Figure: Colliding galaxies. In 3 billion years, the Milky Way and the Andromeda Galaxy will end a 12 billion-year dance, merging together to form an elliptical galaxy. Astronomers have seen many examples of these death dances play themselves out in the cosmos such as this pair of spirals NGC 2207 - IC 2163. (Credit: Hubble Heritage Team, NASA/AURA/STScI)
Contrary to what you might think, when galaxies collide it is very unlikely that even a single pair of stars would collide within the two galaxies. As the galaxies approach one another, the changing gravitational forces distort them, pulling spiral arms containing billions of stars out of the galaxies and sending the stars into completely different orbits. The collisions take over 100 million years, and the outcomes depend on the sizes of the two galaxies and the paths they take. Small galaxies colliding with big ones are usually eaten with little fuss. A head=on collision between galaxies that are roughly the same size, but where one is an elliptical and the other is a spiral-type, can transform the spiral galaxy into a rare 'ring galaxy'. If the collision is off-center, the galaxies get completely shredded by their mutual gravitational fields. The end product may be a single new galaxy, or the two galaxies may continue on their ways, but with their new shapes very different from what they looked like before.
 |
This answer was updated in 2011.
See my books:
The Astronomy Cafe (1998) and
Back to the Astronomy Cafe (2003) for more FAQs in printed form. Author: Dr. Sten Odenwald, Copyright 2011
Return to Ask the Astronomer |