OUR SOLAR SYSTEM

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90°

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240°

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300°330°

3,000

20,000

10,000

30,000

40,000

50,000 light-years

6,000 light-years

Direction of rotation

M2-9

Kappa CrucisNGC 4755

NGC 6397

Owl M97

M71

M10

SN 1572Tycho’s Supernova

IC 1848Soul Nebula

M12

Ring M57

M4

Wild DuckM11

Trifid M20

Lagoon M8Omega M17

Eagle M16

ButterflyNGC 6302

DumbbellM27

North AmericaNGC 7000

AntaresM7

NGC 7293

Omega CentauriNGC 5139

M80

Palomar I

Orion M42Cone NGC 2264

Rosette NGC 2237NGC 7027

Crab M1

Vela

CarinaNGC 3372 Keyhole NGC 3324

NGC 5272

M92

OU T E R A R

M

PE

RS

EU

S

AR M

SA

GI T T A R I U S A R M

O R I O N S P U R

NO

RM

A

AR

M

S C U T U M- C

EN

T

AU

RU

S

AR

M

NEAR

3 K

PC

AR

M

M14

FAR

3 K

PC

ARM

SA

GI

TT

AR

IU

S

AR

M

Globular star cluster

Nebula

Interstellar gas and dust

Molecular cloud

Galactic bulge or center (older star region)

Reference numbers for galaxies, nebulae, and star clustersIC (Index Catalogue)M (Messier)NGC (New General Catalogue)

Coordinate system centeredon galactic center

Younger star region

EARTH

SUNEclipticplane

Galacticplane

Galacticcenter

Halo

Disk

Globularclusters

Dark matter

Halo

Disk

Globularclusters

Dark matter

Earth’s location on the outskirts of the Milky Way means we view the great spiral galaxy from the side, and its 200 billion or more stars look like a glowing band across the sky. This is the galactic equator—the plane on which the solar system orbits the center of the Milky Way every 220 million years. If the pace sounds slow, Earth careens on this cosmic merry-go-round at about 917,000 kilome-ters an hour (570,000 mph). The galaxy has three distinct parts: At the core is a bright, bar-shaped bulge of yellow and red stars. From the center, several spiral arms sweep out to form a disk that contains younger, blue stars, as well as glowing regions of star birth. Earth resides here, 25,000 light-years from the center, and most of the familiar celestial spectacles—supernovae, planetary nebulae, star birth regions—lie relatively nearby. Finally, surrounding the galactic disk is a great halo of dark matter—invisible, but holding most of the galaxy’s mass.

Earth’s orbit around the sun lies at a severe angle to the galactic plane.

Far beyond the galactic disk, yet drawn by its gravity, lone stars and globular clusters wander the galaxy’s halo. Regions of dark matter—unseen but felt through its gravita-tional effects—extend beyond that.

Celestial Wings take FlightDelicate “wings” of superhot gas that once formed the outer layers of a star five times as massive as the sun span trillions of miles of space in the beautiful Butterfly Nebula, also known as the Bug Nebula. The gas is swept outwards at nearly 965,000 kilometers an hour (600,000 mph) by wind blowing from the dy-ing star at the center of this planetary nebula about 2,000 light-years from Earth.

heart oF the Milky Way The mysterious core of our galaxy is revealed in greater detail than ever before in this dazzling mosaic of hundreds of thousands of individual images from the infrared Spitzer Space Telescope. New stars are coalescing in bright regions of yellow and red, while the green haze is gas thrown off from star birth. Blue pinpricks throughout the image are the Milky Way’s older stars. What the image can’t show is Sagittarius A*, a supermassive black hole whose gravity dominates the Milky Way’s core.

stellar nursery in the Carina nebulaA young star is being born in the center of a pillar of dust and gas in the Carina Nebula, an intensely

active region of stellar formation 7,500 light-years from Earth. The nebula’s giant stars range

up to a hundred times more massive than the sun. The biggest of them, Eta Carinae, will soon ex-

plode in a supernova, blasting matter into space and providing seed material for new stars.

CroWded stellar neighborhoodMaximum stars crowd a minimum of space

in the Omega Centauri globular cluster. Upgrades to the Hubble Space Telescope in 2009 revealed previously unseen color detail, providing clues to the life cycles of

the cluster’s ten million stars: Those at midlife are yellow-white, while older ones

are orange, and red giants are nearing the end. Separated by just one-third of a

light-year on average, collisions can occur. By contrast, the sun’s closest neighbor is

over four light-years distant. Though Omega Centauri is unusually large, the Milky Way’s

halo contains many such globular clusters—dense spherical star clouds that orbit the

galactic center like miniature galaxies.

brahe’s “stella noVa” “On the 11th day of November in the evening after sunset . . . I noticed that a new and unusual star, surpassing the other stars in brilliancy, was shining almost directly above my head.” So wrote Danish astronomer Tycho Brahe of the supernova that burned briefly in 1572.

This star explosion some 7,500 light-years from Earth disproved the traditional notion that the heavens were unchanging, and revealed them as highly dynamic. The faded remnant is now a colorful planetary nebula, shown here in an image combin-ing infrared and x-ray orbital telescope views with ground-based optical imagery.

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the milky way

Earth’s Galactic Home

118 Milky Way FFR.indd 118-119 6/30/10 2:02 PM