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The Question

(Submitted September 15, 2006)

What exactly determines the size of a galaxy?

The Answer

Galaxy formation is a very active area of scientific research today and the exact means by which they form and hence their size is determined is not exactly known. The generally accepted view of the process by which a galaxy forms is that dust and gas come together within a region, are gravitationally attracted and coalesce into stars and other celestial bodies inside the galaxy. The amount of material and the angular momentum that material has (that is to say how much spinning movement the particles of dust and gas have) are believed to be the main contributers to the ultimate size of the galaxy.

The story doesn't quite end here however. In recent years there has been observations made about the mergers of galaxies whereby an even larger galaxy can be formed as material from one galaxy is added to another. Whether or not there may be other factors or events in the formation or evolution of a galaxy that can contribute to its size is still being studied.

Some links you might find useful that discuss galaxy formation and evolution:

Hope this helps
Jason and Koji
for "Ask an Astrophysicist"

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