DK - One Million Things Series - A Visual Encyclopedia

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GALAXIES A galaxy is a vast group of stars held together by gravity—it is estimated that there are 100–125 billion in the universe. They are not scattered randomly but exist in clusters, vast distances apart. All the galaxies together take up just two millionths of space. 1 SIZE Galaxies are huge. The largest are more than a million light-years across (one light-year is the distance that light travels in a year). The smallest, called dwarf galaxies, are a few thousand light-years wide. Andromeda measures 250,000 light-years from side to side. *Naresh Jariwala* 2 SHAPE A single galaxy is made of billions or trillions of stars arranged in one of four basic shapes: spiral, barred spiral, elliptical, or irregular. Spirals and barred spirals are disk-shaped with arms of stars. In a spiral, such as Andromeda, the arms wind out from a central bulge, while in a barred spiral, they flow from the ends of a central bar of stars. Elliptical galaxies are ball-shaped. Irregular galaxies have no clear shape. 3 ORBITING STARS Galaxies do not behave like a solid object. Each star follows its own orbit around the center of the galaxy. Stars in a spiral galaxy typically take a few hundred million years to make an orbit. Those farther away take longer than those closest to the core. 4 SPIRAL ARMS Stars exist throughout a spiral galaxy’s disk. The arms stand out because they are full of very bright 7 young stars. 5 CORE The core of a spiral galaxy typically consists of old red and yellow stars, with a supermassive black hole in its center. Andromeda’s black hole is as massive as 30 million Suns.  ANDROMEDA GALAXY 6 DUST LANES Andromeda is one of the closest galaxies to our own, Dense clouds and lanes of dust within the the Milky Way. It is a spiral galaxy 2.9 million light-years away galaxy’s disk hide stars from view. from us—the most distant object that can be seen by the naked eye from Earth. 7 DWARF GALAXY M110 is one of the dwarf elliptical galaxies that orbit Andromeda. It is held in its orbit by Andromeda’s gravity. 131

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