One Million Things - 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.

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.

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