How far does a galaxy extend?
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. This page about the Milky Way has a good picture illustrating the structure of a spiral galaxy.
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.
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