|What is a galaxy?
Galaxys are collections of stars. We are in the
Milky Way Galaxy. Many scientists believe there
are black holes at the centers of galaxies, and
that these objects hold the galaxies together.
This is controversial, so you might want to do
some research on what scientists currently believe.
A galaxy is a collection of about 100 billion
(that's 10^11, or a 1 with 11 zeros) stars, held
together by gravity.
A galaxy is an enormous swarm of a million to a
trillion stars. Our galaxy is medium-large, with
100 billion stars. There are about 100 billion
galaxies in the known universe, so there are
literally as many suns as there are sand grains on
all the beaches of Earth. The nearest large galaxy
(Andromeda) is 2 million light years away. When
you look out into space, you are also looking back
in time. You actually see Andromeda as it was 2
million years ago, because it took that long for
the light to reach Earth.
Stars like to clump together. Each clump is a
galaxy. It turns out that even galaxies clump
together. And that clumps of galaxies, clump
together. In fact, there is a very large scale
structure to the universe that is definitely not
well understood at all.
What holds galaxies
together? Gravity. All of the stars of a galaxy
are all attracted to each other. This attraction
is weak because the stars are very far apart, but
it is enough to keep the galaxies together. There
is also a large amount of matter that has never
been seen (called "dark matter") that helps keep
A galaxy is collection of stars, gas and apparent
dark matter (which is detectable by its influence
on the orbits of stars, but in otherwise not
understood). Galaxies are bound in the sense that
the stars cannot leave the galaxy for the most
part, and orbit the center of mass of the galaxy.
(The sun orbits the Milky Way in about 200 Million
years). Nearly all stars in the universe are bound
into some galaxy.
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