Re: what happens when stars collide?

Dear Mark,

Apologies for this answer taking so long; I was having so much fun researching it that I almost forgot that I had to write the answer up! Anyway, my knee-jerk response to your question was "nobody knows: stars don't go that fast". Most of the stellar collision modeling I'm familiar with is for the case of stars in globular clusters, where collision speeds are something like 20 km/s, not the 3000 or 30000 km/s you were asking about. To figure out the details of what happens you need complicated computer simulations with a lot of physics (gravity, gas dynamics, etc) included. Fully realistic simulations are still in the future, and this is an area of active research. The gist of what happens is that the stars merge and pretty quickly settle down to acting mostly like a regular main sequence star with more mass.

OK, so what about the higher-velocity collisions you were asking about? We could call these `relativistic' collisions, since relativity starts to have important effects once you get above about 1% of the speed of light. Is there any place where stars are actually going that fast? Stars in galaxies have orbital speeds of a few hundred km/s, and because of the large distances between them, are unlikely to smack into each other anyway. But stars can go in faster orbits if they are near the massive black holes in the centers of galaxies, and the galaxy centers are also crowded neighborhoods -- good for collisions.

Here is a nice webpage about the state of the art in stellar collision modelling. Linked to this is a PhD thesis which discusses collisions between main sequence stars in galactic nuclei -- just what you asked about. From reading this quickly, I picked up a couple of useful points: (1) collisions between main sequence stars, even in the crowded regions in galactic centers, are pretty unlikely. (2) the results of the collisions depend on the "impact parameter", that is, whether the collision is exactly head-on or a "glancing blow", and also on the mass ratio of the stars. For equal-mass stars, head-on collisions (small impact parameter) result in the disruption of both stars. Larger impact parameters result in both stars surviving (although they may lose a lot of mass in the collision), and lower velocities can result in the stars merging.

If you want to know more about this topic, check out the above webpages, and also the proceedings of a conference on "Stellar Collisions, Mergers and their Consequences", edited by M. Shara and published by the Astronomical Society of the Pacific. The library at a large research university should have it.

Pauline