Expert: Courtney Seligman - 6/26/2009

Question
QUESTION: seen one of those photographs from the hubble telescope of two galaxies on a collision course and was wondering would it be possible for said galaxies to survive given the enormous distances involved,could they pass through relatively unscathed,do the stars attract or repel each other taking into account time and speed of approach

ANSWER: The stars attract each other, but by and large, not as individual objects. Each star in each galaxy is affected by the gravity of all the stars in both galaxies (more by closer ones and less by more distant ones, but still, by the overall distribution of stars, rather than the individual stars). The distances between the stars are measured in trillions or tens of trillions of miles, while the individual stars are usually only a few hundred thousand miles across. This makes the stars hundreds of millions of times smaller than the distance between them, so collisions of individual stars are virtually impossible. Their usual motions are disrupted by the gravity of the other galaxy, causing some of them to be thrown out into 'tails' and other structures, but in the end, most of them end up back in their own galaxy, and most of the rest are scattered through intergalactic space.

Depending upon the relative mass of the two galaxies, there is a chance that they may merge, and in rich clusters of galaxies, over many encounters between galaxies, there is a strong tendency for the galaxies to merge and grow. But alhough the galaxies will 'survive', albeit in slightly altered forms, and the individual stars will survive, there can be some spectacular interactions which involve the interstellar gases, which may temporarily extinguish all life in one or both galaxies. (For more details, you may find it useful and interesting to refer to my web page on M81 and M82: A Cosmic Train Wreck, at http://cseligman.com/text/galaxies/m81m82.htm)

---------- FOLLOW-UP ----------

QUESTION: so following on from the second part of your reply in your opinion what would be our chances of survival if our own milky way was merging with a similar galaxy at the beginning of our evolution,would we be looking out at a larger more exciting milky way,would we be aware what was happening without disruption to everyday life

Answer
I don't know about at the beginning of our evolution, but if it were to happen now, the odds are that within a few million years of the start of the collision, most if not all life in both galaxies would be extinguished by the huge flood of radiation produced by bright new stars formed from the collision of clouds of gas and dust in the two galaxies. Of course, those stars only last a little while, so within a few hundred million years, as the last episode of violent star formation died out, life could begin anew.