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.