11 Celestial Objects and Events Every Stargazer Should See

Bucket List for
Backyard Stargazers
The
11 Celestial Objects and Events
Every Stargazer Should See
B R I A N V E N T RU D O
THE BUCKET LIST FOR BACKYARD STARGAZERS
11 Celestial Objects and Events
Every Stargazer Should See
by
Brian Ventrudo
Copyright © 2014 by Brian Ventrudo
All rights reserved. This book or any portion thereof may not be reproduced or used in any manner whatsoever without the express written permission of the publisher except for the use of brief quotations in a book review.
Although the author and publisher have made every effort to ensure that the information in this
book was correct at press time, the author and publisher do not assume and hereby disclaim any liability
to any party for any loss, damage, or disruption caused by errors or omissions, whether such errors or
omissions result from negligence, accident, or any other cause.
Published by Mintaka Publishing Inc.
i
PREFACE
“The person who has lived the most is not the
one with the most years but the one with the
richest experiences.” ~ Jean-Jacques Rousseau
This book will show you how, where, and when to see eleven of the
most extraordinary astronomical sights and events that every stargazer-every person-- should see before they pack up their telescope for the last
time. It’s a “Bucket List”, if you will, for those interested in seeing and experiencing the most memorable sights in the heavens.
This list is for all stargazers, but it’s especially aimed at the casual
stargazer, someone who may not have a big telescope or special expertise,
but who has the curiosity and ambition to see some of the most beautiful,
and in some cases, transient sights in nature. To see some of these objects, you need a pair of binoculars or a small telescope. Some require
travel or good timing and luck. And for others, you simply need to look
up. ii
But that doesn’t mean these are all easy to see. Not one person in a
thousand has seen all the objects on this list. I’ve been a stargazer for
some forty years and I have yet to see them all.
Once you’ve seen these eleven sights, you can be assured you’ve seen
some truly remarkable things that few people– even the most celebrated
professional astronomers– ever get to see.
Brian Ventrudo
Publisher, One-Minute Astronomer
www.oneminuteastronomer.com
iii
1
SUNRISE ON THE MOON
Only 24 humans– all Apollo astronauts– have witnessed close up the
rising and setting of the Sun across the stark and airless surface of the
Moon. Further manned missions to the Moon remain unlikely in the near
future, and it will be a long time before Virgin Galactic begins offering
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tickets for a trip to our nearest celestial neighbor. But with a pair of binoculars or a small telescope, you can get a pretty good “astronaut’s-eye
view” of the most striking features of the Moon’s surface. Your first stop
on this celestial bucket list is a view of the sunrise over one of the Moon’s
most fetching craters, the crater Copernicus.
With a diameter of just under 100 km,
Copernicus is a big crater. It’s also quite
young, just 800 million years old compared to an age of three to four billion
years for most lunar craters. Copernicus
is located near the Moon’s equator, just left
of the north-south centerline if seen from the
northern hemisphere, or right of the centerline if seen from the southern
hemisphere. The crater lies south of the large dark region called Mare Imbrium, the “Sea of Rains”. The crater has tall, terraced side walls and a
cluster of peaks near its center. So there are many rolling features that
cast long and striking shadows that can be seen when the Sun rises or
sets on the crater.
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F IGURE 1.1 Location of the crater Copernicus just
south of the Mare Imbrium
The sun rises over Copernicus about nine days after New Moon each
month or about five days before Full Moon. That’s when the crater lies on
the terminator, the line between bright and dark on the face of the Moon.
If the timing is just right, you can watch the sunlight fan out over Copernicus during the course of a few hours in the evening. It’s a beautiful sight
to see the crater’s walls and central peaks catch the sun’s first rays, followed by the low-lying crater floor.
When the sun sets over Copernicus about 5 days after Full Moon,
you can watch the whole thing again, but the shadows will be cast in the
opposite direction, giving you a slightly different view.
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You can certainly see the crater Copernicus with binoculars, but to
see the long shadows cast during sunrise or sunset, you will need a telescope. Pretty much any telescope will work. To locate the crater and
make sure it is visible, use an eyepiece to get a lower magnification of
about 40-75x. Then switch to a higher magnification to get a close up of
the crater. The maximum magnification depends on the steadiness of the
skies and the quality of the telescope’s optics, so a little trial and error
may be required to get the best view. With a telescope of 100 mm - 150
mm aperture, the optimum magnification will likely be in the 125x to
200x range.
Because the Moon goes through phases on a monthly cycle, you can
see the sun rise over Copernicus, and every other feature on the Moon,
every 29.5 days.
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2
A METEOR STORM
A meteor storm! The very term makes an honest stargazer’s heart
beat faster. While a good meteor shower, like the Perseids which occur
every August, may have 50-60 meteors per hour, a meteor storm sprays
shooting stars at a rate of hundreds or thousands an hour. Though spec8
G ALLERY 2.1 Gallery of drawings based on the Leonid meteor storm of 1833.
Painting by Edmund Weiß of the Leonid meteor storm of 1833
over Niagara Falls.
tacular, a meteor storm may be the most difficult to see on this bucket list
because they are extremely brief and rare. But if you are lucky enough to
see one, you will not forget it.
How can you see a meteor storm for yourself? Well, they are hard to
predict. The Leonid meteor shower, which peaks around November 17
each year, has offered stargazers the most reliable opportunity to see a
meteor storm, which astronomers define as a meteor shower in which at
least 1,000 meteors fall per hour. Historical records show the Leonids
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have peaked at multiples of roughly 33 years, in 1799, 1533, 1366, 1202,
and 1037, for example. We now know the peaks correspond to brief periods during which Earth passes through a concentration of debris left in
the path of Comet Tempel-Tuttle. There were also impressive shows in
1966 and 1999, with less pronounced activity in 2001 and 2002.
The great Leonid meteor storm of 1833 was perhaps the most spectacular in recorded history. Visible from eastern North America on the
night of November 12-13, the storm produced as many as 200,000 meteors during the night, startling some 19th-century stargazers and terrifying many others. The storm lasted nearly four hours. According to astronomer Agnes Clerke, “the frequency of meteors was estimated to be
about half that of flakes of snow in an average snowstorm”.
Because the meteors came so quickly during the 1833 storm, and for
so many hours, it was clear to observers that the radiant, or the apparent
source, of the meteors lay towards the Sickle of the constellation Leo, and
that the radiant moved with the stars during the evening. This convinced
scientists that meteors came from outside the Earthʼs atmosphere. Until
then, some believed meteors were an atmospheric phenomenon, which is
why the term “meteorology” refers to the study of the weather.
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The most recent meteor storm was also during the Leonid meteor
shower of 1999. The experts say over the next few decades, the Leonids
will be quiet, but who knows? If the Earth encounters a stream of debris
from a new comet, or from an old comet that has shifted position, we
may get to enjoy an unexpected meteor storm. No one knows for sure, so
to check this one off the “bucket list” you’ll need to be lucky and patient.
By using radar to detect cometary debris fields, astronomers have had
some luck predicting these events several months in advance, so stay
tuned to the science news. And when the time comes, find some dark
sky, lay back, and just look up!
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3
A GREAT COMET
By cosmic standards, a comet doesnʼt amount to much. Itʼs a tiny
remnant of the formation of the solar system, a dusty ice ball a few kilometers across floating unseen far beyond Neptune.
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But from time to time, a gravitational nudge from another comet or
a passing star starts a comet on a long journey towards the inner solar system. Although a half-dozen or more comets come to the inner solar system each year, most are too faint to see without a telescope. But if the timing and mechanics are right, such a comet may put on a spectacular celestial show, developing a swollen head brighter than Venus and a tail which
streaks a quarter of the way across the sky. Such bright and remarkable
comets are informally labeled by astronomers as “great comets”.
On average, one of these bright and truly spectacular comets comes
around about once a decade. In 2011, Comet C/2011 W3 (Comet Lovejoy)
was a spectacular sight for southern-hemisphere stargazers as it grazed
the visible surface of the Sun. So was Comet C/2006 P1 (Comet
McNaught) in 2007. Before that, there were back-to-back appearances of
Comet C/1995 O1 (Comet Hale-Bopp) and Comet C/1996 B2 (Comet Hyakutake) in 1997 and 1996, respectively. And before that, Comet C/1975 V1
(Comet West) put on a lovely show in the pre-dawn skies in 1976. There
have been other dazzlers over the decades, including three comets in the
19th century that were bright enough to see in the daytime sky, and a
fourth that appeared early in 1910.
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G ALLERY 3.1 A gallery of recent “Great Comets”.
Comet C/2011 W3 (Lovejoy) over Queensland, along with the southern
Milky Way, the constellation Crux, and the Magellanic Clouds (credit:
David Liu)
Comet Halley, the most famous comet, also made a spectacular return in 1910. Unfortunately, its next visit to the inner solar system in
1986 was a disappointment. The otherwise reliable comet gave its poorest performance in recorded history. But Comet Halley has a rich historical background that made it rewarding to observe. Known since antiquity, the comet served as an omen both good and bad over the centuries,
and it marked many historical events, including the Battle of Hastings in
1066 (a good omen for the Normans), and the invasion of Europe by
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Genghis Khan in 1222 (a very bad omen for most Europeans). The comet
also gave Edmund Halley a chance to test Newtonʼs newly discovered
laws of gravitation, and to determine that several historical comets were
actually the same comet that returned every 76 years. Halley correctly predicted that this comet would return in 1758. While he didnʼt live long
enough to see the return himself, his name remains associated with this
periodic visitor.
Most bright comets arrive unpredictably from the distant Oort
Cloud, after which they are usually flung into interstellar space by the
Sun’s gravity, never to return. Comets that orbit the Sun with periods of
less than 100 years often come from the Kuiper Belt, a stream of objects
just beyond the orbit of Neptune. Comets are often discovered accidentally by amateur and professional astronomers with large telescopes and
complex imaging equipment.
Bright comets are usually best seen without optics because they are
too large to see all at once in a telescope or binoculars. But some optical
aid is useful when examining structure in the head of the comet. With a
telescope, sometimes changes in the head’s structure can be observed
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F IGURE 3.1 Comet Halley
Comet 1P/Halley during its return in 1910
over the course of a few hours, as jets of material emerge when the comet
is heated by the Sun.
When you see a great comet, you understand why they caused such
fear in more superstitious times. As the comet passes Earth, its coma, or
head, can be larger and brighter than any other celestial sight save the
Moon or Sun, and the tail can display lovely fine structure and filaments
that can change over the course of minutes and hours. No other celestial
object matches its appearance.
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The great Mark Twain was born during the return of Halleyʼs Comet
in 1835. In 1909, he predicted his life would end with the cometʼs return.
“The Almighty has said, no doubt”, wrote Twain, “Now here are these two
unaccountable freaks; they came in together, they must go out together”.
Twain died in April 1910, a day after Comet Halley made its closest approach to the Sun.
Itʼs hard to arrange your life around a comet, as did Mark Twain, but
the appearance of a great comet is a delightful event in the life of any stargazer. When one comes along, make sure you get to see it, observe it at
length, and cross it off your celestial bucket list.
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4
A TOTAL SOLAR ECLIPSE
It’s an amazing coincidence, but our Moon is about 400 times
smaller than our Sun, and is also 400 times closer. That means, when the
alignment is just right, the Moon can pass in front of the Sun and overlap
it almost exactly, blocking out most of the Sun’s light and giving us an
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amazing view of the glowing outer reaches of the solar chromosphere and
corona. This is a total solar eclipse, one of the most memorable and aweinspiring sights in all of nature.
Solar eclipses have struck wonder and fear into the heart of mankind
since prehistoric times. The classical Greek poet Archilochus wrote of an
eclipse:
G ALLERY 4.1 The main types of solar eclipse
A partial solar eclipse during which the Sun and Moon are not exactly overlapped (credit: Rhys Jones)
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“Zeus, the father of the Olympic Gods, turned mid-day into night,
hiding the light of the dazzling Sun; and sore fear came upon men.”
And Mark Twainʼs fictional Connecticut Yankee leveraged a solar
eclipse to escape a tight spot in King Arthurʼs court:
“It got to be pitch dark, at last, and the multitude groaned with horror to feel the cold uncanny night breezes fan through the place and see
the stars come out and twinkle in the sky.”
A solar eclipse occurs only at New Moon when the Moon is between
the Earth and the Sun. But the Moon does not pass exactly in front of the
Sun every month because the Moon’s orbit around the Earth is slightly
tilted compared to the Earth’s orbit around the Sun. So the Moon usually
passes just above or below the Sun each month. But when the alignment
is just right, the Moon appears to cross the face of the Sun and it casts a
dark shadow across a narrow band of the Earthʼs surface. This is the socalled “band of totality”. Just outside this narrow band, an observer sees
the Moon cover only a part of the solar disk. This is a partial solar eclipse.
During some eclipses, because the Moon is in an elliptical orbit, it is a little too far from Earth to cover the Sunʼs disk exactly, so it leaves a slender
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F IGURE 4.1 Solar eclipse paths from 2001 to 2025
The paths of total solar eclipses across the face of the Earth from 20012025
ring of light around the solar circumference. This is called an annular solar eclipse.
A total eclipse presents, by far, a most memorable experience for any
stargazer. As the event unfolds, the Moonʼs limb first crosses the Sunʼs
disk a few hours before totality. Without a telescope, the effect is unnoticeable. Then, a few minutes before the peak of the eclipse, the sky and
Earth darken, the temperature drops, and animals and insects are star-
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tled into their nighttime routine. In the final moments before totality,
bright beads of light appear along the limb of the merged disks– they are
called Baileyʼs Beads— caused by the edge of the Sun shining through lunar valleys. As the Sun shines through a single valley just before and after
totality, a single bright beam of light creates the appearance of a“diamond ring”.
Observing a total eclipse requires great care and caution. During the
few minutes of totality, when only the Sun’s much fainter outer atmosphere is visible, an observer can look at the eclipsed Sun without any filters. The sight is spectacular to the unaided eye, and a low-power telescope gives a close-up view of the corona and chromosphere during totality. But before and after totality, a solar filter is essential to prevent severe damage to your eye or imaging equipment. Annular or partial
eclipses are always too bright to observe without a proper solar filter.
Some sort of solar eclipse happens nearly every year somewhere on
Earth, but not always total, not always in a convenient place, and not always over land. There will be a total eclipse in 2015, and one in 2016 over
Indonesia and the Pacific Ocean. On August 21, 2017, there will finally be
an eclipse over the southern part of North America, the first since 1979.
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The band of this total eclipse will pass across the United States
from Oregon to South Carolina. Another eclipse will be visible in
North America on April 8, 2024 from Mexico to eastern Canada.
If you can, try to make the effort to see a total solar eclipse.
Because words can’t do justice, nor can pictures really, to the
amazing and appalling sight of the Moon sliding over the Sun like
a lens cap, plunging the world into an eerie darkness for a few
minutes to reveal the unearthly colors of the Sun’s outer atmosphere. It is unforgettable.
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5
THE PLANET SATURN
Saturn, the sixth planet from the Sun, has made more people into
stargazers than any other celestial object. Upon seeing Saturn through a
telescope, many first-time observers will ask, “Is it real?” It’s real alright.
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G ALLERY 5.1 Saturn imaged through a small telescope
Saturn and its moon Titan (at top)
Some say the sight of Saturn through a top-notch refracting telescope in
steady sky is pretty enough to make a grown man cry.
Like Jupiter, Saturn is a “gas giant”, a planet made almost entirely of
hydrogen and helium gas and peppered with traces of ammonia, methane, and frozen water. Making its way around the Sun once every 29.5
years, Saturn has a diameter nearly ten times that of Earth. But it’s also
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nearly ten times the distance to the Sun as Earth, so the planet appears
quite small in a telescope.
While Saturn has a system of cloud bands like the planet Jupiter, it’s
far less colorful than its fellow gas giant because of the icy temperatures
in its upper atmosphere. In fact, if not for its ring system, Saturn would
simply look like a smaller and washed-out version of Jupiter. But Saturn
does have rings, an amazingly complex system of rings made of tiny bits
of ice and dust that extend about 7,000 km to 120,000 km above the top
of the planet’s cloud bands. Despite their immense breadth, the whole system is just 20 meters thick. The rings may have formed from material left
over from the formation of the planet, or they may have formed more recently when a small moon wandered too close to the planet and was
ripped to shreds by tidal forces. No one knows for sure.
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You can see Saturn somewhere in the sky for most of the year, but
the best time to observe the planet is a few months before or after opposition, when it rises at sunset and draws closest to Earth. The table below
DATE OF
OPPOSITION
SATURN IN
CONSTELLATION
23 May 2015
Libra
3 June 2016
Ophiuchus
15 June 2017
Ophiuchus
27 June 2018
Sagittarius
9 July 2019
Sagittarius
20 July 2020
Sagittarius
2 August 2021
Capricornus
14 August 2022
Capricornus
27 August 2023
Aquarius
8 September 2024
Aquarius
shows you the dates of the opposition of Saturn through 2024.
Straw-colored Saturn shines brighter than most stars with a steady,
unblinking light. The color, the proportions, the rings suspended in the
blackness of space, the cloud bands, and the little blizzard of moons all
make Saturn just a beautiful object in a telescope. But Saturn can frus27
trate new stargazers. Its image is small even at magnification of 200x. At
higher magnification, the image often becomes blurry and unsteady because of currents and eddies in the Earth’s atmosphere. Some nights
have steadier air, so don’t give up if you can’t get a good view on your
first try. Also make sure you take the time to bring your telescope to the
ambient outdoor temperature so that the optics and air currents in the
tube settle down. Keep looking at the planet through the telescope and
wait for fleeting periods of good seeing when the features of Saturn and
its rings appear to snap into view.
In most conditions, the dark gap of the Cassini Division that separates the outer “A” ring from the middle “B” ring is easily visible. And
even the smallest telescope shows Saturn’s largest moon Titan. Many mistake this large moon for a star, but its motion from night to night relative
to the planet gives away its nature. In a larger telescope, Titan takes on
an orange-yellow color because of its thick atmosphere of nitrogen and
frozen hydrocarbons. Titan is larger than the planet Mercury, and it’s far
more massive than all Saturn’s 61 other moons and rings combined.
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6
THE SOUTHERN/NORTHERN STARS
The astronomer Bart Bok once said, “All the good stuff is in the
southern hemisphere.”
Why is this? The night sky appears so striking south of the equator
because it just so happens the south side of our planet gives us a better
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view into the thickest part of the next big spiral arm in towards the center
of the Milky Way. Here we can see more bright stars, star clusters, and
nebulae, especially along the band of southern constellations from Scorpius and Sagittarius through Crux and Carina, than in any other part of
the sky.
And these constellations are packed with bright and interesting objects to see, including
• The three brightest stars in the sky: Sirius, Canopus, and Rigil Kent
(also known as Alpha Centauri, the nearest star system to Earth)
• The Magellanic Clouds, two irregular dwarf galaxies gravitationally
interacting with our own, and easily visible to the unaided eye
• Omega Centauri and 47 Tucanae, the two brightest globular clusters in the sky, which frankly put all northern globular clusters to shame.
• The Eta Carinae Nebula, the largest and brightest emission nebula
in the sky
• The Coalsack, a big dark cloud of dust nestled in the famous constellation Crux, the Southern Cross.
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G ALLERY 6.1 Celestial sights visible from the deep southern hemisphere
The stars of the constellation Crux, the Southern Cross and the star clouds
of the southern Milky Way. The dark cloud of interstellar dust left and below center is the Coal Sack (credit: David Liu)
Also from the southern hemisphere, the thickest and most beautiful
part of the Milky Way in Sagittarius lies right overhead for nearly half the
year from about May through November. Here you lie back, look up, and
see the symmetrical plane of the Milky Way from our place at the edge of
this vast disk of stars.
Some of the southern constellations are visible just above the southern horizon from mid-northern latitudes. Diligent observers can glimpse
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parts of the constellation Centaurus and the massive and bright globular
cluster Omega Centauri, which rises just above the horizon even from the
southernmost part of Canada. The thick star clouds of Sagittarius and
Scorpius rise low over the southern horizon, so they are frequently obscured by dust and humidity, and much of the Milky Way remains hidden below the horizon for northern observers. The deep-southern sights
such as the Southern Cross and Magellanic Clouds are difficult to see
even from northern tropical latitudes.
So make sure you travel south of the equator, at least once in your
life, to see the dark and clear skies of the southern hemisphere. Bring a
camera. And bring the biggest telescope you can carry.
And if you live in the southern hemisphere, keep in mind the many
fine sights to see in the far northern sky during a trip north of the equator.
In the constellations Perseus and Cassiopeia, for example, in November through March, the night side of Earth points in the direction opposite the center of the Milky Way towards the Perseus Arm, the next spiral
arm away from the galactic center. There are many fine nebulae and star
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G ALLERY 6.2 Celestial sights visible from the northern hemisphere
Star trails around the bright star Polaris, the North Star (credit: Steve
Ryan)
clusters here. And in spring and fall, northern observers look out of the
plane of the galaxy into intergalactic space to see dozens of splendid
nearby galaxies not visible from the southern hemisphere.
Some northern highlights include:
• The dazzling “Double Cluster” in the constellation Perseus, two adjacent groups of blue-white stars born just 5 million years ago
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• The “North Star”, Polaris, the bright star in the “Little Dipper”
which marks the position of the north celestial pole and which has guided
navigators for centuries
• M81, an elegant nearby spiral galaxy and its close neighbor M82,
which is wracked and mangled by a period of rapid star formation. Both
galaxies are visible at once in binoculars or a telescope
• Algol, the “Demon Star”, a two-star system in which the fainter star
passes in front of and dims its brighter companion every 2.87 days like
clockwork.
• The famous “Big Dipper”, part of the constellation Ursa Major, the
Great Bear, which constitutes the nearest star cluster to our solar system
• A half-dozen or more young star clusters in the constellation Cassiopeia, including the “E.T. Cluster”, which resembles the fictional alien in
the Steven Spielberg Movie
And observing from northern latitudes also gives you a better view of
M31 in the constellation Andromeda and M33 in the small constellation
Triangulum. These are the nearest major spiral galaxies to Earth and the
most distant galaxies visible to the unaided eye.
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7
THE GREEN FLASH
If you’ve ever visited the small town of Key West, Florida, you might
have seen as night falls a large crowd gathering in Mallory Square at the
foot of Whitehead Street. Most are there to browse the tourist shops or
to people watch. But a few have come to see a rare and beautiful sight:
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the fleeting “green flash” of light that appears at the Sun’s edge as it sets
below the horizon. Key West is one of many places on Earth to see this
fleeting phenomenon.
Like a rainbow, the green flash is a result of our atmosphere. Air
bends (or refracts) light from the Sun. In fact the atmosphere bends the
setting Sun’s light so much that when we see the Sun’s disk just above the
horizon at sunset, most of it has already set. We’re just seeing an image
refracted from below the horizon.
As with a rainbow, the atmosphere bends green, blue, and violet
light more than red and orange light. So the red light from the Sun’s disk
sets first, followed by orange, yellow, green, blue, then violet light, which
sets last. So why don’t we see a flash of blue or violet light as the last of
the Sun goes down? Sometimes, we do. But unless the air is very clear
and steady, blue and violet light are scattered out of our line of sight by
the same effect-- called Rayleigh scattering-- that makes the sky blue. So
green is the most common color that can make it all the way to your eye.
To see the green flash, you need a clear view of a cloudless horizon
over a great expanse of steady atmosphere. Looking at the sunset over
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the ocean is a good bet. A flat prairie, desert, or mountain range can
work just as well. But the flash is only visible for a couple of seconds, so
you need to look carefully.
But you must be cautious when you try to see the green flash. Donʼt
stare directly at the setting Sun or you may suffer eye damage. Instead,
wait until the Sun is almost down. Glance towards it briefly with your peripheral vision. Once you’ve sensed the Sun has nearly set, take a careful
look for this rare and beautiful sight.
On average, the green flash lasts longer at higher latitudes in the
northern or southern hemisphere because it takes longer for the Sun to
set. You can also see the green flash at sunrise, although it helps to know
exactly where and when the Sun is expected to rise.
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8
THE TRANSIT OF MERCURY
The Moon is not the only celestial body to pass in front of the Sun
from time to time, as it does during a solar eclipse. If the orbits of Earth
and the two innermost planets, Mercury and Venus, were exactly in the
same flat plane, we would see these planets appear to pass in front-- or
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G ALLERY 8.1 Transits of Mercury and Venus across the face of the Sun
Transit of Mercury on November 8, 2006 in white light (credit: Eric S.
Kounce)
transit--- the face of the Sun every 116 days and 584 days, respectively.
But the orbits of the planets are tilted slightly with respect to each other,
so transits of the two inner planets are far more rare events. Venus has
transited the Sun as seen from Earth just seven times since the invention
of the telescope more than 400 years ago. And a transit of Mercury is visible from Earth just 13 to 14 times each century.
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Because Venus is larger and closer to Earth, a transit of Venus is a
striking sight, and it was once a hugely important event for astronomers.
That’s because, in the early 18th century, the British scientist Edmond
Halley found a way, using geometry, to calculate the distance from the
Earth to the Sun by timing the transit of Venus from widely separated locations on Earth. Once the Earth-Sun distance was known, the distances
to other planets could be determined through Kepler’s third law, which
links the distance of a planet to the Sun to its period, the time it takes
that planet to move once around the Sun, and which is easy to observe directly.
Because of Halley’s suggestion, many advanced nations dispatched
astronomers around the world to measure the timing of the transits of Venus in 1761 and 1769. The transit of Venus in 1761 yielded few conclusive
results despite hundreds of attempted measurements. But the transit of
1769 was measured precisely by, among others, Royal Navy Lieutenant
James Cook who led a team to observe the event from Tahiti before sailing on to claim Australia for England. Astronomers used Cook’s measurements to calculate an Earth-Sun distance of 150 million kilometers, very
close to the now-accepted value of 149,597,870.7 kilometers.
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M OVIE 8.1 Transit of Mercury in 2006.
Time lapse of the November 8, 2006 transit of Mercury as photographed
from Southern California. The apparent color and shape of the sun
change as the transit ends near sunset (credit: Dave Kodama)
The last two transits of Venus occurred in 2004 and 2012. But the
next won’t happen until December of 2117. So unless you have a really
good immune system, you’re not going to see it.
But you might have better luck with Mercury. Since Mercury is
closer to the Sun and moves faster than Venus, it transits the Sun more
frequently. The last two transits of Mercury happened in 2003 and 2006.
The next will happen on May 9, 2016 and will be visible from eastern
North America and western Europe. Another will be visible from Africa,
the south Atlantic, and South America on November 11, 2019.
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M OVIE 8.2 Transit of Venus on June 5, 2012 from NASA’s Solar Dynamics Observatory
spacecraft.
Video constructed from extreme ultraviolet light and a portion of the visible spectrum. The red colored sun is the 304 angstrom ultraviolet, the
golden colored sun is 171 angstrom, the magenta sun is 1700 angstrom,
and the orange sun is filtered visible light.
Because Mercury is much smaller and farther away than Venus, the
planet appears as a tiny speck as it crosses the face of the Sun. So if you
want to see it, your best bet is a small telescope with a good and safe solar
filter. With your unaided eye, even with a solar filter, the planet will appear quite tiny and hard to differentiate from sunspots.
A transit of an inner planet unfolds in four stages. First, the leading
edge of the planet appears to contact the edge of the Sun. Then comes
the trailing edge, which is hard to time exactly because of the “black drop
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effect” which appears to bleed darkness from the limb of the planet as it
moves onto the solar disk. The same two stages reverse themselves as
the planet leaves the solar disk. A transit of Mercury takes about five to
six hours from beginning to end.
Despite the challenges of seeing the transit of Mercury, it’s worth the
effort. It’s a great observational project, and it gives you a first-hand look
at the clockwork operation of our solar system.
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9
THE METHUSELAH STAR
The observable universe holds some 100 billion trillion stars, which
by some estimates is more stars than all the grains of sand on all the
beaches of the Earth. But there was a time in the early universe, until a
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few hundred million years after the Big Bang, when there were no stars.
Not a single one.
During its first hundred million years, the universe was as smooth
and uniform as a cup of tea. The universe contained mostly hydrogen and
helium gas, tiny and speedy particles called neutrinos, a mysterious substance called dark matter, and a faint afterglow of light from the Big
Bang. But it was not perfectly smooth. Some patches of gas and dark matter were marginally denser than others, and the gravitational pull of
these denser regions drew in more gas and dark matter. Over tens of millions of years, these areas grew denser and hotter, and some small clouds
of gas within these regions collapsed and become hot enough to burn hydrogen gas into helium gas in their cores and release huge amounts of energy. These were the first stars in the universe.
The first stars were hot and massive, shining mostly with blue and ultraviolet light. They burned fast, exploding as supernovae and seeding
the universe with atoms of heavier elements like carbon and oxygen and
calcium and iron. These heavy elements did not exist anywhere in the universe before the first stars. But they were important for the formation of
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F IGURE 9.1 Location of the Methuselah Star
This annotated image shows the location of the Methuselah star in the constellation Libra (credit: A. Fuji)
the next generations of stars, and they were essential for the formation of
planets and people.
Since they burned out in just a few million years, all the first stars
are long gone. But astronomers have found some very old stars that may
have formed from the ashes of the first stars. One of the easiest of these
ancient stars to find is HD 140283, also known as the “Methuselah Star”.
It’s only 190 light years away and shines at 7th magnitude in the constella47
tion Libra, just north of the stars gamma and beta Librae. You can find it
fairly easily with a good star map and with pretty much any telescope or
pair of binoculars when the constellation Libra is visible. Libra is best
seen in the evening hours from May through August, for example.
The Methuselah Star is not a visually arresting sight. It looks like any
other star. But like many things in astronomy, it’s far more interesting
when you understand what you’re looking at. The Methuselah star is an
elder statesman of the universe. It’s been around since a few hundred million years after the Big Bang, and it has in its atmosphere some of the
heavy elements cooked in the cores of the very first stars. By measuring
the traces of heavy elements in the star-- iron, oxygen, and so forth-- astronomers can get a better idea of the size and lifecycle of the elusive first
stars. This will help them look for these stars as they use the world’s largest telescopes to look far back into the earliest days of the universe.
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10
THE ORION NEBULA
Now to one of the grandest sights in all of nature, the Orion Nebula,
a short-lived blister of glowing gas set alight by blazing newborn stars.
This nebula is as beautiful an object as you will ever see in the night sky.
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I NTERACTIVE 10.1 The Orion Nebula (Messier 42)
M43 (de Mairan’s Nebula)
Trapezium
Dark Dust Lane
The Orion Nebula is essentially a star factory, a region where a new
cluster of stars is forming out of a dark, cold cloud of interstellar gas and
dust. It’s part of a larger cloud in and around the constellation Orion that
has enough material to make about 10,000 stars. The Orion Nebula is
about 1,500 light years from Earth and spans about 25 light years. It
wasn’t there 5 or 10 million years ago, and a few million years from now
it will be gone. That’s because most of the gas and dust will be blown
away by the cluster of new stars forming inside.
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The Orion Nebula, which is one of hundreds of such star-forming regions visible in a small telescope, consists mostly of hydrogen and helium
gas with sprinklings of dust and traces of heavier atoms like oxygen, sulphur, and nitrogen. The atoms within the cloud are excited by the light
from the hot new stars which form nearby, then relax and emit energetic
green and red light at characteristic wavelengths. So these star-forming
regions are sometimes called “emission nebulae”.
You can see the Orion Nebula with just your eyes as the “fuzzy star”
in the middle of the Sword of Orion. The constellation rises in the midevening hours from mid-December through March in most of the world.
Binoculars give an expansive view of the Sword area of Orion, which includes the Orion Nebula and sprinklings of recently-formed blue-white
stars. A small telescope gives a better view, especially at low to moderate
magnifications. Start low, at about 40-50x, and work your way up. The
nebulosity extends much farther than you may first think: use averted vision to glimpse its full expanse. At high magnification you’ll lose the overall shape, but you will see the fine detail in the nebula’s mottled structure
and the beautiful diamond-like stars near the center that sparkle like a
jar full of fireflies.
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The Orion Nebula is often called M42 or Messier 42 because of its
place in the famous “Messier list” of bright nebulae, galaxies, and star
clusters devised by the 18th-century French astronomer Charles Messier.
This is one of the few objects that can, to a telescopic observer, look
better visually than in photographs. It looks like a large, textured, greygreen batwing-shaped cloud surrounding a little collection of blue-white
stars. Some of the new stars within the nebula, especially in the aptly
named Trapezium cluster-- are so hot and energetic they have blown a
bubble inside the nebula and let us have a good look inside.
So have a close look at the Orion Nebula. It’s worth the effort. Astronomy writer Walter Scott Houston said of the Orion nebula, “No amount
of intensive gazing ever encompasses all its vivid splendor”. It’s truly one
of the most beautiful things you will ever see.
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11
A SUPERNOVA
Finally, the last item on this bucket list: a supernova. A supernova is
a massive star that suddenly collapses and explodes, creating more energy in a few weeks than our Sun creates in its entire lifetime. Supernovae are among the most violent and energetic events in the universe.
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They are also one of the most important because they often trigger the formation of new stars, and they seed the universe with the heavy elements
needed for the formation of planets and life.
There are two main types of supernova. When a star five or more
times the mass of the Sun runs of out fuel in its center, it suddenly collapses and quashes its core before explosively snapping back outward.
This is a “Type II” supernova. The core of the star becomes a dense neutron star with a mass of two or three Suns but the size of a small city. If
the core of the star has enough mass, it can collapse into a black hole, a
single massive point from which no light can escape. The so-called “Type
Ia” supernova begins as a white dwarf, which is a remnant of a smaller
star which has run out of fuel and has settled down to slowly cool like a
piece of charcoal in a campfire. But if the white dwarf has a nearby companion star from which it slowly attracts more material, it can gain mass
and suddenly collapse and explode.
Both types of supernova are fearsome events. Much of the star itself
is destroyed, and any planets around a star that becomes a supernova will
be quickly vaporized. Lethal levels of gamma rays and X-rays are released
with enough energy to damage the biosphere of an Earth-like planet
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F IGURE 11.1 A supernova in a distant galaxy
A supernova on the edge of galaxy NGC 4526. The Type Ia supernova
was discovered in 1994. The galaxy lies at a distance of 55 million light
years (credit: NASA/HST).
within an enormous 50 light-year radius. Fortunately, there are no such
stars close enough to the Sun to do us harm in the foreseeable future.
But you would not be here if not for supernovae. That’s because
many of the elements heavier than hydrogen, and all the elements heavier than carbon, were made during the sudden collapse of massive dying
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stars and scattered across the cosmos by a supernova. The iron in your
blood, the gold and silver in your jewelry, the copper and zinc and tantalum in your cell phone, were all created during long forgotten supernova
explosions billions of years ago.
Supernovae only last a few weeks. And most stars, including the Sun,
are too small to explode in this way. So a supernova is quite rare. None
have been seen in our Milky Way galaxy since 1604 when the great astronomer and mathematician Johannes Kepler noticed a bright “new
star” in the constellation Ophiuchus that for a few weeks outshone all
other stars in the sky. Before that, Kepler’s colleague, the great Danish astronomer Tycho Brahe, witnessed an even brighter supernova in 1572 in
the constellation Cassiopeia.
Astronomers estimate there should be a supernova, on average, once
every 50 to 100 years in the Milky Way. So we are long overdue. Many
bright and massive stars in our sky will one day detonate as supernovae:
Betelgeuse in Orion, as well as eta Carinae, Spica, and Antares and
Shaula in Scorpius. Astronomers don’t know exactly when these stars
will blow. It might happen next week, or it might happen in a million
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F IGURE 11.2 The remains of SN1987A
The expanding ring of material around
supernova SN1987A, which exploded in
the Large Magellanic Cloud in 1987.
years. But they all will one day shine bright enough to be seen in our daytime skies and cast shadows by night for weeks before fading away.
Supernovae occur in other galaxies, too, and a few dozen are discovered by astronomers every year using large telescopes and automated imaging systems. Most are too far away to see without a big telescope. But
every few years, a supernova occurs in a nearby galaxy and becomes visible in a small telescope or a good pair of binoculars. In 2014, a star exploded in M82, a galaxy found near the famous “Big Dipper” in the con57
stellation Ursa Major. M82 is about 12 million light years away, which is
close for a major galaxy, so the supernova was bright enough to see in
nearly any small telescope. Another supernova became visible in 2011 in
the galaxy M101, also located near the Big Dipper. M101 is about 20 million light years away, close enough for the supernova to be visible in a
telescope in dark, clear sky. And in 1987, a star exploded in the Large
Magellanic Cloud, a dwarf galaxy visible from the southern hemisphere.
The galaxy is just 160,000 light years away, so the supernova became visible without optical aid for several weeks. It was the only supernova visible to the unaided eye since 1604.
Because they are rare, you need some luck to see a supernova. If one
finally occurs in our galaxy, it will make front-page news and you will
have little trouble finding it using the simplest of maps. It will outshine
every other star in the sky.
Supernovae in other galaxies are a little trickier because they are
much fainter and require optical aid. Instructions and maps to help you
locate such a supernova are usually available at astronomy news websites
and in astronomy magazines. If you get the chance to check this one off
the bucket list, don’t miss it. Seeing a supernova for yourself is an im58
mensely rewarding experience because you’re observing one of the most
energetic and important events in the universe, one which creates the conditions and material to form new stars and planets. Much of the material
that makes up your body was created in a long-forgotten supernova many
billions of years ago.
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C H A P T E R 12
BEYOND THE BUCKET LIST
“Learning never exhausts the mind.”
~ Leonardo da Vinci
Whether you’re a casual or experienced stargazer, try to see the
sights and events on this “bucket list” at least once in your life. You will remember each of them for a long time. And if fortune and time allow, you
might get to see some of them more than once.
But of course there are thousands to millions of sights to see in the
night sky that are accessible to a determined stargazer with binoculars or
a small telescope. You could wander outside with a telescope under a
clear sky every night for the rest of your life and you would not run out of
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things to see. With a simple star map and a bit of experience, you can
sweep a pair of binoculars or a small telescope across the clear night sky
and find...
• Open star clusters of hundreds of new stars which were born together and have emerged from a dusty stellar nursery like the Orion
Nebula
• Planetary nebulae, intricately shaped clouds of glowing gas ejected
by small to mid-sized stars in their last stages of life
• Variable stars that pulse in regular or semiregular cycles every few
days or months, and change their intrinsic brightness by a factor of hundreds or thousands
• The endless motion of the four brightest moons of Jupiter as they
revolve around the big planet and occasionally cast shadows across its
face
• The ice palaces of Uranus and Neptune, the giant outer planets of
the solar system
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• Dwarf planets such as Ceres or Vesta that lurk in the asteroid belt
between Mars and Jupiter
• Aurorae Borealis or Aurorae Australis, the glow of atoms in the
Earth’s upper atmosphere excited by charged particles streaming from
the Sun
With a telescope equipped with a so-called H-alpha solar filter, you
can see flares and prominences and magnetic activity on the blood-red
face of the Sun. Or with a simpler “white light” filter, you can see the dark
blotches of sunspots as they move in and out of view with the Sun’s rotation every 26 days.
And once you acquire a little experience and a bigger telescope, you
can see hundreds or thousands of galaxies, the light from which has traveled across space for tens of millions of years.
If you’re a novice and wish to learn more, your best bet is the latest
edition of the illustrated guide book called Nightwatch by Terence Dickinson. It’s available from most booksellers.
The free short articles in the Guide to the Night Sky and Basic Astronomy at One-Minute Astronomer are also a good place to get started:
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http://oneminuteastronomer.com/stargazing-and-night-sky-guide/
The articles for beginners in the two major astronomy magazines,
Sky and Telescope and Astronomy, are also good places to start.
http://www.skyandtelescope.com/
http://www.astronomy.com/
NASA’s eclipse web page has dates and tables to let you know when
solar and lunar eclipses will occur:
http://eclipse.gsfc.nasa.gov/eclipse.html
The website SpaceWeather.com gives you up-to-date information on
meteor activity, solar flares and sunspots, and auroral activity:
http://spaceweather.com/
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