How far does a galaxy extend? (Intermediate)

I have heard many theories about where the entire galaxy starts, and ends. Does anyone know where it starts and ends?

Recently, there have been many debates about how far out a galaxy actually extends to. The problem is that when we look at a galaxy, what we see is not all that we get! A (spiral) galaxy is composed of a bulge in the center and a flattened disk surrounding the bulge - these are the components of the galaxy that we can see through telescopes. But that's not all a galaxy is composed of: there are globs of dark matter (matter that we can't detect with ordinary observing techniques) that we call the "halo" of the galaxy. You can see a good picture illustrating the structure of a spiral galaxy below, where the left image shows the view from above and the right image shows the view from the side/edge.


Now how do we know that all this mass is there if we can't directly see it? When we look at the velocities of the stars around a galaxy, we find that the velocities of the stars are much faster than predicted. The most likely explanation for this phenomenon is that there is much more mass there than we see - we don't see 50-90% of the mass that's actually there!

So that's what all the fuss is about. Right now, astronomers don't really agree on how far this "dark halo" goes out to - it seems to go on and on and on. So that's why you hear about a lot of debates and theories about where a galaxy ends.

This page was last updated on August 2, 2016

About the Author

Lisa Wei

Lisa graduated from Cornell in May 2004 with a Bachelors in Astronomy. While here she studied frost streaks on Mars and the substructures in the Virgo Cluster. She is now a graduate student in Astronomy at the University of Maryland.

Here is a list of some questions answered by Lisa (2003-2004).