MadSci Network: Astronomy

speed of milky way galaxy

Area: Astronomy
Posted By: Stephen Murray, Post-doc/Fellow, Astrophysics, University of California, Berkeley
Date: Tue Sep 17 19:24:43 1996
Message ID: 842373364.As

Dear Matt,

Unfortunately, there isn't one simple answer to either of your questions. Let's look at your second question first. The Milky Way Galaxy doesn't have a sharp edge that we can easily see. Instead, it just gradually fades as you move away from the center. There might be an outer edge at a radius of around 100 thousand light years, about three times further from the center of the Galaxy than we are.

The Milky Way Galaxy may actually be even larger than that. The number that I gave above is uncertain, because the Galaxy is so sparsely populated at large radii that it's hard to find anything out there. We do see some large star clusters, however, at distances of up to about 300 thousand light years. If they are part of the Galaxy, then the Milky Way must be at least that large.

There also isn't just one time of revolution associated with the Milky Way. Each star in the Galaxy follows its own orbit around the center, and each has its own orbital period. As a rule, stars further out take longer to orbit the center.

Let's use the Sun as an example. It's about 28 thousand light years from the center of the Galaxy. Remember that a light year is the distance that a ray of light can travel in one year, or about 9,460,000,000,000 kilometers. The Sun is orbiting around the center of the Milky Way Galaxy at a speed of 220 kilometers per second. So, the length of time that it takes the Sun to orbit around the Galaxy (I guess that you could call it our "Galactic Year") is about 240 million years. The Sun has orbited around the Galaxy 18 times since it formed 4.5 billion years ago.

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