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The Question

(Submitted April 16, 2010)

Why do the galexies posses different shapes such as elliptical, spherical, spiral, etc?

The Answer

This is a very interesting question, and still an active area of research. We do think we have a pretty good understanding, at least at a basic level, but there's much more to learn in detail.

The main types of galaxies are disk (or spiral) galaxies, elliptical galaxies, and irregular galaxies. "Irregular" covers pretty much everything that isn't a spiral or nice ellipse. The short answer is that galaxies form originally as a spiral disk. As nearby galaxies interact and collide, they eventually end up as an elliptical shape. So the elliptical galaxies are all much older structures.

Irregular ones are probably the result of galaxies being disrupted by gravitational interactions with other galaxies.

There is a nice article about galaxies on wikipedia, which has a lot more detail: http://en.wikipedia.org/wiki/Galaxy_formation_and_evolution

We hope that helps.

-Kevin and Hans,
for "Ask an Astrophysicist"

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