/5.9
470
15h 39.4m
+36o 38’
σ CrB
DS
5.6/6.5
71
16h 14.7m
+33o 52’
ν CrB
DS
5.4/5.6
16h 22.4m
+33o 48’
-
-
Rasalgethi; orange and blue-green
blue-green and garnet
very wide double
Table 2 - Objects to see in the constellations Hercules and Corona Borealis
M13, the Great Hercules Cluster
This by far the most famous sight in this part of the sky. M13 competes with M5 as the brightest
globular cluster north of the celestial equator. But to some, M13 has a finer appearance in a small
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telescope, likely as a result of its looser arrangement of stars. It’s also fairly easy to find, located
between the stars eta (η) and zeta (ζ) Herculis on the western side of the “Keystone” of Hercules.
At magnitude 5.8, the cluster is easily visible in binoculars or finder as a distinctly un-starlike
smudge. In dark sky the cluster is visible without optics. In a 4” scope at 100x or so, the cluster
resolves into a halo of stars about 20’ across, with a gradual brightening towards the core. An 8”
scope resolves the cluster all the way to the core, revealing an amazing arrangement of tiny pinprick white and yellow stars of 11th and 12th magnitude. Take a quick glance to the side with
averted vision, and the hundreds more stars snap into view in an almost shocking manner.
The cluster contains some 500,000 stars with an age of 13 billion years, not much younger than
the universe itself. While M13 lies a distant 25,000 light years from Earth, Kurt Vonnegut wrote,
accurately, “Every passing hour brings the Solar System forty-three thousand miles closer to
Globular Cluster M13 in Hercules — and still there are some misfits who insist that there is no such
thing as progress.”
NGC 6207
Once you’ve taken a good look at M13, try for something a little more challenging: the galaxy NGC
6207 just 2/3 of a degree to the northeast. At magnitude 11.6, this is not an easy object to spot,
though it does fit in the same low-power field of view as M13. Dark sky is a huge help in finding
this object. In a 4-8” scope, it will appear simply as an oval grey smudge. But you are getting a
great depth of field here. The individual stars in the foreground are perhaps a few hundred light
years away, M13 is 25,000 light years away, and NGC 6207 is about 46 million light years from
Earth!
M13, with galaxy NGC 6207, lower left, in the same field of view
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Deep-Sky objects in the tour of the constellation Hercules
M92
Poor M92. Overshadowed by its beautiful and famous sister M13, it sits alone and forgotten in the
northern reaches of Hercules. But if M92 were in most other constellations, it would rank as a
showpiece object. This is a splendid cluster in a small scope. At moderate magnification in a 4”
scope, it easily resolves into individual stars in the halo, and a larger scope reveals a core of fine
diamond dust. The cluster appears somewhat oval compared to M13, and perhaps slightly more
concentrated near the center. M92 is about the same distant away as M13, so its slightly smaller
apparent size is a consequence of its smaller diameter.
The cluster is located north of the Keystone, about 7o north of pi (π) Herc. It forms a nearly
equilateral triangle with pi and eta with each side measuring 7o or so. In very dark and clear sky,
those with keen eyesight may see M92 without optical aid.
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NGC 6210
South of the Keystone, about 4o northeast of the star beta (β) Herculis, you will find the little-known
planetary nebula NGC 6210. Robin’s-egg blue, this tiny planetary is a challenge for 3-4” scopes, in
which at low-power it’s often mistaken for a star. As always with small planetaries, to distinguish it
from background stars, try higher magnifications to resolve the nebula’s disk. Another trick... hold
a nebula filter up to your eyepiece and look at the apparent star field. The filter will dim and
discolor stars while leaving the nebula unaffected.
This planetary is some 3,600 light years away. In large scopes, or in long-exposure photographs,
the nebula takes on a twisted oval shape that leads some to call it the “Turtle Nebula”.
NGC 6210
Dolidze-Džimšeleljšvili 9
Named after its Georgian discoverers, this pretty little open cluster is a pleasant surprise in this part
of the sky, where such objects are rare. To find this group, look for the line of stars starting at delta
(δ) and extending to lambda (λ), mu (μ), and xi (ξ). Follow this line about 2.5o beyond xi to find the
cluster. In your telescope at low power, you’ll see about 25 stars scattered over half a degree. The
group has no concentration near the center, but it’s arranged in two curved lines running north and
south. The brightest star in the field of view is 104 Herculis. South of the cluster is another pair of
7th magnitude stars that make a nice color contrast. The distance to this cluster has not been
published.
Alpha Herculis
Now to a series of binary stars in this part of the sky, starting with the best-known, alpha (α)
Herculis. Also known as Rasalgethi (Arabic for “The Head of the Kneeler”), this is a splendid star
for visual observation. The components have a stirring color contrast of orange and blue-green… a
very pretty sight.
You’ll need a scope at 100x to resolve these two: they’re separated by about 4.6” (arc-seconds),
which is fairly tight. These stars take nearly 3,600 years to revolve around each other.
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The brighter star of alpha Herculis is an enormous red giant that’s 400 times the diameter of the
Sun and some 800 times as bright. The pair have magnitudes of 3.5 and 5.4 roughly, and are
some 380 light-years away. The fainter star is itself a closely-space double with an invisible white
dwarf companion.
Rasalgethi
Kappa Herculis
This double star is to alpha Herc what M92 is to M13: overlooked. This is a lovely, easy-to-split
yellow-and-garnet-red combination that’s split by a generous 27”, so it can be resolved in any
telescope at low power. A 4-6” scope gives the best view of the color contrast. Smaller scopes
do not collect enough light to fully stimulate the color center of your retina, while larger scopes
collect enough light to overwhelm the eye.
Despite their appearance, the two stars are not physically associated. The brighter 5th magnitude
component of this optical double is 390 light years away, while the fainter 6th magnitude star is
470 light years distant.
95 Herculis
On now to the subtle binary star 95 Herculis. It is a lesser known double star, but its color contrast
can be even more vivid than Rasalgethi. At a distance of 470 light years, the pair is separated by 6
arc-seconds and shines at magnitude 4.8 and 5.2. These are both evolved stars that have moved
on to burning heavier fuels in their core. While separately, the stars should appear white and
yellow, their proximity tricks the eye into seeing more vivid color. Admiral Smythe, a prolific 19th
century amateur astronomer, described the pair as “apple-green and cherry red”. What colors do
you see?
Another tip for observing double stars cannot be repeated enough... do not use averted vision.
You want to saturate the color-sensitive part of your retina.
Zeta Corona Borealis
Next door to Hercules, in tiny Corona Borealis, you will find a handful of pleasing doubles. Perhaps
the best is zeta (ζ) CrB, a pretty blue-white and greenish-white double that is little observed. The
spacing is a little tight, about 6.3”, but still resolvable at moderate magnification in any telescope on
all but the most turbulent nights. The 5th and 6th magnitude pair lies in the northern edge of the
constellation. It makes an equilateral triangle with theta (θ) CrB and delta (δ) Bootis.
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Sigma and Nu Corona Borealis
This pair lies on the eastern edge of the constellation about 4 degrees north-northeast of iota (ι),
the star in the eastern tip of the crown. Sigma is also about 2o northeast of rho (ρ) CrB. The
magnitude 5.6 primary of sigma (σ) CrB is described as blue-white or yellow white, though most
see it as yellow. Same with the magnitude 6.5 secondary. At a separation of 7.1”, the stars are a
fairly easy split at 100x.
Directly 1.5o east of sigma is the much wider double star nu (ν). This 5th-magnitude pair is
extremely roomy with a separation of 360”. You can split these yellow-orange stars with
binoculars, or with keen eyes, without any optics at all.
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Tour 3 - Lyra
While Lyra, the Lyre, is a small constellation, it’s one of the most recognizable in the night sky.
Legend holds that the lyre belonged to Orpheus, who played music of such beauty, the god of the
underworld, Pluto, was persuaded to allow the wife of Orpheus, Eurydice, to return to the world of
the living. However, Pluto demanded that Orpheus not look back until both reached the light of the
world. Orpheus looked back just before the couple emerged from the underworld, and lost
Eurydice forever. He wandered the world in sadness for the rest of his days, and the gods placed
his lyre in the sky to honor him and his beautiful music.
The constellation takes the shape of a small parallelogram formed by the stars delta (δ), zeta (ζ),
beta (β), and gamma (γ), with blue-white Vega marking a jewel in the head of the instrument. While
small, Lyra is situated in a fine star field and holds many lovely sights for small telescopes.
Object
Type
Mag.
Dist. (ly)
RA
Dec
Comments
M57
PN
8.8
2300
18h 53.5m
33o 02’
Ring Nebula
M56
GC
8.2
31000
19h 16.6m
30o 11’
δ1,δ2
DS
4.2/5.6
1000
18h 54m
36o 55’
Nice color contrast
Stephenson 1
OC
3.8
1000
18h 54m
36o 53’
~10 stars, including deltas
β Lyrae
VS
3.3-4.3
900
18h 50.1
33o 22’
Eclipsing binary
ε Lyrae
DS
5.0/6.1
160
18h 44.3m
39o 38’
The “Double Double”
150
18h 44.6m
37o 36’
19h 09m
34o 40’
5.3/5.4
ζ Lyrae
DS
4.3/5.7
Σ2470, Σ2474
DS
7.0/8.4
Optical “double double”
6.8/7.9
Table 3 - Objects to see in the constellation Lyra
M57 - The Ring Nebula
The Ring Nebula is perhaps the most famous object in Lyra and the best-known of all planetary
nebulae. Even a 3” scope at 50x shows the tell-tale “smoke-ring” shape of M57. M57 is located
on a line between gamma and beta Lyrae, about 3/5 of the way from the former to the latter.
The nebula can be seen even in binoculars, though it appears star-like at such low magnification.
Telescopes give the best view. M57, like most planetaries, can take a good deal of magnification,
so experiment with your equipment and try low to high powers to get the best view.
19
The outer ring of M57 is lighter than the inner portion, which itself is lighter than the background
sky. The central star, which is intermittently blowing off its outer layers to form the nebula, is visible
only in larger telescopes (10” to 12” or more). The nebula is likely shaped like an hourglass; we are
looking at it from the top.
The blue-green light of the nebula is created by rarified oxygen (OIII) ions as they change to a lower
energy state. Red light is created by the same process in hydrogen atoms. An OIII or UHC filter
can improve the contrast of this nebula (and most planetary and emission nebulae) in light-polluted
sky.
The Ring Nebula (M57) as it appears in a small telescope
M56 - A Dim Globular
The “other Messier” object in Lyra is the faint globular cluster M56. Located about 4o eastsoutheast of gamma Lyrae, this overlooked glob shares the field of view with a 6th-magnitude redorange star. The cluster appears small because it is fairly far away, about 31,000 light years. In a
small scope, it remains unresolved... just a grainy smudge that brightens towards the center. In a
6-8” scope at 200x or more, with good seeing, you may resolve some stars in the outer halo. The
cluster sits in a lovely star field.
The cluster spans about 9’, so given its distance, it is about 85 light years across.
Delta (1,2) Lyrae
The northwest star in the parallelogram of Lyra is the optical double δ1 and δ2 Lyrae. These stars
are aligned by chance, and are not physically associated. Delta 1 is about 1000 light years away,
while delta 2 is 100 light years closer. The pair makes a lovely color contrast. Brighter Delta 2 is a
gigantic red giant star of magnitude 4.2, well past its youthful phase and surely burning heavier
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elements in its core. Delta 2 is a blue main sequence star, shining at an apparent magnitude of
5.6. The pair is easily split in any telescope.
A map of the constellation Lyra
Stephenson 1
Delta Lyrae is surrounded by a little-known but attractive star cluster called Stephenson 1. The
cluster holds some 50 stars, with perhaps 10-15 visible in a 4” scope. The cluster occupies 1/4
degree around the deltas, although most of the stars lie just to the south of this pair, scattered in a
haphazard fashion with no concentration towards the center. The blue delta-1 Lyrae may belong
to the cluster.
Beta Lyrae
Beta Lyrae, also known as Sheliak, is a fine variable star and a remarkable example of an eclipsing
binary. Sheliak is the southeastern star in Lyra’s parallelogram. The two stars of this system are
“semi-detached”, which means they revolve about their center of mass at such close proximity that
they are almost touching. The fainter star, a blue-white main sequence star, is transferring mass to
its nearby companion, which has a disk of accreted matter spinning around it. Both stars are
distorted into ellipsoids by gravitational forces and rapid rotation.
No telescope can directly resolve the components of beta Lyrae; astronomers determine the
presence of each star spectroscopically. But you can easily detect the effect of one star passing in
21
front of the other by visually observing the change in brightness of the pair. Sheliak changes
continuously from magnitude 3.3 to magnitude 4.3 over a 12.9 day period. The other star at the
bottom of the parallelogram, gamma Lyrae, is also magnitude 3.3 so it serves as an excellent
reference star to observe the eclipse.
You can see two more stars within 1.5’ (arcminutes) of beta Lyrae. The closer is magnitude 7.2
and the farther is magnitude 9.9. The stars are not physically connected to the beta Lyrae system.
Zeta Lyrae
The northeastern star of the parallelogram of Lyra is also an interesting double star. Zeta 1 and
zeta 2 Lyrae lie about 44” apart. The pair lack the color of delta, but are still pleasing to see and
are easily separated by any scope at 20-30x. The pair appears white to yellow-white to most
observers.
The brighter star of Zeta Lyrae has an abundance of metallic elements in its spectrum, possible a
result of its slow rotation and the force of radiation pressure. It has some five times the abundance
of iron in its atmosphere compared to our Sun.
Epsilon Lyrae
The showpiece multiple star of Lyra is the famous “Double Double”, also known as epsilon Lyrae.
The star is easily found just 1.7o east-northeast of Vega.
There are four stars here, all gravitationally interacting with each other. Sharp-eyed observers can
resolve the two distant pairs of stars, which are 208” apart (about 3.5’). The northernmost star is
ε1, and the southernmost is ε2. Binoculars or any telescope easily separate this pair.
Both epsilons are themselves double stars. ε1 consists of two stars of magnitude 5.0 and 6.1
separated by a tight 2.6”. ε2 is even tighter at 2.4”, with two evenly-matched components of
magnitude 5.3 and 5.4. Decent atmospheric seeing is required to split the two pairs of of stars. A
3” telescope at 100x will do the job, and higher magnification will separate the components nicely.
While there is not much color in these mostly-white stars, the system makes a striking image in a
small telescope. ε1 and ε2 are 0.16 light years apart, and so interact weakly. The stars take at
least 500,000 years to revolve around each other.
More detailed measurements with larger telescope reveal more stars in this system. A fifth
component orbits the pair ε2 , and there may be other nearby stars bound to the system as well,
for a total of ten stars!
22
Epsilon Lyrae as is appears in a small telescope at 90x, approximately
Struve 2470 and 2474
Not many know this, but Lyra has a second “Double Double”, the two stars Σ2470 and Σ2474.
The pair lies just west of the quadrilateral and make for a striking sight in a small telescope.
(The “Σ” refers to a double star in the extensive catalog of F. W. G. Struve, a 19th-century Russian
astronomer who measured thousands of double stars. The star names are pronounced Struve
2470, for example).
The pair are fainter and wider than epsilon Lyrae, and lie some 10’ apart compared to epsilon’s 3.5’
separation. Each pair is itself wider and fainter than epsilon. Σ2470 is 14” apart, with components
of magnitude 7.0 and 8.4. Σ2474 is 16” apart with components of magnitude 6.8 and 7.9. While
Σ2470 and Σ2474 are likely true binary stars, they are probably not related to each other and are
aligned by chance along the same line of sight.
23
Tour 4 - Ophiuchus and Serpens
Ophiuchus, the Serpent Bearer is a large dim constellation between Hercules to the north and
Scorpius to the south. The constellation is identified with the ancient healer Asclepius, who
according to legend, discovered the healing arts when he saw one snake lay healing herbs on the
head of another. Artful star atlases show Ophiuchus grasping a serpent, which is represented by
the constellation Serpens to the west. Serpens is split into two discontinuous sections, the head
(Serpens Caput) and the tail (Serpens Cauda).
Ophiuchus also falls along the ecliptic, the band of sky in which we find the Sun, Moon, and
planets. The 12 constellations along the ecliptic are called the zodiacal constellations. Ophiuchus
was not included because ancient astrologers considered it unlucky to include 13 constellations in
the zodiac.
While there are no bright stars in Ophiuchus, the constellation is chock-a-block with globular and
open star clusters, as well as dark nebulae in the southern extremes of the constellation near the
border with Scorpius. It holds some interesting double stars as well.
Taurus Poniatowski
Even a casual look at the night sky south of Hercules reveals a small group of stars that looks like a
mad little bull charging east towards the Milky Way. This horned beast is a striking sight, and,
frankly, a little unsettling. But it points the way to some good star fields that make a lovely sight in
binoculars or a small telescope.
This little star pattern (or asterism) is called Taurus Poniatowski, or Poniatowski’s Bull. It was
named in 1777 by Abbe Poszobut after King Stanislaus Poniatowski of Poland. For a time, this
little group was considered a constellation, although it’s now part of Ophiuchus. While “Taurus P”
didn’t make the cut as a modern constellation, the name of this star group remains.
And it’s a pretty little group. In binoculars, the background is flecked with fainter 9th and 10thmagnitude stars that straggle off the western edge of the Milky Way. The V-shaped head of the
bull consists of three stars: 67, 68, and 70 Ophiuchi. The two stars at the back end of this little
beast are gamma and beta Ophiuchi (Cebalrai).
Object
Type
Mag.
Dist. (ly)
RA
Dec
Comments
70 Oph
DS
4.2/6.0
16
18h 5.6m
+2o 30’
88 year orbital period
67 Oph
DS
4.0/8.6
2,000
18h 0.6m
+2o 56’
IC 4665
OC
4.2
1,400
17h 46.0m
+5o 40’
Large, loose, lovely
IC 4765
OC
5.4
1,300
18h 39.0m
+5o 27’
Large; use low magnification
24
Object
Type
Mag.
Dist. (ly)
RA
Dec
Comments
NGC 6633
OC
4.6
1,000
18h 27.7m
+6o 34’
Smaller than IC 4756
Σ2375
DS
6.4/6.7
700
18h 45.5m
+5o 50’
Tight double star
Cr 350
OC
6.1
-
17h 48.1m
+1o 18’
NGC 6572
PN
8.0
2,000
18h 12.1m
+6o 51’
M10
GC
6.6
14000
16h 57.1m
-4o 06’
M12
GC
6.6
16000
16h 47.2m
-1o 57’
M5
GC
5.8
24000
15h 18.6m
+2o 05’
5 Ser
DS
5.0/10.0
81
15h 19.3m
+1o 45’
M19
GC
7.2
28000
17h 02.6m
-26o 16’
NGC 6293
GC
8.2
28000
17h 10.2m
-26o 35’
M9
GC
7.7
24000
17h 19.1m
-18o 31’
B64
DN
-
17h 17.0m
-18o 33’
-
“Emerald Eye” Planetary
Bright globular cluster
Dark nebula near M9
Table 4 - Objects to see in the constellation Ophiuchus
70 Ophiuchi
The nearby star 70 Ophiuchi in Taurus Poniatowski is one of the best-known and widely studied
binary star systems. The two components of 70 Oph have magnitude 4.2 and 5.9; the brighter
star is a yellow-gold while the fainter looks orange-red, with some observers reporting a tinge of
violet. Move your telescope out of focus just a touch to see the colors well. Each star has an
intrinsic brightness only a fraction that of our Sun.
The components complete a revolution about each other in just 88 years. And because the system
is only 16 light years away, the stars are close enough to resolve in a backyard telescope. So this is
one of the few double stars you can see revolve appreciably during a human lifetime.
The average distance between the stars is about the same as the distance between the Sun and
Uranus. The stars were closest together in 1989. Since then, their separation has quickly increased
from 1.7 arc-seconds to about 5 arc-seconds. You’ll need about 100x to resolve them cleanly with
a telescope.
67 Ophiuchi
Just across the ‘V’ of Taurus Poniatowski lies another pleasing double star 67 Ophiuchi. Both
components are blue-white and are widely separated by 55”. But there is a large difference in
brightness: the primary star is magnitude 4.0 and the fainter star just to the southeast is magnitude
8.6, a difference of 70x. So look carefully to see it.
25
The asterism Taurus Poniatowski at the north end of Ophiuchus
IC 4665
Now look for the lovely open star cluster IC 4665 just 1.3o northeast of Cebalrai (beta Ophiuchi). In
dark sky, IC4665 is just visible to the unaided eye. If you’ve got a little light pollution, you’ll need
binoculars to spot it. The cluster is spread out over a full degree, more than twice the diameter of
the full Moon, so it looks fainter than its integrated magnitude of 4.7
In binoculars or your finderscope, you’ll see perhaps a dozen stars; a small telescope at 25-35x
shows a few dozen blue-white stars. At an age of 35 million years, this little cluster is young
compared to robust grand-dad open clusters like the Beehive (M44) which looks similar but is
more than 600 million years old. The youth of IC4665 means few if any stars have evolved into red giants or supergiants. So the
color of its stars is fairly uniform. But the apparent structure of the cluster invites a long gaze.
Look for arrangements among the stars, especially short intertwined arcs. If southwest is “up” in
your field of view when you look at IC4665, look carefully at the inner stars. They form the pattern
of the word “HI”, like a big friendly cosmic greeting. While not obvious at first, it’s a little unnerving
when the pattern finally jumps out at you! On the east side of the cluster lies the wide double star Σ2216. The stars are magnitude 8 and 10,
approximately. IC 4665 is about 1,400 light years away.
26
IC 4756
About 5o northeast of the little bull’s eastern horn lies the large, loose cluster IC 4756. This is an
excellent cluster for binocular and low-power telescopic viewing. The cluster spans a full degree of
sky, so unless you have a wide field of view, IC 4756 may overfill your field and not appear like a
cluster at all. If you can take it all in, you will easily see a lovely patch of 80-90 stars set in a rich
background. The cluster was missed by early telescopic stargazers like Messier because their
telescopes simply did not have the field of view to show the object as a cluster. Even the
venerable Norton Star Atlas did not include this sprawling star cluster. And visual observers often
passed it by because it is set in such a rich portion of the Milky Way that it does not immediately
stand out. But it is a splendid, ancient cluster, more than 600 million years old and packed with
stars arrayed in all types of shapes and patterns.
NGC 6633
Just west of IC 4756, look for its equally splendid neighbor, the cluster NGC 6633. The two
clusters are sometimes called the “Ophiuchus Double Cluster”, like the more famous Double
Cluster in Perseus. NGC 6633 appears much smaller than IC 4756, so it is easier to spot in a
telescope. Lower magnification is still recommended. Look for 30-40 stars arranged in a thick bar
that extends northeast to southwest, and another dozen stars outside the bar.
The two clusters are about the same age and same distance from us, so NGC 6633 is truly smaller
in size, spanning about 5.8 light years to IC 4756’s 15.2 light years. What NGC 6633 lacks in size
it makes up for in sheer beauty: many amateurs rank this cluster among their favorites upon first
seeing it. There are many visible patterns among these stars, including a very evident “hookshape” pattern moving from the middle of the cluster and arcing eastward, ending at a bright 6th
magnitude star.
Both IC 4756 and NGC 6633 are some twenty times older than IC 4665, so there will be some
aged orange stars here evolved off the main sequence.
27
IC 4756 (left) and NGC 6633, the “Ophiuchus Double Cluster”
Struve 2375
Immediately east of the cluster IC 4756 (about 1.6o), look for the fairly tight double star Σ2375.
Both components are whitish, one of magnitude 6.4 and one of 6.7. The separation is just 2.5”, so
use magnification of 100-125x in a 2.4” or larger scope. Each component is itself a very closely
spaced pair, too close to be resolved directly in a telescope.
Collinder 350
Look for the obscure open cluster Collinder (Cr) 350 about 1.4o south of the star gamma Ophiuchi.
At low magnification, say 20x, you will see perhaps 30 stars arrayed over 50’ of sky. The stars
range from magnitude 9 to 11, and seem to splay out from the middle of the cluster in four spidery
arms. The cluster itself is magnitude 6.1, visible (barely) in dark sky.
NGC 6572
Before you leave the region of Taurus Poniatowski, take in the pretty little planetary nebula NGC
6572, sometimes called the “Emerald Eye Planetary”. The name comes from the somewhat
greenish hue of this small bright gem-like nebula. The color comes from the light emitted by OIII
oxygen ions excited by the nebula’s hot central star.
NGC 6572 is 1.5 magnitudes brighter than M57 in Lyra and five times smaller, so it has a high
surface brightness and is easily seen in a small telescope. But because it is so small, it appears
star-like at low magnification (<50x). The nebula reveals a small disk only at 70x or higher. Try as
much magnification as you can to bring out detail in this tiny object. At 100x, you may begin to
see a diffuse elliptical halo. Try an OIII or UHC filter if you have one to bring out more contrast and
detail.
28
Most planetary nebulae last for just 50,000 years or so before they expire, which is not long in
astronomical terms. But NGC 6572 is young even by these standards: just 2,600 years old. So it
will likely expand, dim slightly, and change its shape slowly over the coming centuries.
Look for this little celestial emerald above the horns of Taurus Poniatowski, and about 1.3o
southeast of the star 71 Ophiuchi.
The globular clusters of Ophiuchus
M10
Now lets move south into the body of Ophiuchus and find some excellent globular clusters,
beginning with Messier 10. The glob lies some 14,000 light years from Earth. Look for the cluster
just 1o west of the orange star 30 Ophiuchi. M10 appears brighter than its neighbor M12, and
certainly more concentrated towards the center. The cluster is visible in binoculars or a
finderscope as a dim smudge. In a 4” scope at 70x, you will see some granularity on the outer
29
edges of the cluster. A 6” or larger scope resolves a couple of dozen stars of this relatively nearby
star cluster.
M12
M12 is just 3o northeast of M10. The two are visible in the same finder field of view, and even in a
fast telescope at low magnification. The cluster is certainly less dense than M10, and a good 6”
scope may resolve some stars near the core. Try a range of magnification to see what works best.
Some see in globular clusters a very subtle range of hues, with the core appearing straw colored
and the stars in the halo somewhat blue-white. This may be a matter of perception, because most
globular cluster have very few blue stars. With ages of 10-12 billion years, the blue stars have long
ago ended their lives, with only old, low-mass, yellow-white stars remaining.
M12 is about 16,000 light years away from us. But it must be just 2,000 light years away from
M10. A planet around a star in either cluster would see the the other cluster shine with a quite
bright magnitude 2.0 or so.
M5
Now to Messier 5, the brightest globular cluster north of the celestial equator (yes, it’s little brighter
than the more famous M13 in Hercules). The cluster is located about 7.5o southwest of alpha
Serpentis, the brightest star in Serpens.
This cluster is a splendid object in a large telescope, with a tight sparkling core and innumerable
tiny stars spraying throughout the halo in lines and arcs. A small telescope gives a lovely view as
well, with fewer stars resolved but an “electric spark” seeming to emanate from the core of the
cluster. Most agree the cluster is perceptibly non-circular, or at least non-symmetric, with the core
appearing somewhat brighter to the north.
At 13 billion years of age, M5 is one of the oldest globular clusters. And it’s one of the largest, at
130 light years across. The cluster is 24,000 light years away... almost twice as far as M10 or
M12, yet easily outshining both.
Globular cluster Messier 5 in Serpens
30
5 Serpentis
Look for a bright double star just south of M5, within the same low-power field of view. This is 5
Serpentis, also known as Σ1930. The primary is magnitude 5, and the companion is magnitude 10
and lies about 11” to the northeast. The difference in brightness makes this a good challenge of
your observing skills. A small scope can split the stars at 70-80x or more, but the brighter star
tends to overwhelm the fainter.
M19
Now to the southernmost regions of Ophiuchus, where star clouds and knots of dark nebulae are
thick. Find the bright red star Antares in Scorpius (the brightest star in this part of the sky), and
look east about 3/5 of the way to the moderately bright star theta Ophiuchi. Here is where to look
for 7th magnitude M19. It’s not as impressive as M5, but it still presents very clear brightening
towards the core and some resolution of halo stars at 100x or more in a small telescope. The
cluster is clearly non-circular. In fact, M19 is one of the most distended globular clusters in our
galaxy, with a drawn-out oval shape. This may be a result of tidal interactions with the core of the
Milky Way, which is just 5,000 light years from M19.
NGC 6293
Just 1.7o east of M19 is the fainter globular cluster NGC 6293. The pair fits into the same lowpower field of view in most telescopes. The cluster is much smaller than M19, and more difficult to
resolve.
M9
Yet another globular cluster in this starry region of sky, M9 shines at a modest magnitude 7.7. It is
just 3.5o southeast of the star eta Ophiuchi. At just 3’ across, the cluster needs at least 100x to
give up a few resolved stars. The eastern edge is more easily resolved. The region below and to
the east of M9 holds many dark nebulae, which appear as voids in the star fields.
Barnard 64
Just west of M9, with low magnification, look for the small dark nebula Barnard 64. This is a patch
of cold dust and gas in the foreground that blocks out the most distant stars in the Sagittarius arm
of the Milky Way. It takes a bit of practice to see dark nebulae, since you are accustomed to
looking for something, not nothing! Larger dark nebulae are best seen with binoculars or without
optics altogether. B64 is one of the few that easily fit into the field of view of most telescopes. And
the contrast with M9 makes for an even more beautiful sight.
31
Globular cluster Messier 9 (left) and dark nebula B64 (right)
32
Tour 5 - Scorpius
Scorpius, the Scorpion is one of the handful of constellations that look like their namesakes. The
great cosmic scorpion makes a grand entrance “claws first” over the horizon each June, followed
by the bright red star Antares that marks its heart. Then his long winding body slowly arcs into the
sky, followed by a long, curved stinger. The constellation is a dazzling sight. There are many bright
blue-white stars visible in Scorpius, and several of the best star clusters in the night sky. Most of
the stars belong to the Scorpius-Centaurus OB association, a group of bright blue white stars that
share a common origin and still move together through space.
The constellation is low on the southern horizon for northern stargazers, and those in Canada and
northern Europe may not be able to see its southernmost reaches. In the southern hemisphere,
the constellation arcs high in the southern winter sky and makes for excellent viewing in this region
near the rich center of the Milky Way.
Legend almost always associates Scorpius with Orion. In one tale, the mighty hunter Orion
boasted he would kill every animal on Earth. Artemis, goddess of the hunt and of wild animals,
was distraught and sent a scorpion to deal with Orion. The battle was fierce, but the scorpion
killed the great hunter. Zeus placed them in the sky to remind mortals of the dangers of excessive
pride.
Object
Type
Mag.
Dist. (ly)
RA
Dec
Comments
β Scorpii
DS
2.6/4.9
530
16h 05.4m
-19o 48’
Graffias
ν Scorpii
DS
4.0/6.3
440
16h 12.0m
-19o 28’
Antares
DS
1.1/5.5
600
16h 29.4m
-26o 26’
M4
GC
5.9
7000
16h 23.6m
-26o 32’
Near Antares
NGC 6144
GC
9.0
30000
16h 27.3m
-26o 02’
Near Antares and M4
M80
GC
7.2
32000
16h 17.0m
-23o 00’
False Comet
OC
2.5
-
16h 54m
-42o 00’
Near zeta Sco; includes NGC 6231
M6
OC
4.2
2000
17h 40.1m
-32o 13’
Butterfly cluster
M7
OC
23.3
800
17h 53.9m
-34o 49’
Table 5 - Objects to see in the constellation Scorpius
33
Beta Scorpii
The star beta Scorpii goes by two names, Achrab and Graffias. It’s a fine double star in a small
telescope, and easily split in a 3-inch scope at 50x. The main components are about 14” apart
and shine at magnitudes 2.6 and 4.9. Each component is a hot and massive young star which will
one day, like most bright stars in this part of the sky, explode as supernovae. Each component of
Graffias is itself a close double star, and the brighter may have another star associated with it. So
Graffias is a five-star system.
The constellation Scorpius, including M4, M80, M6, M7, and the “False Comet”
Nu Scorpii
Just half a degree northeast of Graffias (or Achrab), look for a more challenging double star nu (ν)
Scorpii. The two brightest components are easy to split. But look closely at the fainter
component. It, too, is a double star. You’ll need at least a 4” scope at 130-150x to split this star,
which is separated by 2.4”. The brighter star is also a double and is even harder to split, with a
34
separation of just 1.3”. This is near the limit of a 3-4” scope, and often near the limit of average
seeing conditions: if you split this star, you have done well! The entire nu Sco system is comprised
of blue sub-giant and main sequence stars. There may be as many as seven stars associated with
the system.
Antares
Antares is the brightest star in the constellation Scorpius and one of the brightest stars embedded
in the sweeping arc of the Milky Way. It’s beautiful from the northern hemisphere, and like much of
Scorpius, even more dazzling from the south where it lies almost directly overhead in June and
July.
Many new stargazers mistake Antares for the planet Mars. So did the ancient Greeks who named
it ant-Ares, which means “compared to Ares” (Ares was the Greek name for Mars, the god of war).
Like all red supergiants, Antares will soon run out of fuel. It has burned hydrogen, then helium,
then carbon in its core. Then it will burn and neon and magnesium into heavier elements like iron. But it will go no further: fusing iron into heavier elements creates no energy. So when the fuel finally
runs out, Antares will collapse and detonate as a supernova, the biggest explosion known in the
universe. From our sky, it will shine bright enough to cast shadows in the dead of night for many
weeks.
And this could happen anytime, though forecasting supernovae is less exact than forecasting
earthquakes. Astronomers believe Antares could explode in a week from now, perhaps, or in a
century, or in a million years. But whenever it goes, it will be quite a show. Antares, which is slightly variable like many end-stage stars, is listed as the 15th or 16th brightest
star in the sky at visual wavelengths. Intrinsically, it’s about 10,000x as bright as our Sun, and
some 60,000x brighter if infrared wavelengths are included.
Antares presents an excellent challenge for backyard stargazers with a telescope. The star has a
companion, a blue-giant star called Antares B, that’s nearly 3 arc-seconds away from the reddish
Antares A, but some 370x fainter. Seeing the fainter star is not easy… it’s like trying to look for a
firefly in the glow of a bright streetlight. Still, it’s worth a try. You’ll need a good 8-10″ scope to
separate the two stars, and a magnification of perhaps 200x.
The Moon often passes near Antares, and occasionally passes in front of it (an occultation). When
this happens, bright Antares A is blocked for a brief time, making it easy to spot Antares B, even
with a good 3″ scope. Many observers say the fainter star looks greenish compared to its brighter
red companion.
35
The comparative size of Antares and other well-known stars.
M4
Just 1.5o directly west of Antares lies Messier 4, the brightest of the globular clusters of Scorpius.
M4 is a splendid 6th-magnitude cluster. It appears much more “open” than most globular clusters,
which makes it easier to resolve in a small telescope.
The cluster is bright enough to be observable with the unaided eye. But the nearby glare of
Antares makes it all but impossible to do so. In binoculars, the cluster is easy quarry and appears
round and bright and as large as the full Moon.
Even a 3-4” scope at 80x will resolve some stars, at least in the halo. And an 8” scope will resolve
the cluster to its core and reveal hundreds of yellow-white stars. Most observers notice a loosely
defined “bar” of stars in M4 than runs north-south. To some, the cluster looks a little like a cat’s
eye, with the bar taking the form of the eye’s pupil.
With so many resolvable stars, you will begin to see an amazing array of patterns in M4, defined
not just by the position of stars but the positions of their absences. The dark lanes, with no stars,
seem to radiate and ripple from the center of the cluster. De-focus your telescope slightly and relax
your eye to see this remarkable effect.
At at a distance of just 7,000 light years, M4 one of the closest globs. If you have dark sky and a
4” or larger scope, try to see the much more distant globular cluster NGC 6144 just 1/2 a degree
36
northwest of Antares. The neglected cluster is more than 30,000 light years away. But it makes a
striking though challenging sight between large M4 and bright Antares.
Antares, left, and the globular cluster M4, right. The two are separated by 1.5o. To the upper right of Antares
is the fainter and much more distant globular cluster NGC 6144.
M80
M80 is the other major globular in Scorpius, though it’s a little more challenging to see. It’s a
compact and rich cluster, and intrinsically quite dazzling. But it’s 32,000 light years away and so
appears quite dim in our skies. You will find the cluster almost exactly halfway between beta
Scorpii and Antares. At magnitude 7.2, it’s not visible without optics. The brightest stars of this
cluster shine at magnitude 13, so it is very difficult to resolve any individual stars in a telescope of
less than 8” aperture. Still, it’s a pleasure to take in the 10-billion-year-old yellowed light from so
distant an outpost of our galaxy.
37
The False Comet
Now to the splendid complex of bright stars and star clusters known as the “False Comet”. This
sparkling group of associated stars in a rich section of the Milky Way presents the uncanny
appearance to the naked eye of a small comet. It's a beautiful sight.
The “False Comet” is a group of two star clusters next to a string of brighter stars in the tail of the
constellation Scorpius. The collection starts at zeta (ζ) Scorpii and spans some 2 degrees of sky.
The aggregation appears like a small comet with a curved tail pointing northward into the Milky
Way.
Easily visible with the naked eye, the “False Comet” has been known since antiquity, although
comet hunter Charles Messier was too far north to include it in his famous catalog. The star cluster
NGC 6231 forms the “head” of the comet; the large open cluster Trumpler 24 forms the tail. While
cataloged as separate clusters, these stars are physically associated and formed out of the same
massive nebula only 6-8 million years ago. The collection is roughly 6,000 light years from Earth.
If the head of the False Comet, NGC 6231, were as close as the Pleiades, it would appear about
the same size. But the stars would be 50x brighter, some as bright as Sirius!
This star collection only looks like a “False Comet” visually: looking at it through a telescope breaks
the spell. But a wide-field scope show a wonderfully bright set of new stars set against the rich
background of the Milky Way. Use your lowest magnification to sweep this fine region.
Unfortunately for far northern observers, the False Comet lies in the most southern reaches of
Scorpius, so it’s hard to see if you’re north of 40o latitude.
The False Comet, north of zeta Scorpii
38
M6, the “Butterfly Cluster”
M6 is among the brightest of dozens of open star clusters that fleck the Milky Way in Scorpius and
Sagittarius. Look for it about 4o north of the bright star Shaula (λ Scorpii) in the Scorpion’s tail. M6
is a respectable 4th magnitude, though its light spreads over an area as large as the full Moon. You’ll see the cluster without optics in dark sky; a full Moon or city lights make it harder to see,
especially if the cluster is low on the horizon.
M6 is often called the Butterfly Cluster, and a glance through a small telescope reveals why. At
40-50x, the cluster has 3 bright stars running through the center (the body of the butterfly), with
two irregular loops of stars on either side (the wings). A little imagination reveals the butterfly’s
“antennae” to the northeast. Experiment with different eyepieces to get the best view. There are
not many foreground or background stars here.
The cluster lies some 1,600-2,000 light years away from us towards the galactic center. It holds a
little more than 300 stars, though you’ll see just a few dozen in binoculars, and perhaps 100 stars
in a 6-inch scope. It’s a young cluster… about 100 million year old… so it contains mostly blue
stars, with one orange-giant star (BM Scorpii) in the northeast corner.
M7, lower left, and M6, upper right
M7
Look 3.5 degrees southeast of M6 to find the cluster M7 set in one of the richest sections of the
Milky Way. Though they’re close in the sky, the two clusters are not physically associated. M7 is
closer, just 800 light years away. To the unaided eye, in the words of Stephen J. O’Meara, the
spray of light from M7 looks “like the eruption of distant fireworks.”
39
M7 appears larger and brighter than M6, though both were known in antiquity. But M7 is the
southernmost Messier object, so it’s rarely seen well at northern latitudes, and it presents a real
challenge for observers in northern Europe. The view of this cluster from the southern hemisphere,
however, where it’s high overhead, is jaw-dropping.
The cluster spans more than a full degree of sky, twice the size of the full Moon. So stick with
binoculars or a low-power eyepiece. At 30x, the center of M7 looks square, or to some, crossshaped. Since M7 is twice as old as M6, some of its 80 stars have begun to evolve off the main
sequence and turn orange-red. The hottest and most massive blue stars have, presumably,
burned out long ago. Despite its proximity to M6, which is set in a spartan patch of sky, M7 has a
fantastic setting in a blazing star field in the southern Milky Way.
40
Tour 6 - Sagittarius (Part 1)
Sagittarius, the Archer, is depicted in Greek legend as a centaur, a half-man and half-horse, who is
drawing his bow to fire an arrow westward into the heart of Scorpius, the Scorpion. Some legends
tell the centaur is the noble Chiron, teacher and mentor to many Greek heroes. Others hold that
Chiron is represented by the constellation Centaurus. The Babylonians also identified these stars
as the god-like centaur Pabilsag.
For most stargazers, it’s hard to see in these stars the image of a centaur drawing a bow. To most
modern observers, the main body of the constellation takes on the unmistakable shape of a
teapot.
As seen from Earth, the star clouds of the Milky Way are thickest and densest in Sagittarius
because this is the direction of the center of the galaxy. We can’t see the center of the galaxy from
Earth because dust and gas obscure the view. But we do get a good view of the Sagittarius Arm
of the Milky Way, the next-nearest spiral arm to the galactic center. We get the full view here:
clouds of countless stars, ribs and eddies of dark dust, dozens of open star clusters and emission
nebulae, a few planetary nebulae, and many globular clusters centered around the nucleus of the
galaxy. This is one of the richest regions of the heavens.
Object
Type
M24
-
Mag.
Dist. (ly)
RA
Dec
Comments
4.5
9400
18h 16.9m
-18o 29’
Small Sagittarius Star Cloud
B92, B93
DN
-
-
-
-
Within M24
M18
OC
6.9
3900
18h 19.9m
-17o 08’
M17
EN
6.0
4900
18h 20.8m
-16o 11’
Swan Nebula
M16
EN
6.0
8200
18h 18.8m
-13o 47’
Eagle Nebula
M22
GC
5.1
10000
18h 36.4m
-23o 54’
Splendid globular
M55
GC
6.3
17000
19h 40.0m
-30o 58’
NGC 6822
GL
8.8
1.6 million
19h 44.9m
-14o 48’
Barnard’s Galaxy
NGC 6818
PN
9.3
5500
19h 44.0m
-14o 09’
“Little Gem”
Table 6 - Objects to see in the constellation Sagittarius
Along with Scorpius, the constellation Sagittarius is in the southernmost portion of the zodiac, so it
lies low on the horizon for observers in the late northern-hemisphere summer. In the southern
hemisphere, however, it lies high in the southern winter sky where observers can better see its
many deep-sky sights.
41
The M24 Star Cloud
Let’s begin the tour of Sagittarius with what’s called the “Small Sagittarius Star Cloud”, M24. M24
is unlike any other entry in Charles Messier’s famous catalog. While it looks like a rich open star
cluster, M24 is simply a distant cloud of stars in a spiral arm of the Milky Way.
To be more specific, M24 is a small section of the Sagittarius Arm of our galaxy that’s clearly visible
because we’re looking through a gap in the obscuring dust along the plane of the Milky Way. If
there was no dust or cold gas, the entire Milky Way from Cygnus to Scutum and into Sagittarius
(and beyond into Centaurus and Crux in the southern hemisphere) would appear as bright and
luminescent as M24.
Dark sky is essential to see M24, and indeed all of the Milky Way. M24 appears as a brightening in
the Milky Way just 4 degrees north of mu (μ) Sagittarii, and spans a fairly small 1ox2o rectangular
patch of sky. It is splendid in binoculars and in a telescope with a field of view of at least 2o. The
individual stars range from magnitude 6 down to invisibility in a small telescope. The cloud appears
to shimmer and take on a 3-dimensional quality in a good scope, and some observers see the
color as blue or even green. As you gaze at the star cloud, you may also see a network of dark
lanes and channels as your eye and brain try to make sense out of the profusion of patterns
formed by thousands of sparkling stars.
M24 is over 330 light years wide and lies a fairly distant 9,400 light years from Earth, nearly as
distant as some globular clusters.
Barnard 92 and 93
M24 has within it several dark nebulae, and the bright background of the cloud make these
nebulae easier to observe. Perhaps the most straightforward to see is Barnard 92 (B92), a 1/4degree oval of darkness along the middle of the northwestern edge of M24. Once you find B92,
examine it with a range of magnification to see the numerous bright stars around it, as well as a
single lone foreground star apparently embedded in the nebula. If you can spot B92, try for B93
just 1/3 of a degree to the northeast. This nebula is narrower than B92 but nearly as long. It’s a
little trickier to see because its borders are less defined.
42
The Small Sagittarius Star Cloud (M24) runs from upper left to lower right in this image. B92 is at bottom,
slightly left of center; B93 is just to its right.
M18
Now let’s follow a short chain of remarkable sights up from mu (μ) Sagittarii to the northeast. The
first stop is the fairly sparse open cluster M18. Were this object in another constellation, it would
rank as a worthy sight. But in Sagittarius, with so many bright and famous objects nearby, M18 is
often overlooked.
The cluster is located just less than 1o above the northern edge of M24. It’s easily visible in
binoculars, though it just escapes detection by most visual observers. At low magnification in a
telescope, the cluster appears more concentrated, but any more than 25x will reveal the cluster as
a fairly sparse agglomeration of a dozen stars about 8-9 arcminutes across. Three of the brightest
stars form an arc-like shape along the cluster’s northern edge. The cluster is worth examining at
higher magnification as well to bring out the background star field and add some dimensionality.
M17 (The Swan Nebula)
As you gaze at M18, you are no doubt distracted by a hazy silver glow just 1o to the northwest.
This is Messier 17, and it goes by several names which hint as to how some observers perceive its
shape: the Omega Nebula, the Horseshoe Nebula, the Swan Nebula, the Checkmark Nebula, and
the Lobster Nebula. The shape of a Swan is the easiest to discern, so we will use that name
here...
The Swan Nebula is one of a handful of emission nebulae bright enough to see with the unaided
eye. It is a remarkably beautiful object, especially in dark sky or even in suburban skies with a
good light pollution filter. The brightest part of the nebula, visible in any telescope, is a long silver
bar that extends west-northwest to east-southeast. The neck of the swan is also easily seen
extending southward from the western end of the body. These two sections give the nebula its
43
“check mark” appearance. But with a 3” or larger scope, the neck extends around into a curve to
complete the swan-like shape. There is more nebulosity above the swan’s body, and below and off
the back of the tail.
As an emission nebula, the Swan is set aglow by new stars forming within its central region. Many
such nebulae, including our next stop, M16, are coincident with a cluster of newly-born stars. But
the Swan reveals few of its progeny, although astronomers estimate there are more than 600 stars
embedded within the nebula. M17 lies about 4,900 light years from Earth.
M17, the Swan Nebula in Sagittarius
M16 (The Eagle Nebula)
Now let’s go out of bounds a little, just beyond the borders of Sagittarius into Serpens Cauda.
Here, just 2.5o north-northwest of M17 lies the equally beautiful Messier 16, the “Eagle Nebula”,
another fine emission nebula along the plane of the Milky Way.
Officially, M16 refers to the star cluster embedded within the nebula, which itself is cataloged as IC
4703. The cluster reveals 30-40 blue-white stars in a 4” telescope at 80-100x. This is a young
cluster, certainly less than one million years of age, with some stars considerably younger than that.
The nebula around the cluster is subtler than M17 and not nearly as bright. Time-exposure images
reveal the shape of the nebula as an eagle with wings spread in flight. This shape is harder to see
in a small telescope, though the nebula clearly shows two perpendicular arms that make up the
body and the wings. Most interesting is the dark nebulosity at the nebula’s heart. One “L-shaped”
finger of darkness may be discerned at higher magnification. This structure is sometimes called
“The Star Queen’s Throne”. Close up images reveal many more dark lanes within the nebula that
are blown about by the winds and strong ultraviolet radiation from new stars, and which may
harbor new protostars in the process of formation. The Hubble Space Telescope revealed a series
of three long fingers within the Eagle Nebula that are famously referred to as the “Pillars of
Creation”.
44
The Eagle Nebula is about 20,000 times the extent of our solar system and lies about 9,000 light
years away.
The cascade of Messier objects northeast of mu (μ) Sagittarii: M24, M18, M17, and M16. M22 is just
northeast of Kaus Borealis.
M22
We turn now southward in Sagittarius, just a degree east-northeast of lamba (λ) Sagittarii. Here
you will see the fine spectacle of the globular cluster M22, one of the most visually appealing such
objects in the heavens. The cluster is not nearly as dense at its core as M5, or even M13. So
even a 2.4” or 3” telescope resolves stars in the outer halo of this fine globular, and an 8” or larger
scope resolves the cluster to the core. In a telescope, the image is one of a crushed diamond,
with its dust and fragments crumbled against black velvet.
45
M22 is sometimes called the “Arkenstone of the Stars”, after the description of J. R. R. Tolkien in
The Hobbit of the jewel called the Arkenstone of Thrain, which “... was as if a globe had been filled
with moonlight and hung before them in a net woven of the glint of frosty stars”.
The cluster spans half a degree of sky, as much as the full Moon. The brightness and size of the
cluster are rivaled only by 47 Tucanae and Omega Centauri, two globulars in the deep southern
sky. The brilliance of M22 is somewhat diminished for northern observers because it is never far
above the thick air and dust near the horizon. It is a marvel from the southern hemisphere.
Though even there, you can’t see the cluster’s full splendor because its light is diminished by light
years of dust near the galactic plane. M22 is about 10,000 light years from Earth.
M55
Let’s move to another part of Sagittarius, the somewhat less star-rich region trailing the handle of
the teapot. The first stop in this neglected part of the constellation is the globular cluster Messier
55. Find this cluster by extending a line southeast from the stars sigma (σ) and tau (τ) in the
handle a distance of 2.5x their separation. It’s easily visible in binoculars or a finderscope. This
6th-magnitude cluster is a little more than half the size of M22, though it is still fairly loose
compared to M5, for example. The cluster reveals some interesting dark lanes and arcs in its outer
halo, especially with a 6” scope at 100x or more.
M55 (lower left), and NGC 6822 (Barnard’s Galaxy) and NGC 6818 (the Little Gem) (upper left)
46
NGC 6822 (Barnard’s Galaxy)
Now to a “challenge object”, NGC 6822, one that’s hard to find in almost any telescope but is well
worth the effort to see. NGC 6822 looks like another nebulous patch, or perhaps a tight and hardto-resolve star cluster in the eastern reaches of Sagittarius. But it’s really an object external to our
own galaxy, and is considered a small galaxy in its own right, one that is gravitationally bound to
our own.
NGC 6822 was first discovered by E. E. Barnard in 1884 when he was still an amateur astronomer.
In 1920, Harlow Shapley measured its variable stars to determine it was well outside our own Milky
Way, much farther than globular clusters. While a distant glob might lie 25,000 light years from
Earth, NGC 6822 is 1.6 million light years away!
Barnard’s Galaxy is about 16’x14’ in apparent size, and shines with an integrated magnitude of
8.8, so its surface brightness is fairly low. Look for it just south of a line extended from the stars pi
(π) and rho (ρ) Sagittarii. Try searching at low magnification for the galaxy before powering up.
Dark sky is all but essential.
NGC 6822 is one of a handful of small irregular galaxies bound to our own. The famous Magellanic
Clouds are two much brighter irregular galaxies associated with the Milky Way, though they are
visible only from the southern hemisphere.
NGC 6818
Less than a degree north of Barnard’s Galaxy is a fine little planetary nebula NGC 6818, often
called “The Little Gem”. This small, fairly bright planetary lies within the same field of view as NGC
6822, and appears as a distended star at 30-40x. Higher magnification may bring out the
aquamarine color of this nebula and perhaps a little patchy structure in the tiny halo. Like NGC
6572 in Ophiuchus, the Little Gem is young... just 3,500 years old.
47
Tour 7 - Sagittarius (Part 2)
Let’s continue the tour of the rich region in and around Sagittarius, beginning with two extremely
complex and subtle emission nebulae just above the spout of the “Teapot”.
Object
Type
Mag.
Dist. (ly)
RA
Dec
Comments
M8
EN+OC
5.8
5200
18h 04.0m
-24o 20’
Lagoon Nebula; Includes star cluster (NGC6530) and nebula (NGC
6523)
M20
EN+OC
5.6
5000
18h 02.5m
-23o 00’
+RN
Trifid Nebula; includes star cluster
CR360, nebula NGC6514, and dark
nebula B85.
M21
OC
5.9
4000
18h 04.6m
-22o 30’
NGC 6520
OC
7.6
5300
18h 03.4m
-27o 54’
M28
GC
6.8
18300
18h 24.5m
-24o 52’
Burnham 245
DS
5.8/8.0
360
18h 10.1m
-30o 44’
Piazzi 6
DS
5.4/7.0
900
17h 59.1m
-30o 15’
The “Castaway Cluster”
Table 7 - More objects to see in the constellation Sagittarius
M8, The “Lagoon Nebula”
Look for a tight little patch of silver mist above the spout and just northwest of the star lambda (λ)
Sagittarii. This is Messier 8, the “Lagoon Nebula”. Along with the Orion Nebula and the Eta
Carinae Nebula, the Lagoon is one of the three brightest in the sky and a rewarding object for
stargazers with any size telescope. M8 is visible easily in dark sky and even from many suburban
locations with the unaided eye. Its splendor is somewhat diminished by its low altitude in northern
latitudes, and like so many things, looks better from the southern hemisphere.
Like most emission nebulae, the Lagoon contains a fine star cluster. This one is cataloged as NGC
6530. Try a magnification of 80-90x to find 20-30 stars of magnitude 7 and fainter. The pattern is
arranged in a triangular shape and spans perhaps 1/5 of a degree. Larger telescopes reveal more
than a hundred stars in this cluster.
The nebular component of M8 is NGC 6523. You will see nebulosity around the brighter star 9
Sagittarii just west of the cluster. Look for an hourglass-shaped knot just a little west of 9. A
second patch of nebulosity engulfs the cluster NGC 6530 itself. Between the two nebulous
patches, look for a remarkable black gulf or channel that gives the Lagoon nebula its name. The
48
whole view is best taken in at lower magnification, say 25-50x. The contrast of the nebulosity is
greatly improved by a UHC or OIII filter, even in darker skies.
Within the M8 complex, look just 1/3 of a degree west of the cluster for the 5th-magnitude star 7
Sagittarii, and the double star Argelander (Arg) 31. Here, the nebulosity begins to fade. Arg 31 is a
7th and 9th magnitude star about 34” apart.
Look also for more nebulosity north and south of NGC 6530. Even a 2.4” or 3” scope gives a
remarkable view of the nebula and dust lane. Whatever your optics, make sure to use averted
vision. When you do, an astonishing amount of fleeting detail will pop out. The Lagoon is a object
that should be examined carefully, a little at a time, night after night. In time, it will reveal to you
nearly as much detail as you see in time exposure images. Though you will not see any color in the
nebula... there is not enough light to stimulate the color-detecting cells in your retina.
The main components of the M8, the Lagoon Nebula (bottom), and M20, the Trifid Nebula (top)
M20, The “Trifid Nebula”
The Trifid Nebula is just 1.2o north of the Lagoon. Another splendid sight for a small telescope, the
Trifid is smaller and fainter than the Lagoon but has a more complex network of dark dust channels
that make for challenging and fascinating viewing. The nebula is centered on the 7th-magnitude
star H N 6, which has a magnitude-8.7 companion about 11” to the southwest. The emission
nebula is brightest around this star, and even binoculars will reveal the nebulosity. The stars
embedded in this section set the nebula aglow by ionized hydrogen, which emits reddish light
when it recombines into a lone proton and electron. The nebular part of the Trifid is NGC 6514.
49
The Trifid gets its name from the dark nebula Barnard 85, which appears to split the nebula into
three sections. A 3” scope at moderate magnification (60-90x) can show this “trifurcation”, though
it is not dead easy to see and requires careful viewing to first glimpse it. Again, a nebula filter may
help with contrast. In photographs, B85 appears to split the emission nebula into four sections;
this can be seen visually in larger telescopes.
Images of M20 also show a blue region of nebulosity just north of the reddish region. This is
associated with the star forming region, but it shines by a different process. Fine dust particles
surrounding a 7th-magnitude star reflect its bright blue light into our line of sight, so this type of
nebula is called a “reflection nebula”. It exhibits the same optical spectrum as the star, so an OIII or
UHC filter will not improve contrast here. But the reflection nebula is visible with some effort in a
small telescope, though it is certainly fainter than the emission nebula just to the south.
The reflection nebula is included as part of NGC 6514. The sparse cluster of stars associated with
M20 is often listed as Collinder 360.
Some believe the Trifid and Lagoon might be part of the same star-forming complex. The former is
about 5,000 light years distant, while the latter is about 5,200 light years away. But they may also
simply be aligned by chance.
M21
It will hardly escape your notice that there is another star cluster less than a degree to the
northeast of M20. You may stumble upon it as you observe M20. This star cluster is M21, a fine
object to observe, though it is often passed over for its flashier neighbors just to the south. A short
cascading arc of three or four 7th magnitude stars appear to lead from M20 to M21. The two
objects are not physically associated, however. M21 is about 1,000 light years closer than the
Trifid.
M21 is a fairly loose cluster, without any obvious edges or borders. Even a small scope at 50x
reveals 20 or so stars, while an 8” scope might spot 50 stars spread over a quarter of a degree.
Higher magnification will peer into the heart of the cluster and may reveal a faint mist of more stars.
Look also for a the group of 6th and 7th magnitude stars connecting M20 with M21. This is
sometimes called “Webb’s Cross” after Rev. T. W. Webb, a 19th-century astronomy writer who first
noted it. The cross has its base in the Trifid Nebula and its tip in M21. The somewhat crooked
arms stick out in an east-west direction.
50
The Castaway Cluster
Now to a truly under-observed gem of an open star cluster, the lovely NGC 6520. The cluster lies
south of M8, about 2/3 of the way from M8 to the star Alnasl at the tip of the spout of the Teapot.
You can see the cluster in binoculars as a bright blister in an amazingly rich section of the Milky
Way towards the galactic bulge. The cluster was named “The Castaway Cluster” by Stephen J.
O’Meara because it looks like a tiny island in a tempestuous sea of stars. It reminded him of the
story of the castaway Robinson Crusoe, shipwrecked on a small island near the coast of Venezuela
in 1659.
NGC 6520 is a distant cluster, about 5,300 light years away, which means it appears quite tiny in
our skies. So while you can see it in binoculars, you’ll need a telescope with at least 150-200x to
resolve this tight family of fairly new stars. You may resolve as many as 15-30 stars, although there
are about 60 in total.
References differ as to the age of NGC 6520. But its preponderance of blue stars suggest it is a
young cluster, perhaps 100 million years old. Within the field of view, you will also see bright red
stars that are likely not associated with the cluster but simply share the same line of sight.
Just west of the cluster, in good sky, you can also see the conspicuous dark nebula Barnard 86.
The nebula is easier to see than many of its kind because of the bright tapestry of background
stars. Averted vision may reveal other dark nebula here, and certainly untold number of distant
stars.
The star cluster NGC 6520 and dark nebula Barnard 86.
M28
Take a look for the easy-to-find globular cluster M28 just northwest of lambda (λ) Sagittari, the top
of the Teapot. The 7th-magnitude cluster is not nearly as impressive as M22, but it is intrinsically
just as spectacular and only appears dimmer because it is 18,300 light years away, nearly twice as
far as M22. Once you find this cluster, use increasing degrees of magnification to resolve some of
51
the outer halo stars. Some stargazers report seeing two curved “pincers” stretching out northward
from the core.
Two Fine Doubles
Let’s finish our tour of Sagittarius with a couple of double stars near the tip of the spout of the
Teapot. Burnham 245 is just inside the spout, about 1o east-southeast of Alnasl, or gamma (γ)
Sagittarii. There are many, many stars in this region, but Burnham 245 is the brightest. The
yellow-orange primary shines at magnitude 5.8, and the 8th-magnitude yellow companion lies 3.9”
to the north. Try 100x or more to split this fairly tight star.
Just outside of the spout, about 1.5o west of Alnasl lies the pretty but obscure double star Piazzi 6.
The star consists of a red-orange primary shining at magnitude 5.4, and a 7th magnitude yelloworange secondary about 5” to the east. A 3” scope and 80x should split it cleanly.
M8, M20, and M21; the Castaway Cluster NGC 6520
52
Tour 8 - Scutum and Aquila
Northeast of Sagittarius and east of Ophiuchus lies the small, dim constellation Scutum, the Shield.
This modest group of stars was first named Sobieski’s Shield after a 17th-century Polish king. It’s
the only constellation with political origins still in use today.
The stars of Scutum form a small distorted triangular shape. You can find the constellation trailing
to the southwest of the tail of Aquila, the Eagle, the larger constellation just to the northeast. While
it has no stars brighter than 4th magnitude, Scutum’s location in the thick band of the Milky Way
makes it a favorite stop for stargazers with binoculars and small telescopes. There are many fine
sights here.
Object
Type
Mag.
Dist. (ly)
RA
Dec
Comments
M11
OC
5.8
6100
18h 51m
-6o 16’
“Wild Duck” cluster
H VI 50
DS
6.2/8.2
460250
18h 49.7m
-5o 55’
Σ2391
DS
6.5/9.6
500 (?)
18h 48.7m
-6o 00’
B111
DN
18h 50m
-5o 00’
NGC 6664
OC
7.8
3800
18h 36.5m
-8o 11’
M26
OC
8.0
5200
18h 45.2m
-9o 23’
NGC 6712
GC
8.1
22500
18h 53.1m
-8o 42’
NGC 6709
OC
6.7
3500
18h 51.3m
-10o 19’
-
-
Encompasses B110, B113
In Aquila
Table 8 - Objects to see in the constellations Scutum and Aquila
M11, the “Wild Duck” Cluster
The open star cluster M11 is the finest open cluster in this part of the sky. It’s the favorite of many
experienced stargazers, and quite rewarding to observe in detail. The cluster is located in Scutum,
southeast of the tail of Aquila and just southeast of beta Scuti.
Unlike many other bright open clusters, M11 is quite compact and requires substantial
magnification to resolve. In binoculars, it’s a bright 6th-magnitude smudge. In a 3” scope at
30-40x M11 remains mostly unresolved, though its smoky haze spreads across 1/5 of a degree of
sky. Crank up magnification to 100x and above, and the cluster finally expands into a fan-shaped
spray of dozens of white 9th and 10th-magnitude stars.
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With a steady gaze, you may perceive a wedge-shaped pattern amongst the stars of M11. British
Admiral William Smyth imagined M11 as a V-shaped configuration of stars that reminded him of
wild ducks flying in formation. That’s why M11 is often called the “Wild Duck” cluster.
M11 is a distant cluster, nearly 5,500 light-years away and roughly 300 million years old. Most of its
2000-3000 stars are luminous giants that shine much brighter than our own sun. From Earth, the
stars in M11 are 9th to 11th magnitude. If our Sun lived in M11, it would appear as a barely-visible
16th-magnitude star.
Unlike many open star clusters, M11 has not yet been dispersed by passing stars and gas clouds
because it’s massive enough to hold itself together through the mutual gravitational attraction of its
constituent stars. If you found yourself on a planet near the center of M11, you’d see a sky chock
full of brilliant stars, more than 50 of which would range from 3 to 50 times brighter than Sirius, the
brightest star in our sky.
M11, the “Wild Duck” cluster in Scutum
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Aquila and Scutum
Two Double Stars Near M11
Once you’ve inspected the marvelous Messier 11, look for two wide and colorful double stars just
half a degree to the cluster’s northwest. The wider pair is Herschel (H) VI 50. This pair makes a
good test in binoculars, but splits in a telescope at lowest magnification. The primary of H VI 50 is
golden, while its 8th-magnitude companion is yellow-white. The pair is believed to be an optical
double, and not truly associated in a gravitationally-bound system.
Within the same field of low-power field-of-view of H VI 50 is the slightly more closely-spaced
Σ2391. With a separation of 38”, the pair is too close to split with binoculars, but comes apart
easily in a telescope. The primary is blue-white, while the fainter companion is whitish. This pair
may also be an optical double. But the two stars make an intriguing sight in the same field as M11
at low magnification.
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The region around M11 and the Scutum Star Cloud
Scutum Star Cloud
M11 is just on the northern edge of a lovely bright blister of stars about 1.5o square. This is the
Scutum Star Cloud, which E. E. Barnard called “the gem of the Milky Way”. The somewhat
hammer-shaped cloud is packed with faint stars of the Sagittarius Arm of the Milky Way, which
here curves towards us and grows brighter and closer. Like M24, the Small Sagittarius Star Cloud,
this cloud appears brighter than the rest of the Milky Way here because there is less obscuring
dust in the way. Peruse the cloud at low magnification to get an idea of the fantastic number of
stars in this small patch of sky.
Just north of the cloud and extending northeast and southwest, look for the dark band of the
“Great Rift”, which appears to bisect the Milky Way in two from Cygnus, though Scutum and into
Ophiuchus. This rift is opaque cold gas and dust that blocks our view of the stars of the Milky
Way, but which will serve over eons as raw material for new stars, star clusters, and emission
nebulae.
Barnard 110, 111, and 113.
Let’s remain in the area of M11 and examine the large dark nebula just north of the Wild Duck
cluster and the Scutum Star Cloud. In binoculars or very wide-field telescope, look for a large
patch of darkness about 2o across that pushes north and curves slightly east. This is Barnard 111,
a large dark nebula complex that has H VI 50 on its southern edge. Within B111 is the smaller
coal-black splotch of B110 about 1 degree north of H VI 50. Two 8th-magnitude stars lie just to its
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southeast. B113 is just half a degree to the northeast of B110, with a few foreground stars
between the two. Many more dark nebulae can be seen in this area, especially in very dark, clear
sky.
The Scutum Star Cloud and nearby dark nebulae
NGC 6664
Let’s move a little southwest of the region around the Scutum Star Cloud to the star alpha (α)
Scuti. Specifically, to the sadly overlooked little gem of NGC 6664, an open star cluster some
3,800 light years away, which appears just 1/3 of a degree to the east of alpha Scuti. Like M11,
NGC 6664 needs a fair bit of magnification to resolve. And like M11, the cluster is nearly 1/5 of a
degree across. But NGC 6664 has far fewer stars, and is two magnitudes fainter than its more
famous neighbor. The cluster displays just 25-30 stars at 100x in a 4-inch scope. But it’s still a
pretty sight, especially in the lovely star field just west of the great star cloud. Even in binoculars,
NGC 6664 presents a pleasant soft white glow.
M26
While you’re in the area, check another Messier object off your list. The sparse open cluster M26
is less than a degree southeast of delta (δ) Scuti, and is on a line extending from alpha (α) to delta
(δ). This 8th-magnitude cluster is a pale shadow of M11. It shows perhaps a dozen stars in a 3-4”
scope at 80-100x. An 8” scope shows not much more. The cluster spans perhaps 8-10
arcminutes. But the cluster is set in a fine region, and is worth passing inspection. The stars
appear to be arranged in two rows with a dark lane in between.
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NGC 6712
Just a degree northeast of M26, look for the faint glow of the globular cluster NGC 6712. Again,
the cluster itself is too distant and faint to make much of an impression. It’s tough to resolve many
stars, even in the halo, except perhaps for one faint star to the northeast.
NGC 6712 has had an interesting time over the past 10 billion years. Like all globular clusters, it
revolves in an elongated and inclined orbit about the core of the Milky Way over the course of tens
of millions of years, and so sometimes passes through the plane of the Milky Way. But the orbit of
this cluster has taken it through the galaxy’s plane more often than most. As it passes through the
plane, the gravitational attraction from massive dust clouds and stars strip away lighter stars from
the globular, leaving it only with heavier stars. Most of the stripped-off stars from globulars
continue in their orbit about the galaxy’s core, so the Milky Way has a halo not only of globular
clusters, but of lonely ancient stars stripped away from their home clusters, doomed to tour the
central region of the galaxy alone forever.
NGC 6709
All the sights in this section so far have been in the small constellation Scutum. North and east of
Scutum lies a larger, brighter constellation Aquila, the Eagle. Aquila has fewer sights of interest for
backyard observers, but it does have some gems. The bright star Altair is beautiful sight without
optics, of course, and forms the apex of the famous “Summer Triangle” of Altair, Vega, and Deneb
(this is sometimes called the “Winter Triangle” or “Northern Triangle” in the southern hemisphere).
In the western wing of Aquila lies the lovely open cluster NGC 6709. This nearly-forgotten cluster
lies in a fairly rich region of stars, just off the Great Rift of the Milky Way. The cluster is about 5
degrees southwest of zeta (ζ) Aquilae, and makes a right-angle triangle with zeta and epsilon (ε).
The cluster has an angular diameter of 1/4 degree, and shines at magnitude 6.7. It’s a very
challenging object for the unaided eye. But it’s easily visible in binoculars, and partially resolvable
upon careful gaze. In a telescope at low power and wide field, the cluster sits amidst a remarkable
background of stars that seems to flash and fluoresce with averted vision. It’s hard to sort out
which stars belong to the cluster, but perhaps 25-30 stars are visible in a 3-4” scope. Look
carefully at the stars, and you will perceive many apparent double stars within the cluster. Most are
likely optical doubles.
The cluster is about 3,900 light years away.
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Tour 9 - Cygnus
The constellation Cygnus is one of the most recognizable in the mid-northern sky, and can be seen
from most parts of the southern hemisphere in July through October.
Cygnus is an ancient constellation. Ptolemy listed the group in his original 48 constellations in the
2nd century A.D. The main part of the constellation forms the asterism known as the “Northern
Cross”, with Deneb (alpha Cygni) at the top of the cross and Albireo (beta Cygni) at the base. The
binary star Albireo is a stunning example of contrasting colors and one of the finest double stars in
the heavens. The 3rd-magnitude component of this star shines a golden-yellow; the fainter 5th
magnitude component is a sapphire-blue.
Object
Type
Mag.
Dist. (ly)
RA
Dec
M39
OC
4.6
900
21h 32.2m
+48o 26’
NGC 7000
EN
-
1800
20h 00m
+44o
Northern
DN
-
20h 45m
+40o
-
Comments
“North America” Nebula
Coalsack
M29
OC
6.6
4000
20h 23.9m
+38o 32’
NGC 6910
OC
7.4
3700
20h 23.1m
+40o 47’
Coincident with nebula IC 1318
61 Cygni
DS
5.2/6.0
11.4
21h 06.9m
+38o 45’
“Flying Star”
NGC 6826
PN
8.8
5100
19h 44.8m
+30o 31’
“Blinking Planetary”
Veil Nebula
EN
2500
20h 51m
+31o
Includes NGC 6960, 6992, 6995
-
Table 9 - Objects to see in the constellation Cygnus
In Greek legend, Cygnus was a friend of young Phaeton who met his demise trying to drive the
sun-chariot across the sky. When Phaeton fell into the river Eridanus, Cygnus begged Zeus to turn
him into a swan so he could fly down to retrieve his friend’s body. In doing so, however, he gave
up immortality. So touched was Zeus by the selfless act of Cygnus in honoring his friend, he
placed the swan in a place of honor in the night sky.
And what a place of honor! Cygnus lies in a thick band of stars of the Milky Way and is split down
the middle by the dark “Great Rift”. The constellation has nowhere near the number of deep-sky
sights as Sagittarius or as Centaurus in the southern hemisphere, but there are some memorable
and unique sights in Cygnus that await the inspection of a patient stargazer with a small wide-field
telescope.
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M39
Let’s begin the tour of Cygnus behind the Swan’s tail (or above the cross, if you wish). The target
is the open cluster Messier 39, located about 3/4 of the way on a line between Deneb and pi-2 (π2)
Cygni. The cluster is visible in dark sky with the unaided eye. In fact, its discovery is assigned to
the great Aristotle in the 4th century B.C.
The cluster is an easy sight in binoculars. And a small telescope at 40-50x will show 15-20 blue
stars. They are arranged in a triangular formation, and a lovely double star is visible at its center.
The stars are of a nearly uniform brightness against the rich background of the Milky Way. The
cluster is relatively close by for such an object, about 900 light years.
Deep sky sights in Cygnus
The “North America” Nebula
The emission nebula NGC 7000 is a famous object and a favorite target for astrophotographers.
Its red light is spread over nearly 3o of sky... about six times the diameter of the full moon. And in
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photographs it takes on the unmistakable shape of the continent of North America, especially the
areas corresponding to Florida, Mexico, Central America, and the Gulf of Mexico.
The so-called “North America Nebula” has a low surface brightness, which makes it a non-trivial
object to detect visually. Dark, clear, moonless sky is a must for this object. Under optimum
conditions, the nebula is visible in binoculars, and many sharp-eyed observers have seen the
nebula without optical aid. Not all telescopes are suited for observing the whole complex: any
scope with a focal length longer than 700 mm to 800 mm will have too small a field of view to
accommodate the entire nebula. It is possible to observe the nebula piece by piece, and the bright
regions of “Florida” and “Mexico” are the brightest and easiest to see.
Though even under optimum conditions, this is a tricky and subtle object. You may be staring right
at it for some time before it pops out at you. Center your telescope’s field of view about 3o east of
Deneb and 1o west of xi (ξ) Cygni, and start looking carefully and patiently. Tap the telescope a
little to jiggle the image, and try averted vision. And make sure your observing eye is fully dark
adapted (a process that takes 20-30 minutes). A nebula filter (OIII or UHC) is a big help with NGC
7000.
If you can spot the North America Nebula, try for the even fainter “Pelican Nebula”, IC 5067. It is
immediately west of NGC 7000 within the same low-power field of view of a fast scope.
Both glowing complexes are regions where ionized hydrogen is recombining to emit light. But
what is ionizing the hydrogen? At one time, Deneb was the suspect. But now astronomers
believe the culprit is the very hot blue star HD 199579, which is embedded in the northern part of
the nebula. NGC 7000 is about 1,800 light years from Earth.
The North America Nebula, NGC 7000, upper left; and the Pelican Nebula, lower right
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The “Northern Coalsack”
Southern-hemisphere observers are familiar with the “Coalsack”, a dark nebula of cold gas and
dust that blocks a patch of the background star field in the constellation Crux. But northern
observers have a coalsack, too. The Northern Coalsack extends over a 6ºx5º span of sky in
Cygnus between the stars Deneb, Sadir, and Gienah in northeastern Cygnus. There are many dark
nebula in the area. But the Northern Coalsack marks the beginning of the Great Rift of the Milky
Way, a huge dark cloud of gas and dust stretching from Cygnus into Sagittarius. This cloud, which
gives the northern Milky Way a mottled appearance, contains some 1 million solar masses of
material.
The Northern Coalsack is easy to spot if you have dark sky. You’ll need no optics, though
binoculars give you a closeup view of a few foreground stars and the attenuated light from
background stars. Don’t bother with a telescope… it will show you only a tiny fraction of the cloud. Many believe the Northern Coalsack is not nearly as dark at the southern version in Crux. But
accurate measurement show each is similar in darkness. The southern Coalsack simply appears
darker by contrast because it appears in a brighter part of the Milky Way.
Also look for the larger dark region “Le Gentil 3” between M39 and Deneb. Like the Northern
Coalsack, this mass is best seen with the unaided eye and inspected with binoculars.
M29
This open cluster must be one of the least impressive in Messier’s catalog. Many new stargazers
are unsure they see this object even when they are staring right at it. The cluster is located just
1.7o south of Sadr (gamma Cygni), so it should not be hard to find. But with only six bright stars,
the cluster can be mistaken for an accidental clump in the Milky Way. You can tell if you’re seeing
M29 by the arrangement of the six bright stars in two groups of three that look like curved brackets
facing each other like this... )(
A larger scope, say 6-8”, will reveal more stars, but it’s hard to sort them out from the fairly rich
background.
NGC 6910 & IC 1318
Just half a degree to the northwest of the star Sadr lies another open star cluster NGC 6910. This
Y-shaped cluster is a little more attractive than M29, and features two 7th-magnitude yellow-white
stars at the top of each tip of the “Y”. A somewhat fainter blue star lies below the “Y”.
NGC 6910 itself is embedded in a section of the wide patchwork of nebulosity known as IC 1318.
Two or three bright sections lie within 1.5o of the star Sadr, and the whole complex spans more
than 3o. So use as low a magnification as possible to look for this. This is an emission nebula
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excited by a very energetic Wolf-Rayet star, a massive evolved star that’s loosing mass quickly and
nearing an unstable phase near the end of its life. A nebula filter may help spot this visually
challenging complex.
The IC 1318 region, also called the “Sadr Nebula” is another favorite for astrophotographers.
IC 1318. The brightest star is Sadr (gamma Cygni). The cluster NGC 6910 is to the lower-left of Sadr.
61 Cygni
While it’s not much to look at, the star 61 Cygni is worthy of inspection. That’s because it’s one of
the 14 closest stars to Earth, and one of the four closest stars visible with the unaided eye. The
star is just 11.4 light years away. It was the first star to have its distance measured. The German
astronomer Friedrich Wilhelm Bessel accomplished the feat in 1838 by measuring the parallax of
this modest star.
61 Cygni is actually a binary star. Each component is an orange dwarf star, one of magnitude 5.2,
one of magnitude 6.0. They are separated by 30”, and are easy to split in any telescope at low
power.
The star is interesting for another reason: it has a large “proper motion”, which is its apparent
motion compared to more distant background stars. 61 Cygni has changed its position by 0.5o
over the past 400 years in the direction of the star sigma (σ) Cygni. This is a huge amount of
movement compared to nearly any other star. In the 18th century, the Italian astronomer Guiseppe
Piazzi named 61 Cygni “The Flying Star”.
The “Blinking Planetary”
Now to an intriguing planetary nebula off the northern wing tip of the Swan. The nebula, NGC
6826, is just south of the star iota (ι) Cygni, and just west of a line extended from kappa (κ) to iota
with a length equal to the distance between the two. The planetary appears nearly star-like at 50x,
and you can determine if you have indeed found the nebula by increasing magnification to enlarge
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the disk. At 120x or more, the nebula will reveal an obviously oval shape and a subtle blue-green
color. Unlike many planetary nebulae, the 10th-magnitude central star of NGC 6826 is quite
obvious in a small telescope.
NGC 6826, is an object that most dramatically demonstrates the effect of averted vision. Stare
directly at this blue-green planetary nebula for several seconds and you see only the central star.
Look slightly to the side and the faint nebula around the star appears suddenly. When you switch
from straight on to averted vision, the nebula appears to blink on and off. It’s darned impressive.
Using a nebula filter increases the contrast of the nebula against the background sky, but ruins the
blinking effect.
The Veil Nebula
The final stop on this tour of Cygnus is the Veil Nebula, a sprawling supernova remnant and one of
the most intricate and intrinsically beautiful objects in our galaxy.
The Veil Nebula is the shattered remains of two stars that exploded 18,000 and 5,000 years ago.
The stars were only 2,500 light-years away, so each likely appeared as bright as the crescent
moon in the skies of our prehistoric ancestors (and probably scared the living heck out of them).
The Veil is also called the Cygnus Loop, which consists of three structures that span an impressive
3.5 degrees across the sky… roughly 7x the diameter of the full moon. You’ll find the Veil a few
degrees off the star epsilon (ε) Cygni.
The eastern section of the nebula, also called NGC 6992 and NGC 6995, is the brightest and
easiest to see. You can spot it in 7×50 binoculars if you have extremely dark sky.
The fainter western section of the nebula is cataloged as NGC 6960. Though harder to see, this
section is set against the lovely 4th magnitude foreground star 52 Cygni, so this section may be
the easiest to find.
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The Veil Nebula
To see the Veil through a telescope, a light-pollution filter is a huge help. Use your low power
eyepiece. Unless you have a field of view of 3.5o or more (a large field for a telescope), you’ll only
see one section at a time, especially in scopes with longer focal ratios. Even the brightest part of
the Veil, NGC 6992, is roughly 1 full degree across.
The delicate lacework in the Veil Nebula becomes visible in 4” or larger telescopes with a UHC or
OIII filter. Use higher magnification to see the fine structure. You can spend all night just examining
this wonder.
The bright optical filaments in the Veil Nebula are caused by the supernova blast wave colliding
with interstellar gas clouds. As the shock wave slows down, it loses energy by emitting visible light.
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Tour 10 - Vulpecula, Sagitta, and Delphinus
Between Cygnus and Aquila, you will see three small constellations Vulpecula, the Fox; Sagitta, the
Arrow; and Delphinus, the Dolphin. Because this part of the sky is so close to the Milky Way, there
is a profusion of stars here and many deep-sky objects which are often passed over for sights in
better-known constellations. But there is enough here for much pleasant stargazing on late
northern summer nights.
Sagitta is an ancient constellation. It was included among Ptolemy’s original 48. The shape of the
constellation has caused many cultures to interpret this group as an arrow of some sort. The
ancient Greeks took the constellation to be the arrow Hercules used to kill the eagle which upon
Zeus’ command consumed the liver of poor Prometheus over and over again.
Delphinus, too, is ancient. Like Sagitta, it is clearly shaped like its namesake, in this case the little
Dolphin that carried the poet Arion to safety after his shipmates conspired to rob and cast him
overboard.
Vulpecula is a relatively recent creation. Hevelius included this completely non-descript group of
stars on one of his late 17th-century star maps.
Object
Type
Mag.
Coathanger
AS
~6.0
NGC 6802
OC
9.0
M27
PN
8.0
M71
GC
H20
Dist. (ly)
RA
Dec
Comments
-
19h 26m
+20o
Collinder 399
-
19h 30.6m
+20o 16’
Same FOV as Coathanger
1000
19h 59.6m
+22o 43’
“Dumbbell Nebula”
8.2
13000
19h 53.8m
+18o 47’
OC
6.8
5000
19h 53.2m
+18o 20’
gamma Del
DS
4.4/5.5
100
20h 46.7m
+16o 07’
NGC 7006
GC
10.6
140000
21h 01.5m
+16o 11’
Toadstool
AS
9.0
21h 07.4m
+16o 18’
-
Very distant globular
Table 10 - Objects to see in the constellations Vulpecula, Sagitta and Delphinus
The “Coathanger”
Just south of Albireo in Cygnus, look for the 4th-magnitude star Anser, the brightest star in
Vulpecula. This orange star has another fainter orange star just 6’ to the northeast. Now imagine
a line drawn from Albireo though Anser and extended beyond the latter for about 1.5x their
separation. This is the location of a remarkable asterism sometimes called Brocchi’s Cluster,
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sometimes Collinder 399, but which is better known as the “Coathanger”. One look at the cluster
through binoculars or a low-power telescope reveals why.
The “Coathanger” (Collinder 399). NGC 6802 is at extreme left.
You can glimpse the group with your unaided eye in good sky as a little cloud. The longest axis of
the group is 1.4o, so you will need a field of view of at least that much to see the whole object. If
your telescope’s lowest-power eyepiece can’t fit it all in, try observing with your finderscope. It
may give a better view.
Six stars make the flat bar of the hanger, and four more make up the hook. While it was once
thought to be a true star cluster, astronomers know understand this is simply a chance alignment
of stars.
Here’s a bonus object, one that few stargazers know about. Just 1/3 of a degree east of the most
eastern star in the coathanger lies the true open cluster NGC 6802. The cluster is less than 1/10
of a degree across and shines at 9th magnitude. It makes a wonderful little splash of light in this
rich star field.
The Dumbbell Nebula
While the Coathanger is well known, our next stop on this tour is even more famous. Messier 27,
the Dumbbell Nebula, is on the list of most beginning stargazers. But it’s a beautiful planetary
nebula, so more experienced amateur astronomers enjoy stopping by this old favorite.
Even in binoculars or a finder, the nebulous nature of M27 is obvious. It’s easier to find from the
star gamma (Υ) Sagittae, the tip of the arrow Sagitta, than any star in Vulpecula. Look for the
nebula about 3o due north of gamma.
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Through even a 2.4” scope, M27 looks rectangular. Unlike most such objects, M27 gives us an
edge-on perspective to an evolved planetary nebula. We see gas ejected from the central star in a
bipolar fashion, possibly because of the effects of the star’s magnetic field on the stellar winds. At
75x, the “dumbbell” or “apple core” shape of M27 becomes quite evident. A brighter streak across
the long diagonal of the nebula can also be seen, along with texture and brightness variations
across the whole apparent surface.
Compared to other planetaries, M27 is close (about 1,000 light years), and large (more than 1 light
year across). So it appears far larger in our sky than most other such nebulae. Astronomers have
had a good look at this object and distinguished two separate expanding shells of gas from the
central star. A shell of ionized oxygen is hurled into space at 14 km/s, and one of ionized nitrogen
expands at 30 km/s. The nebula is expanding at a rate of 6 arcseconds per year as seen from the
Earth.
The central star is of 14th magnitude and quite a challenge in most telescopes. Remember that to
see the central star of nearly any planetary nebula, you should not use a nebula filter since it will
reduce the star’s brightness appreciably.
M27, the Dumbbell Nebula
M71
Now let’s move a little south of Vulpecula into Sagitta. Here is another Messier object far less
famous than M27, the globular cluster M71. Astronomers pondered for decades the true nature of
M71. Some though it was a tightly packed open cluster. Others believed it was a loose globular
cluster, which is now the accepted wisdom.
The cluster is close by, about 13,000 light years. Although at magnitude 8.2, it is not terribly bright,
especially compared to the grand M22 in Sagittarius which is only slightly closer. In a 3-4” scope,
the stars are unresolved and there is a clear brightening towards the core at low power. Crank it
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up to 100x or more, and many stars become resolved in the halo. The shape of the cluster has
been described as “arrow-like” or “Y-shaped”.
Look for M71 just south of the line between gamma (Υ) and delta (δ) Sagittae, slightly closer to
gamma.
Deep sky sights in Sagitta, Vulpecula, and Delphinus
Harvard 20
After you’ve had your fill of M71, turn your gaze just 1/2 degree to the south-southwest. Here lies
the sparse and rarely-observed open cluster H (Harvard) 20. Yes, this really is a cluster. The stardense background makes it hard to distinguish, but astronomers have determined the 30-millionyear-old cluster has 100 members. By eye, through a small scope, it’s hard to tell which stars are
cluster members. You will see perhaps ten 9th and 10th magnitude stars in an irregular splotch,
and two 8th magnitude stars on the cluster’s west edge. H 20 makes an interesting contrast with
M71 in the same low-power field of view.
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Gamma Delphinus
Now to Delphinus, the little celestial dolphin. The star at the dolphin’s snout, gamma (γ) Delphinus
is a pretty double. The primary is a yellow-gold star of 4th magnitude, while the secondary is of
5th magnitude and looks pure yellow. The pair is separated by 9” and can be resolved under
average seeing conditions at 50x. Now look about 1/4 of a degree to the south-southeast of
gamma for a second double star. This is Σ2725. It consists of two yellow-orange stars of
magnitude 7.5 and 8.2 separated by 6”. So here again is an lovely example of a “double double”
star, though in this case, the two pairs are not physically related.
NGC 7006
Now to a globular cluster just 3o west of gamma Delphinus. This is a challenging object for a small
telescope because it is quite faint (about magnitude 10.6) and tiny (about 3’ across). In a 3-4”
scope, it will be nothing more than a dim smudge in a good star field. In a 6-8” scope, it will be a
brighter smudge but still unresolved. Even a 12” scope cannot resolve the stars of this faint cluster.
NGC 7006 appears so faint because of its extraordinary distance, some 140,000 light years. The
orbit of this cluster about the center of the Milky Way is quite eccentric. At one end of its orbit it
comes 60,000 light years from the galactic core, and at the other extreme it is some 330,000 light
years away. That’s 1/6 the distance to the Andromeda Galaxy and further away than the
Magellanic Clouds. At present, NGC 7006 is one of the most distant of all the Milky Way’s 150
known globulars. So while it’s not a striking object visually, NGC 7006 is worth observing for its
extraordinary position as a distant intergalactic outpost of our own Milky Way.
The distance to globular clusters is determined by observing a select group of the cluster’s variable
stars called RR Lyrae stars. The period of variability of these stars is proportional to their
brightness. So when astronomers measure nearby RR Lyrae stars and calibrate the periodbrightness relationship, they can use this relationship to indirectly measure the distance to objects
for which there is no other means to do so.
This fortunate situation was used by the famous astronomer Harlow Shapley in the early 20th
century to determine the distance to many globular clusters, and to determine such clusters are
primarily gathered about the core of the Milky Way in a giant halo. That’s why we see more
globular clusters in Sagittarius, near the galactic center, than we do in Ursa Major, for example,
which is near the galactic pole.
The “Toadstool”
Now an interesting little asterism less than 2o east of NGC 7006, or about 5o east of gamma
Delphinus. This unrelated group of stars looks like a tipped over mushroom or “toadstool”.
whole structure is just 13’ across (a little less than 1/4 of a degree). It won’t be apparent in
The
70
binoculars or a finder scope, but in a telescope at 30x or so, you will see it. Experiment with
different eyepieces to get the best view. The top of the mushroom’s cap faces southwest.
The “Toadstool” in Delphinus
71
Appendix - Easy Math for Astronomers
Astronomy is one of the most mathematical of the sciences, and professional astronomers need to
know math extremely well. Backyard stargazers, however, can see thousands of beautiful sights
without burdening themselves with a single complicated equation if they choose. But there are a
few simple relationships that will help you select and properly use your telescope and accessories.
By understanding the magnification and field of view of each of your eyepieces, especially, you will
be much better equipped to find your way around the sky and observe the many deep-sky objects
in this guide.
The following diagram defines the telescope objective diameter (D), the focal length of the
telescope’s objective (F), and the effective focal length of the eyepiece (f). While the diagram
illustrates these quantities for a reflecting telescope, the same arguments and formulae apply to
reflecting telescopes.
Focal Ratio. The focal ratio of your telescope is a critical number. It influences magnification, field
of view, and photographic capability of your telescope.
Focal Ratio (F#) = Focal Length of Objective/Diameter of Objective
If, for example, you have a 6-inch (150 mm) scope with a focal length of 36 inches (900 mm), then
the focal ratio is f/6. The focal ratio is also called the F# (“F-Number), much like the number you
see on a camera lens.
Magnification. This is also a simple thing to calculate. To find out how much your telescope
magnifies an image:
72
Magnification=Focal Length of Telescope/Focal Length of Eyepiece
So take the above example with a 900 mm focal length telescope, using an eyepiece with 15 mm
focal length, you get 900/15=60x (60 power).
Exit Pupil. This is an important one. The exit pupil is the diameter of the beam of light coming out
of your eyepiece. If the exit pupil of your eyepiece exceeds the size of your eye’s pupil, which is 6-7
mm at best, then you waste light from your telescope. The exit pupil is:
Exit Pupil = Focal Length of Eyepiece /Focal Ratio of Telescope
With our example of an f/6 scope, if we use the 15 mm eyepiece, the exit pupil is 15/6=2.5 mm.
That’s good… all the light will enter your eye. But a 45 mm eyepiece gives you a 7.5 mm exit pupil.
Your eye can’t take that in, so using such an eyepiece with an f/6 telescope gives you a low
magnification and a wide field, but it’s a waste of light.
Resolution. Resolution tells you how good your telescope is at seeing fine detail. The resolving
power of a telescope:
Resolving Power (in arc-seconds) = 116/Diameter of Objective (in mm)
A 150 mm telescope can resolve features, such as a tightly-spaced double star or fine details on a
planet), as close as 116/150=0.77 arc-seconds apart. That’s pretty good… but it takes nearperfect atmospheric seeing conditions to get close to such resolution. On an average night, you
might expect a resolution of 1-2 arcseconds or more, depending on the atmosphere, no matter
how big your telescope.
True Field of View. The true field of view tells you how much of the sky you can see with a
particular eyepiece through a particular telescope. If you know how much sky you are looking at
with each of your eyepieces, then you can navigate around the sky much better. Each type of
eyepiece has an apparent field of view, which is what you see when you look through the lone
eyepiece. This number is often printed on the eyepiece itself. To get the “true field of view”:
True Field of View = Apparent Field of View/Magnification
So if a 15 mm eyepiece has an apparent field of view 50 degrees, and you use it with our scope
with 900 mm focal length to get 60x, then the true field of view is 50/60=0.83 degrees, which is
about 1.6x the size of the full moon. You should calculate the true field of view of each of your
eyepieces… it helps you know how big a circle of sky you can see with each eyepiece.
73
Appendix - Whole-Sky Star Maps
The following three maps show the sky at seen from a latitude of 45oN at 10 p.m. on July 1,
August 1, and September 1. The maps give you an idea where the major constellations are
situated each month. Once you locate the major constellations, you can follow the tours in this
guide. Most of the tours cover constellations near the meridian during the month, that is, the
constellations are at their highest point above the horizon. These maps are useful for other times
as well. Keep in mind the stars advance westward by about one hour every two weeks. So, for
example, the map of the sky on August 1 at 10 p.m. is also accurate for August 15 at 9 p.m. and
for July 15 at 11 p.m.
Map for July 1 at 10 p.m.
74
Map for August 1 at 10 p.m.
75
Map for September 1 at 10 p.m.

What To See Telescope(Jul-Sept) v1 - One

Transcription

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