your guide to planets stars and galaxies - astronomy.com

500FACTS INSIDE!

Your guide toplanets, 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

Saturn

Mars

Mars’ ruddy appearance arises because the sand on the planet’s surface consists largely of iron oxides — rust. NASA/JPL/MSSS

Saturn’s rings consist of icy particles ranging in size from tiny motes to house-sized icebergs. NASA/The hubbLe

heriTAge TeAM (STSci/AurA)

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Mars boasts the largest vol-canoes 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.

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Earth 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 pos-sess 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.

Planets of thesolar system

2 Your guide to planets, stars, and galaxies

Planet Distance from Sun Orbital period Diameter Mass Density (Earth=1) (Earth=1) (Earth=1) (water=1)

Mercury 0.39 87.97 days 0.383 0.055 5.43Venus 0.72 224.70 days 0.949 0.815 5.24Earth 1.00 365.26 days 1.000 1.000 5.52Mars 1.52 686.98 days 0.532 0.107 3.93Ceres 2.77 4.60 years 0.075 0.0002 1.93Jupiter 5.20 11.86 years 11.209 317.832 1.33Saturn 9.58 29.46 years 9.449 95.159 0.69Uranus 19.20 84.01 years 4.007 14.536 1.27Neptune 30.05 164.79 years 3.883 17.147 1.64Pluto 39.48 247.68 years 0.187 0.002 1.75Note: Ceres and Pluto are officially considered to be dwarf planets.

Solar system planets

Mercury's high density means more than half of it must be made of the heavy elements iron and nickel. NASA/JPL/USGS

Venus

Pluto & Charon

Jupiter

Mercury Earth

Uranus Neptune

Nearly three-quarters of Earth’s surface is covered with water. It’s what makes our home world conducive to life. NASA

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

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

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

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

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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.

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Sunlight takes just eight min-utes to reach Earth but more than four hours to cross the void to Neptune and Pluto.

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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 small-

est 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 diam-eter of the dwarf planet — the largest ratio in the solar system.

The hefty moons of the gas giants garner most of the atten-tion. 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 ter-rain. 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 sys-tem. If their long, looping orbits bring them close to the Sun’s warmth, they shed gas and dust. The Sun then blows this mate-rial 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.

Small bodies of the solar system

Titan

Europa

Io

Titan’s hazy atmosphere glows as it scatters incom-ing sunlight. The atmosphere of Saturn’s moon is thicker than Earth’s and, like ours, contains mainly nitrogen. NASA/JPL/SSI

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

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


Moon Planet Distance from Orbital period Diameter Density planet (miles) (days) (miles) (water=1)

Moon Earth 238,900 27.32 2,160 3.3Io Jupiter 262,100 1.77 2,263 3.5Europa Jupiter 417,000 3.55 1,940 3.0Ganymede Jupiter 665,100 7.15 3,271 1.9Callisto Jupiter 1,169,900 16.69 2,994 1.8Enceladus Saturn 147,900 1.37 313 1.6Tethys Saturn 183,100 1.89 660 1.0Dione Saturn 234,500 2.74 698 1.5Rhea Saturn 327,500 4.52 949 1.2Titan Saturn 759,200 15.95 3,200 1.9Iapetus Saturn 2,212,600 79.33 913 1.1Ariel Uranus 118,600 2.52 719 1.7Umbriel Uranus 165,300 4.14 727 1.4Titania Uranus 271,100 8.71 980 1.7Oberon Uranus 362,600 13.46 946 1.6Triton Neptune 220,400 5.88 1,681 2.1Charon Pluto 12,200 6.39 753 1.9

Major moons

Io ErosHale-Bopp

Callisto

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

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

PhobosLike most small moons in the solar system, Mars’ Phobos measures just a few miles across and has an irregular shape. NaSa/JPl/MSSS

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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.

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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

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

Triton

Enceladus

Geyser-like plumes deposited the dark streaks seen on Nep-tune’s moon Triton. At a temperature of –390° F, Triton has the coldest surface known in the solar system. NaSa/JPl

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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.

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Main sequence solar-type star

Protostar Red giant Asymptotic-giant-branch star Protoplanetary nebula Planetary nebula White dwarf

All 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 collaps-ing — 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 gener-ate 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 to about eight times the Sun’s mass eventually puff off their outer layers and form glowing gas clouds known as planetary nebulae. The star itself settles down as a white dwarf.

More massive stars typically die in supernova explosions. Such explo-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.

Stars in our galaxy

The Sun

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

The life of a Sun-like star

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)F U N

Astronomers divide stars into seven main spectral classes. Generations of students have learned the sequence by using the first letters in the sentence: “Oh, Be A Fine Girl (or Guy), Kiss Me.”

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Main sequence solar-type star

Protostar Red giant Asymptotic-giant-branch star Protoplanetary nebula Planetary nebula White dwarf

Spectral Mass Temperature Main sequence Examples class (Sun=1) (Fahrenheit) radius (Sun=1)

O 20–120 greater than 55,000° 12–25 Zeta (ζ) Puppis

B 4–20 17,100°–55,000° 4–12 Rigel, Spica

A 2–4 12,300°–17,100° 1.5–4 Sirius, Vega

F 1.05–2 10,300°–12,300° 1.1–1.5 Canopus, Procyon

G 0.8–1.05 9,000°–10,300° 0.85–1.1 Sun, Capella

K 0.5–0.8 6700°–9000° 0.6–0.85 Aldebaran, Arcturus

M 0.08–0.5 3100°–6700° 0.1–0.6 Antares, Betelgeuse

Star characteristics

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 hydro-gen 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

Surface of the SunCat’s Eye Nebula

Cone Nebula

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

When the Sun dies in 5 billion years, it may resem-ble 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)

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

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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.

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Scutum-Centaurus Arm

Norma Arm

Central bar

Sagittarius ArmPerseus Arm

Orion

SpurSun

The Milky Way Galaxy Head 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 com-bined 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 mea-sures 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 mil-lion years to complete one circuit of the galactic hub.

The most obvious sights of the galaxy are stars. Astronomers esti-mate between 200 and 400 billion populate the Milky Way Galaxy (most are hidden from view or extremely faint, so a precise count isn’t possible).

Because the hottest, brightest stars 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 mil-lion 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.

Structure of the Milky Way

The Pleiades M3

Star formation in Cygnus

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Most naked-eye stars are massive and highly luminous ones that shine across great distances. But this gives a dis-torted view of the galaxy as a whole. In actuality, cool, dim, M-type stars make up about two-thirds of all stars in the Milky Way.

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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)

Open star clusters like the Pleiades contain dozens to hundreds of stars. These groups lie in our galaxy’s spi-ral 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 light-years across. S. KAfKA and K. hoNeyCutt,

INdIANA uNIverSIty/WIyN/NoAo/NSf

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)8 Your guide to planets, stars, and galaxies

Perseus ArmOrio

n Sp

ur

Structure of the Milky Way

M3

Star formation in Cygnus

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Much of the Milky Way Galaxy and its structure remain hid-den to earth bound observers because dust chokes the spi-ral arms. It’s like being in the woods and trying to discern the forest’s form.

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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.

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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

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A century ago, astrono mers 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.

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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.

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The 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 sev-eral 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 gal-axies 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.

Galaxiesin theuniverse

The Mice

NGC 4414

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)

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

M87A 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)

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

M84 & M86


NGC 68221,760 kly

Ursa Minor Dwarf225 kly

Draco Dwarf248 kly

Milky Way

SMC 189 kly

NGC 1852,020 kly

NGC 1472,460 kly

Andromeda Galaxy (M31)2,510 klyPinwheel Galaxy (M33)

2,770 kly

Leo I 818 kly

Leo II750 kly

SextansDwarf

293 kly

LMC 160 kly

The Local GroupThe 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

The Mice

M87

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If you examine the brightest galaxies, some 75 percent are spirals, 20 percent ellipticals, and 5 percent irregulars. Includ ing faint dwarfs skews the numbers to 30 percent spirals, 60 percent ellipticals, and 10 percent irregulars.

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Andromeda GalaxySlightly 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

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

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The Large and Small Magellanic Clouds (LMC and SMC) are satellite galax-ies to the Milky Way. They lie deep in the southern sky and were not seen by Europeans until Magellan’s around-the-world voyage.

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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.

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M84 & M86

How 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 origin some time ago. The far-ther 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 expan-sion 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 uni-verse 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 creat-ing 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 discov-ered the echo of the initial fireball, which has cooled to a few degrees above absolute zero.

An even more shocking discovery came at the end of the 20th century. By looking at supernova explosions 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 universe is not only queerer than we imagine, it is queerer than we can imagine.”

Probing the distantuniverse

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Galaxies do not spread evenly, but gather in clusters that themselves form vast filaments, leaving huge voids in between.

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Abell 1689Thousands of galaxies cluster together in Abell 1689. The concentration of luminous matter and dark matter (stuff we can’t see but which adds to grav-ity) creates a fun-house mirror of arcs and wisps. NASA/ESA/thE ACS SCiENCE tEAm

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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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Dark energy 72%

Atoms 5%Dark matter 23%

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Quasars — short for quasi-stellar radio sources — are the brightest objects in the universe. They radiate as much energy as an entire gal-axy from a volume no bigger than our solar system.

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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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Cosmic microwave background

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

The Great Attractor

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

Abell 1689

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?

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 eve-nings. 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.

You and theuniverse

Conjunctions

Solar eclipses

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

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

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

Meteor showers

Meteors storm from the sky at speeds of up to 44 miles per second. Fric-tion with Earth’s atmosphere heats the tiny dust particles until they flare into incandescence. John ChumaCk

annual showers

Name Peak dateQuadrantids Jan. 3lyrids april 22eta aquarids may 6Perseids aug. 12orionids oct. 21leonids nov. 17Geminids Dec. 14

your guide to planets stars and galaxies - astronomy.com

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