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Brief Galactic Overview
The Milky Way is a galaxy that contains at least 200
billion stars. It is an average-size galaxy of the barred spiral class.
This means that there is a central bulge, with arms, which extend for
many light-years*.
However, the term "barred" means that its central bulge is elongated;
in the Milky Way, this elongation is at a ratio of 2:3, meaning that it
is about one and a half times as long as it is wide.
The Milky Way has five distinct arms protruding from the
central bulge. Their names are as follows: the Cygnus Arm, Centarus Arm, Sagittarius
Arm, Orion Arm, and Perseus Arm. Our solar system lies
in the Orion Arm.
The Milky Way belongs to a group of galaxies that is known
as the Local Group. The Local Group contains about 30 galaxies, two of which
are the Andromeda Galaxy and the Triangulum Galaxy. The Local Group is 6 million
light-years (3.5x1019 miles) in diameter.
In turn, the Local Group belongs to a much larger group
of galaxies known as the Virgo Supercluster. The Milky Way lies on the outer
edge of the supercluster. The Virgo Supercluster is about 100 million light-years
(6x1020 miles) in diameter.
The Milky Way is 100,000 light-years (6x1017 miles)
in diameter. The Milky Way takes approximately 240 million years to rotate,
clockwise, even though the arm that we are in is whipping around the center
at about 465,000 miles an hour. At that speed, a space ship from Earth
would only need a half an hour to get to the moon!
The known universe consists of about 200 to 300 billion
galaxies.
*One light-year is about 5,680,368,000,000
miles; see the glossary.
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