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Your guide to
planets, stars,
and galaxies
by Richard Talcott
A supplement to Astronomy magazine
618129
© 2012 Kalmbach Publishing Co. This material may not be reproduced in any form
without permission from the publisher. www.Astronomy.com
50
IN 0
S
FA
CT
S
ID
E!
Saturn
Planets
of the
solar system
Saturn’s rings consist of icy particles
ranging in size from tiny motes to
house-sized icebergs. NASA/The Hubble
Heritage Team (STScI/AURA)
E
arth may seem extraordinary to those who call it home,
but it’s not a land of superlatives. Earth is neither too hot
nor too cold, too big nor too small. It’s just right in so
many ways — the perfect “Goldilocks” planet. Of course,
as the only known abode of life in the universe, Earth
does have one major claim to being special. The other planets in
the solar system leave their marks in different ways.
The planets divide into two broad categories: terrestrial and
jovian. The small, rocky terrestrial planets include Mercury,
Venus, Earth, and Mars. Mercury, the closest to the Sun, bakes
at temperatures up to 800° Fahrenheit at noon. But Mercury’s
razor-thin atmosphere can’t hold heat; at night, the temperature
plummets far below freezing. Venus most resembles Earth in
mass and diameter, but a thick atmosphere of carbon dioxide
has led to a runaway greenhouse effect. Venus’ surface remains
a scorching 865° F year-round.
Earth and Mars are the water worlds of the solar system.
Our home planet is the only one with liquid water at the surface
now, but spacecraft observations during the past 15 years leave
no doubt that Mars once had loads of surface water. Even now,
Mars has permafrost and permanent polar caps of water ice.
Winds up to 70 mph blow around the ubiquitous martian dust,
creating shifting seasonal patterns.
The jovian planets — Jupiter, Saturn, Uranus, and Neptune
— are all gaseous behemoths. They consist mostly of hydrogen
and helium, the most abundant elements in the universe. Jupiter
dwarfs the others: It contains more than twice as much matter
as all the other planets combined. All the jovian planets possess ring systems, but only Saturn’s appears bright. Its icy rings
span 170,000 miles and measure just 100 feet thick. Uranus
and Neptune are the true twin planets of the solar system, with
nearly equal diameters, masses, compositions, and rotations.
Most scientists no longer consider small, distant Pluto to be a
major planet. A mixture of ice and rock, this world more closely
resembles the thousands of so-called Kuiper Belt objects that
lurk beyond Neptune. In 2006, astronomers demoted Pluto to a
“dwarf planet,” a category that also includes the asteroid Ceres.
2
Your guide to planets, stars, and galaxies
Mars
F
U
N
Mars boasts the largest volcanoes in the solar system,
although they’re all extinct.
The biggest — Olympus
Mons — spans nearly 400
miles and rises 13 miles
above the surrounding plains.
F
Mars’ ruddy appearance arises
because the sand on the planet’s
surface consists largely of iron
oxides — rust. NASA/JPL/MSSS
A
C
T
Jupiter
Jupiter is so big that it would take 11 Earths wedged side by side to cross the
giant’s girth and more than 1,000 Earths to fill its volume. NASA/JPL/University of Arizona
Venus
F
U
Thick clouds blanket Venus, so astronomers use radar to see
its surface. The atmospheric pressure there is nearly 100 times
that at Earth’s surface. NASA/JPL
N
Saturn has the lowest density
of any planet. In fact, if you
filled a solar-system-sized
basin with water, the ringed
world would float.
F
Mercury
A
C
T
Earth
Pluto & Charon
F
U
N
Sunlight takes just eight minutes to reach Earth but more
than four hours to cross the
void to Neptune and Pluto.
F
Mercury's high density means
more than half of it must be made
of the heavy elements iron and
nickel. NASA/JPL/USGS
Nearly three-quarters of
Earth’s surface is covered with
water. It’s what makes our home
world conducive to life. NASA
Uranus
Neptune
A
C
T
The orbit of dwarf planet Pluto (left; along with its moon, Charon) brings it closer
to the Sun than Neptune for 20 years out of its nearly 250-year-long circuit. ESA/NASA
Solar system planets
Planet Distance from Sun Orbital period Diameter
Mass
Density
(Earth=1)
(Earth=1) (Earth=1)(water=1)
Uranus’ bland cloud tops mask
the fact that its rotation axis lies in
its orbital plane, so night and day at
the poles last 40 years each. NASA/JPL
Storms rage in Neptune’s atmos­
phere, as they do in the massive
atmospheres of most of the jovian
planets. NASA/JPL
Mercury
Venus
Earth
Mars
Ceres
Jupiter
Saturn
Uranus
Neptune
Pluto
0.39
0.72
1.00
1.52
2.77
5.20
9.58
19.20
30.05
39.48
87.97 days
224.70 days
365.26 days
686.98 days
4.60 years
11.86 years
29.46 years
84.01 years
164.79 years
247.68 years
0.383
0.949
1.000
0.532
0.075
11.209
9.449
4.007
3.883
0.187
Note: Ceres and Pluto are officially considered to be dwarf planets.
0.055
0.815
1.000
0.107
0.0002
317.832
95.159
14.536
17.147
0.002
5.43
5.24
5.52
3.93
1.93
1.33
0.69
1.27
1.64
1.75
Titan
Small bodies
of the solar
system
A
fter the Sun and planets, there’s not much else in the
solar system — certainly not in terms of mass. But
in sheer number (and in a few notable instances,
prominence), the small objects hold their own. The
biggest of the small bodies actually outrank the smallest planet. Both Ganymede, a Jupiter moon, and Titan, a Saturn
moon, have diameters larger than Mercury. More than 170
moons have been discovered orbiting the eight planets, although
the vast majority are little more than flying boulders.
The smaller planets tend to have fewer moons. Earth has just
one, which formed when an object the size of Mars struck the
proto-Earth, ejecting debris that eventually coalesced. Mercury
and Venus have no moons, and Mars possesses just two small
ones. Oddly enough, Pluto’s large moon, Charon, is half the diameter of the dwarf planet — the largest ratio in the solar system.
The hefty moons of the gas giants garner most of the attention. Jupiter’s four big moons — Io, Europa, Ganymede, and
Callisto — form a miniature solar system. Io ranks as the most
volcanically active object in the solar system. Europa hides an
ocean of liquid water — perhaps larger than all of Earth’s oceans
— beneath its frigid ice crust. Giant Ganymede also may harbor
an ocean and has a surface covered with intriguing grooved terrain. And Callisto sports more craters than any other object in
the solar system. At the top of Saturn’s family of moons is Titan,
which possesses a significant atmosphere and methane lakes.
More than half a million asteroids also inhabit the solar system.
The biggest, Ceres, has a diameter of 600 miles. Yet most are far
smaller: If you add them all up, asteroids don’t equal the weight
of Earth’s Moon. Most asteroids circle the Sun between the orbits
of Mars and Jupiter, although a few wander into Earth’s vicinity.
Perhaps the most spectacular small bodies are comets.
Billions of these “dirty snowballs” lurk in the outer solar system. If their long, looping orbits bring them close to the Sun’s
warmth, they shed gas and dust. The Sun then blows this material back to create a long tail. Although a comet’s nucleus may be
only a mile or two across, its tail can stretch millions of miles.
4
Your guide
Your
guideto
toplanets,
planets,stars,
stars,
and
and
galaxies
galaxies
Titan’s hazy atmosphere glows as it scatters incoming sunlight. The atmosphere of Saturn’s moon is
thicker than Earth’s and, like ours, contains mainly
nitrogen. NASA/JPL/SSI
Europa
Ridges crack the surface of Jupiter’s moon
Europa. Such ridges could be sites where
slushy water erupted through the icy surface
and then froze. NASA/JPL
Io
More than 100 active volcanoes dot the surface
of Jupiter’s moon Io. The plumes can reach 100
miles high and spread debris over thousands of miles. NASA/JPL
Callisto
Triton
Geyser-like plumes
deposited the dark
streaks seen on Neptune’s moon Triton. At a
temperature of –390° F, Triton
has the coldest surface known in
the solar system. NASA/JPL
Enceladus
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Saturn’s enigmatic outer
moon, Iapetus, has a split
personality: Half of its surface
appears as dark as freshly laid
asphalt while the opposite
hemisphere reflects as much
light as newly fallen snow.
F
A
Saturn’s icy moon Enceladus reflects
more than 90 percent of the sunlight
that reaches it, the highest percentage
of any object in the solar system. NASA/JPL
Hale-Bopp
C
T
Multi-ringed impact basins, some stretching more than
1,000 miles, formed on Jupiter’s Callisto when massive
impacts left concentric fractures and faults. NASA/JPL
Eros
One of the brightest comets of the past
40 years, Hale-Bopp wowed observers
in 1997. It had a nucleus 25 miles wide
and a tail that stretched more than 100
million miles. Bill and Sally Fletcher
Potato-shaped asteroid Eros, some 20 miles long, looks like
a lot of other modest-sized asteroids, but this object might
one day wander dangerously close to Earth. NASA/JHUAPL
F
U
Major moons
N
Io’s active volcanoes and
Europa’s underground ocean
of liquid water both stem
from enormous tidal forces
— which flex and heat the
moons’ interiors — exerted
by Jupiter’s massive gravity.
F
A
C
Moon
Planet
T
Phobos
Like most small moons in the solar
­system, Mars’ Phobos measures just a
few miles across and has an irregular
shape. NASA/JPL/MSSS
Distance from planet (miles)
Orbital period Diameter Density
(days)
(miles) (water=1)
Moon Earth 238,900
Io
Jupiter 262,100
EuropaJupiter 417,000
GanymedeJupiter
665,100
Callisto Jupiter1,169,900
EnceladusSaturn
147,900
Tethys Saturn 183,100
Dione Saturn 234,500
Rhea Saturn 327,500
Titan Saturn 759,200
Iapetus Saturn2,212,600
Ariel Uranus 118,600
UmbrielUranus 165,300
TitaniaUranus 271,100
OberonUranus 362,600
TritonNeptune220,400
Charon Pluto 12,200
27.32
1.77
3.55
7.15
16.69
1.37
1.89
2.74
4.52
15.95
79.33
2.52
4.14
8.71
13.46
5.88
6.39
2,1603.3
2,2633.5
1,9403.0
3,271
1.9
2,994 1.8
313 1.6
6601.0
6981.5
9491.2
3,2001.9
913 1.1
7191.7
727 1.4
9801.7
946 1.6
1,6812.1
7531.9
The Sun
Stars in
our galaxy
A
ll stars begin their lives in the vast clouds of gas and
dust that litter galaxies like the Milky Way. A single
cloud can produce hundreds, or even thousands, of
stars. Something triggers the cloud to start collapsing — perhaps strong winds from a massive star or
a nearby supernova explosion — and gravity works its magic.
The cloud fragments, and each pocket of material continues to
contract and heat up.
The contracting star becomes stable when it starts to generate energy by nuclear fusion. Four hydrogen atoms combine to
form one helium atom. Because one helium weighs slightly less
than the four hydrogens combined, the reaction creates energy
according to Einstein’s equation E=mc2.
The biggest stars contain up to about 120 times as much
material as the Sun. They burn hot and use their fuel rapidly.
These luminaries may have a surface temperature of 70,000° F,
radiate nearly a million times the Sun’s light, and survive only
a few million years.
The Sun shines at about 10,000° F and will last some 10
­billion years (it’s about halfway through now). The smallest
stars have 8 percent of the Sun’s mass and glow at only 3000°
to 4000° F — so dim that they can shine for a trillion years.
Once a star exhausts its hydrogen fuel, the end is nigh. First,
it swells into a red giant, expanding to a diameter of hundreds of
millions of miles and cooling to a few thousand degrees. It may
tap into more nuclear reactions, converting helium to carbon,
for example, but eventually those
fuels run out as well. Stars with up
F
U
N
to about eight times the Sun’s mass
Astronomers divide stars into
eventually puff off their outer layers
seven main spectral classes.
and form glowing gas clouds known
Generations of students have
as planetary nebulae. The star itself
learned the sequence by
using the first letters in the
settles down as a white dwarf.
sentence: “Oh, Be A Fine Girl
More massive stars typically die
(or Guy), Kiss Me.”
in supernova explosions. Such exploF
A
C
T
sions scatter the heavy elements built
up during the star’s life, forming
the raw material for new stars and,
perhaps, planets. The collapsed remnant of the exploded star
becomes either a rapidly spinning neutron star or a black hole,
whose gravity is so strong that not even light can escape.
6
Your guide to planets, stars, and galaxies
Most people think of the Sun as the anchor of our solar system — and
that’s certainly true. It contains 99.8 percent of all the matter in the solar
system. But to astronomers, the Sun has even more importance. It is the
only star in the universe that appears as more than a point of light
through a telescope. Detailed observations of the Sun led scientists to
understand how stars shine, how they radiate energy, and even how
huge storms wrack their surfaces. NASA/SOHO
N49
Heavy elements forged
in a massive star spread
out at thousands of miles
per second in supernova
remnant N49. One day,
these elements may be
included in a new stellar
generation. NASA/The Hubble
Heritage Team (STScI/AURA)
The life of a Sun-like star
Protostar
Cat’s Eye Nebula
Surface of the Sun
F
U
N
To shine as brightly as it does
and nourish life on Earth, the
Sun must convert 600 million
tons of hydrogen into helium
every second.
F
A
C
T
When the Sun dies in 5 billion years, it may resemble the symmetric Cat’s Eye Nebula. Here, glowing
strands of ionized gas mark where a dying star
repeatedly shed its outer layers. NASA/ESA/HEIC/
The Hubble Heritage Team (STScI/AURA)
Cone Nebula
An intricate honeycomb on the Sun’s surface marks regions
where heat (bright areas) rises and cooler material (dark areas)
sinks in a process called convection. Royal Swedish Academy of Sciences
Star characteristics
New stars form from clouds of gas and dust such as
the Cone Nebula. Hot stars ionize the surrounding
hydrogen gas, which glows with a characteristic red
color. NASA/ESA/The ACS Science Team
Spectral class
Mass
(Sun=1)
Temperature
(Fahrenheit)
Main sequence radius (Sun=1)
Examples
O
B
A
F
G
K
M
20–120
4–20
2–4
1.05–2
0.8–1.05
0.5–0.8
0.08–0.5
greater than 55,000°
17,100°–55,000°
12,300°–17,100°
10,300°–12,300°
9,000°–10,300°
6700°–9000°
3100°–6700°
12–25
4–12
1.5–4
1.1–1.5
0.85–1.1
0.6–0.85
0.1–0.6
Zeta (ζ) Puppis
Rigel, Spica
Sirius, Vega
Canopus, Procyon
Sun, Capella
Aldebaran, Arcturus
Antares, Betelgeuse
Stars like the Sun condense out of a gaseous cloud. The growing protostar develops a disk (which may form planets)
and shoots out material before settling down as a main sequence star, converting hydrogen to helium. Once the hydrogen runs out, the star swells to a red giant and becomes unstable as an asymptotic-giant-branch star before puffing off
its outer layers as a planetary nebula. The star’s core remains as a dense white dwarf. ASTRONOMY: ROEN KELLY
Main sequence
solar-type star
Red giant
Asymptotic-giant-branch star
Protoplanetary nebula
Planetary nebula
White dwarf
Star formation in Cygnus
Open star clusters like the Pleiades
contain dozens to hundreds of stars.
These groups lie in our galaxy’s spiral arms and disperse over billions
of years. Jason Ware
Globular star clusters have existed
as long as the Milky Way. M3 packs
500,000 stars in a sphere 160 lightyears across. S. Kafka and K. Honeycutt,
Indiana University/WIYN/NOAO/NSF
Structure of the Milky Way
Norma Arm
Ce
nt
ra
l
ba
r
Sun
ion
Spu
r
um
ut
Sc
s Arm
uru
nta
e
-C
itt
ar
g
Your guide to planets, stars, and galaxies
M3
Sa
8
The Pleiades
Or
H
ead outside on a clear, dark summer’s night, and your
eyes will be greeted by thousands of stars. All of them
belong to our galaxy, as does virtually everything
else you can see with the naked eye. If you let your
eyes adjust to the darkness, you’ll see a gauzy, whitish
band running across the sky. This is the Milky Way — the combined light of countless stars — and the feature that lends its
name to our galaxy.
The Milky Way is a giant barred spiral galaxy that stretches
about 120,000 light-years from end to end but whose disk measures only some 1,000 light-years thick. The central bar extends
28,000 light-years. The Sun lies about halfway between the gal­
axy’s center and edge and revolves at approximately 150 miles
per second, taking roughly 225 million years to complete one circuit of
F
U
N
the ­galactic hub.
Most naked-eye stars are
The most obvious sights of the
massive and highly luminous
galaxy are stars. Astronomers estiones that shine across great
distances. But this gives a dismate between 200 and 400 billion
torted view of the galaxy as a
populate the Milky Way Galaxy
whole. In actuality, cool, dim,
(most are hidden from view or
M-type stars make up about
extremely faint, so a precise count
two-thirds of all stars in the
isn’t possible).
Milky Way.
Because the hottest, brightest stars
F
A
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T
are also short-lived — and the spiral
arms are the only place in the galaxy
with active star formation — the arms stand out. The clouds
of gas and dust from which stars form also call the spiral arms
home, as do the open star clusters that emerge from them.
The nuclear bulge of the galaxy consists mostly of old stars.
It measures about 12,000 light-years across. At the galaxy’s heart
lies a supermassive black hole that weighs approximately 4 million Suns. Surrounding the bulge and disk is a vast spherical
halo that stretches some 300,000 light-years.
The most prominent members of the halo are globular
clusters. These ancient collections of up to a million stars each
were born at the same time as the galaxy, some 12 billion to 13
billion years ago. They contain few heavy elements because they
formed before supernova explosions had enriched the inter­
stellar medium with them.
A stellar nursery in Cygnus harbors many massive young stars. Invisible in
optical light, the DR21 complex shows up when viewed in dust-penetrating
infrared radiation. NASA/JPL-Caltech/A. Marston (ESTEC/ESA)
ius
Arm
Ar
us
e
s
Per
m
The Milky
Way Galaxy
The Sun lies in the Orion Spur, one of several arms and smaller appendages
where our galaxy creates stars. Astronomers name the spiral arms after the
constellation where they appear prominent. NASA/JPL-Caltech/R. Hurt (SSC-Caltech)
F
U
N
In measuring distances in
the galaxy and the universe,
astronomers use a unit
known as the light-year. It
represents the ­distance a
beam of light travels in one
year. At 186,000 miles per
second, light traverses 5.9
trillion miles in a year.
F
F
U
N
Much of the Milky Way Galaxy
and its structure remain hidden to earth­bound observers
because dust chokes the spiral arms. It’s like being in the
woods and trying to discern
the forest’s form.
F
A
F
C
U
T
N
A century ago, astrono­mers
thought the Sun occupied
the center of the galaxy. But
careful studies of globular
clusters, which orbit the
Milky Way’s center and tend
to gather in the constellation
Sagittarius, show we live
halfway to the edge.
F
A
F
C
U
T
N
How do astronomers know
a black hole resides at the
Milky Way’s center? They have
found stars near the central
hub orbiting so fast that they
must be circling an invisible
object containing 4 million
solar masses.
F
A
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T
A band of dust cuts through the Milky Way, blocking light
from distant stars. If not for all the dust, the galaxy’s center
would shine brighter than the brightest star. Steve Thornton
A
C
T
NGC 4414
Galaxies
in the
universe
T
he collections of stars, gas, and dust known as galaxies
form the building blocks of the observable universe.
Roughly 125 billion galaxies populate the cosmos, and
they come in all shapes and sizes. Astronomers divide
galaxies into three major categories: spirals, ellipticals,
and irregulars. A spiral has a broad disk containing clouds of gas
and dust and from two to several spiral arms, a nuclear bulge of
old stars, and a spherical halo that envelops both.
Approximately one-third of spirals exhibit central bars — a
symmetric concentration of stars, and sometimes gas and dust,
that crosses the nucleus and connects with the outer spiral arms.
(Recent studies show the Milky Way possesses a significant bar.)
The diameters of spirals range from roughly 20,000 to more
than 100,000 light-years, and they contain anywhere from several billion to several hundred billion stars.
Elliptical galaxies appear spherical or flattened in shape.
They possess little of the gas and dust seen in the disks of spiral
galaxies, so they don’t generate any new stars. Ellipticals show
the widest range in size of any galaxy type. Giant ellipticals can
span 1 million light-years and contain several trillion stars;
dwarf ellipticals may be only a few thousand light-years across
and have millions of stars. An important intermediate type of
galaxy has a disk like a spiral galaxy but contains no gas or dust.
Astronomers call this type of galaxy a lenticular.
Irregular galaxies don’t show any symmetry or organized
spiral structure. The category exists basically as a catchall for
galaxies that don’t fit either the spiral or elliptical classification.
Irregulars can be big, containing up to 100 billion stars, or as
small as dwarf ellipticals. Astronomers think most irregulars
result from the collisions or mergers of two or more galaxies.
The gravitational interactions disrupt normal spiral or elliptical
structure, leaving behind a chaotic appearance.
Most galaxies belong to groups with dozens of members, or
to clusters with up to thousands of members. The Milky Way
joins with the slightly larger Andromeda Galaxy to form the
cornerstones of the Local Group, a collection of roughly 50 galaxies that spans several million light-years. The vast majority of
Local Group galaxies are dwarf ellipticals and irregulars. Small
groups generally have a few dozen member galaxies, but clusters
can contain several thousand galaxies. The Virgo cluster, located
50 million light-years away, is the nearest large cluster to Earth.
10 Your guide to planets, stars, and galaxies
Multiple spiral arms
wind out from the nucleus of
NGC 4414. Young blue stars throng the arms
while older, redder stars populate the nuclear
bulge. NASA/The Hubble Heritage Team (STScI/AURA)
M84 & M86
Giant elliptical galaxies M84 (left) and M86 (right)
each contain a trillion stars. These two dominate the central region
of the nearby Virgo cluster, a collection of some 2,000 galaxies. NOAO/AURA/NSF
The Mice
The “Mice” are two spiral galaxies in the process of merging. Gravity has
pulled material out of each to form long tails while compressed gas clouds
fuel new star formation. NASA/ESA/the ACS Science Team
M87
A high-speed jet shoots from the heart of the giant
elliptical galaxy M87 (upper left) in the Virgo cluster. A black hole of some
3 billion solar masses drives this activity. NASA/The Hubble Heritage Team (STScI/AURA)
F
U
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If you examine the brightest
galaxies, some 75 percent are
spirals, 20 percent ellipticals,
and 5 percent irregulars.
Includ­ing faint dwarfs skews
the numbers to 30 percent
spirals, 60 percent ellipticals,
and 10 percent irregulars.
F
A
C
T
F
U
N
The Andromeda Galaxy may
seem a good neighbor to
the Milky Way, but it won’t
always be so. Astronomers
think that in approximately 5
billion years, our two galaxies
will collide and merge.
F
Andromeda Galaxy
F
Leo I
818 kly
Leo II
750 kly
Sextans
Dwarf
293 kly
Milky
Way
C
U
T
N
The Large and Small
Magellanic Clouds (LMC
and SMC) are satellite galaxies to the Milky Way. They lie
deep in the southern sky and
were not seen by Europeans
until Magellan’s around-theworld voyage.
Slightly bigger than the Milky Way, the Andromeda Galaxy (M31) contains
some 500 billion stars. Located 2.5 million light-years away, the galaxy can be
glimpsed with the naked eye. Michael Stecker
The LMC
A
Ursa Minor Dwarf
225 kly
Draco Dwarf
248 kly
F
A
C
T
LMC
160 kly
SMC
189 kly
NGC 185
2,020 kly
The Large Magellanic
Cloud (LMC) is an irregular
galaxy about 160,000
light-years from Earth. The
reddish cloud at top right
is the Tarantula Nebula,
the largest known region
of star formation. Luke Dodd
The Local Group
NGC 6822
1,760 kly
The Milky Way and Andromeda galaxies
rule the Local Group, accounting for more
than half its mass. Distances from our galaxy
are given in thousands of light-years (kly).
ASTRONOMY: ROEN KELLY
Pinwheel Galaxy (M33)
2,770 kly
NGC 147
2,460 kly
Andromeda Galaxy (M31)
2,510 kly
Probing
the distant
universe
H
ow did the universe get to be the way it is today? It
might seem a hopeless question at first, at least until
scientists invent a time machine to take us back. But
astronomers have invented such an instrument — they
call it a telescope. Here’s how it works: Because light
travels at a finite speed (186,000 miles per second), the light we
receive on Earth left its place of o
­ rigin some time ago. The farther we look into space, the further we peer back in time.
When astronomers first studied galaxies in the early 20th
century, they found the farther a galaxy was from Earth, the
faster it appeared to be moving away. If you think of this expansion as a movie and run it backward, then all of the galaxies
must have been much closer together in the past. This led to
the idea of the Big Bang — the theory that all matter in the universe started out together and then something triggered a rapid
expansion that continues today.
But is there any proof of such an extraordinary beginning?
Yes — lots of it. Astronomers looking ever deeper into space
find that the universe was surprisingly different. Instead of the
stately spiral and elliptical galaxies we see now, there were lots
of galactic fragments that were colliding, merging, and creating general havoc. The activity dumped fuel onto supermassive
black holes at the centers of nascent galaxies, creating highly
luminous quasars. All this action took place — in fact, could
only take place — in a universe much smaller than today’s.
Radio astronomers have even discovered the echo of the initial fireball,
F
U
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which has cooled to a few degrees
Galaxies do not spread
above absolute zero.
evenly, but gather in clusters
that themselves form vast
An even more shocking discovery
filaments, leaving huge voids
came at the end of the 20th century.
in between.
By looking at supernova explosions
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in the distant universe, astronomers
discovered that the blasts did not
appear as bright as expected. The conclusion: The universal
expansion is accelerating. In essence, a long-range repulsive
force must be driving the universe to expand at ever greater
speeds. As the Scottish geneticist J. B. S. Haldane once famously
said: “The u
­ niverse is not only queerer than we imagine, it is
queerer than we can imagine.”
12 Your guide to planets, stars, and galaxies
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For decades, astronomers
have known that the universe
contains lots we can’t see.
This mysterious dark matter
surrounds galaxies and binds
clusters. Scientists suspect
exotic subatomic particles
are the culprit.
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Abell 1689
Thousands of galaxies cluster together in Abell 1689. The concentration of
luminous matter and dark matter (stuff we can’t see but which adds to gravity) creates a fun-house mirror of arcs and wisps. NASA/ESA/the ACS Science Team
Cosmic microwave background
The microwave background glows at a nearly constant temperature of 4.9° F
above absolute zero. Tiny variations in the glow are subtle density fluctuations
that gave rise to the galaxies and voids we see in the universe. NASA/WMAP Science Team
What is the universe made of?
Dark energy 72%
Atoms 5%
Dark matter 23%
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The atoms that make up stars, planets, and us add up to just 5 percent of the
universe. Invisible dark matter makes up 23 percent more, while the dark energy
that drives the accelerating cosmos accounts for 72 percent. ASTRONOMY: ROEN KELLY
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Black holes are so dense that
their gravity prevents even
light from escaping. They
range in size from monsters
in galactic cores to star-sized
remnants of supernovae.
Some scientists think there
even may be mini-black holes
left over from the Big Bang.
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The Great Attractor
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Quasars — short for quasistellar radio sources — are
the brightest objects in the
universe. They radiate as
much e­ nergy as an entire galaxy from a volume no bigger
than our solar system.
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The biggest concentration of matter in the nearby universe is a string of
huge galaxy clusters known as the Great Attractor, which is pulling the Local
Group and the Virgo cluster in its general direction. European Southern Observatory
Conjunctions
You and the
universe
When a crescent Moon passes a bright star or planet during the twilight
hours, it’s a sight that thrills any skywatcher, experienced or not. Randall Wehler
Solar eclipses
Y
ou can make a connection to the universe at large
on any clear night. Simply head outside and look up.
You don’t need binoculars or a telescope (although
they won’t hurt) — all you need are your eyes. The
simplest thing to do is trace the patterns of stars. In
winter, look for the commanding figure of Orion the Hunter.
Spring brings the Big Dipper, probably the most recognizable
stellar group in the sky.
In the summertime, look for the Northern Cross and trace the
path of the Milky Way, which appears most prominent this time
of year. The Great Square of Pegasus beckons on autumn evenings. You can find these patterns and, from them, hunt down
the less conspicuous constellations with the help of the circular
star map that appears at the center of every issue of Astronomy.
Next, use “The Sky this Month” in the magazine to home in
on the most visually arresting events in a given month. It could
be a nice meteor shower, where you might see a “shooting star”
every minute or so. Or maybe there’ll be a pretty conjunction
of the Moon with a bright planet or two (events like this usually
happen several times a month). Or perhaps you’ll be lucky and
get to witness a solar or lunar eclipse. After all, in the world of
backyard astronomy, the sky is the limit.
Meteor showers
When worlds align, observers can expect a treat. Here, the Moon passes in
front of the Sun, blocking the brilliant solar disk and revealing the corona. Such
displays are possible because, by cosmic coincidence, the Sun is 400 times
­farther from Earth than the Moon and 400 times bigger than the Moon. Don Folz
Lunar eclipses
Annual showers
Name Peak date
Quadrantids
Jan. 3
LyridsApril 22
Eta AquaridsMay 6
PerseidsAug. 12
OrionidsOct. 21
LeonidsNov. 17
Geminids
Dec. 14
Meteors storm from the sky at speeds of up to 44 miles per second. Friction with Earth’s atmosphere heats the tiny dust particles until they flare
into incandescence. John Chumack
A ruddy Moon signals a total lunar eclipse, when the Moon dips completely
into Earth’s shadow. Earth’s atmosphere acts like a filter, scattering out blue light
and bending red light into the shadow, producing the striking color. Jason Ware