This false-color image shows the near-infrared sky as seen by the COBE satellite. The dominant source of light at this wavelength is stars in our galaxy. So, you end up with an image that shows the thin disk and the central bulge of the Milky Way Galaxy. Our Sun lies in the plane of the disk, 28,000 light years from the center which is why the Milky Way disk appears edge-on to us.
Click on image for full size

How far across is the Milky Way?

At a given time how can one locate the center of our galaxy as we orbit the Sun and the Sun orbits the galaxy? Is our solar system moving away from the center of the galaxy? How many years does it take the Sun to orbit the center of the galaxy? Specifically, where in the universe can our solar system be found?

The Milky Way galaxy is the home of the Sun and our solar system. There are 200 billion other stars in the Milky Way galaxy too. Our galaxy is a spiral galaxy, with a bulged center and arms that start in the center and form a flat pinwheel shape. The galaxy is about 90,000 light-years across. The Sun is located about two-thirds of the way out from the center in the Orion Arm.

The Sun (and our solar system) is revolving around the center of the Galaxy at a speed of half a million miles per hour, but it still takes 200 million years for it to go around once. As far as we can tell, the Sun and the solar system are not moving away from the center of the galaxy. The Sun has made less than 25 trips around the galaxy in its lifetime.

The center of our galaxy is located about 28,000 light-years away, beyond the constellation Sagittarius (actually just beyond the border of Sagittarius and Scorpio). So, if you can locate these two constellations in the sky, you'll be looking toward the center of our galaxy!

The Milky Way is part of a set of galaxies known as the Local Group, which includes several dozen different galaxies within 3 million light-years. Only one of these, the Andromeda galaxy, is close to the size of the Milky Way. This Local Group is part of a supercluster, known as the Virgo supercluster, which has at least 5,000 member galaxies and is roughly 100 million light-years across. Beyond this level of organization, not much is known about our position in the universe.

Last modified July 18, 2001 by Jennifer Bergman.

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