A galaxy is a
collection of dust, gas, and stars. Galaxies were formed soon after the birth of the
universe and has been held together by its gravity ever since then. A galaxy is constantly
traveling through space.
Galaxies vary in
size. Some of the smaller galaxies contain only a few million stars, but the biggest
galaxies may contain a trillion stars. They come in a wide variety of shapes too. Some
galaxies look like irregular globs of stars while others look like footballs, basketballs,
or tight spirals.
Our galaxy, the
Milky Way, is a spiral galaxy that is 100,000 light-years across and contains about
200,000,000,000 stars, including the Sun. The Sun is in one of its spiral arms. The galaxy
nearest to the Milky Way, the Sagittarius dwarf galaxy, is about 60,000 light-years from
Earth but cannot be seen without a telescope. The reason it can't be seen without a
telescope is because it is behind the central bulge of the Milky Way. In fact, it is
slowly being swallowed by the Milky Way's center. Our solar system is about 30,000
light-years from the Milky Way's center, in a spiral limb known as the Orion Arm. If you
view the Milky Way from its side, its center becomes clearly visible.
Some galaxies
have nicknames because they look like certain things that you can find on Earth, like a
sombrero. The Sombrero Galaxy is one of the many galaxies named for their looks. Others
include the Whirlpool, the Spindle, and the Antennae. Edwin Hubble was the first
astronomer to classify galaxies based of their appearance (see Types Of Galaxies). Today,
a newly discovered galaxy is named by, or even for, the person who discovered it. Other
galaxies, such as NGC 1365, are known by letters and numbers only.
Sometimes, two
galaxies have a head-on collision. The Hubble Space Telescope took a picture of one of
these collisions in 1997 when it took a picture showing the collision of two galaxies
known as the Antennae. The accident wasn't only destructive: As the two galaxies rammed
into each other, clouds of dust and gas inside them merged to form new stars. The Milky
Way is expected to crash into the Andromeda Galaxy in a few million years.
Some people
believe that a quasar is the center of a young but incredibly distant galaxy. Quasars
shine with the energy of 1,000 galaxies. The are the most energetic objects in the
universe. Some people believe that a quasar is fueled by massive amounts of matter
disappearing into a black hole. The brightest quasar seen from Earth is 3C273, which is
1,800,000,000 light-years away from Earth.