I don't mean when stars in a binary system spiral in and fuse together. I mean when they undergo a fast head on collision in an elliptical galaxy. I assume it's much rarer for stars to undergo that type of collision in a spiral galaxy than in an elliptical galaxy because in a spiral galaxy, every star is pretty much moving at velocity that depends on its position in the galaxy. Has anyone observed a collision of that sort? Do the stars explode into a ring nebula, that is a nebula that's expanding too fast for its gravitational attraction to itself to pull it into a crab nebula, because they collide at such a high speed that gravitational attraction is not enough to make them combine into a larger star?

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    $\begingroup$ How frequently do you think two stars travel towards each other in a straight line with no angular momentum around their centre of gravity? $\endgroup$ – Rob Jeffries Nov 30 '17 at 19:53
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    $\begingroup$ I bet this is really pretty rare, but when it happens it must be pretty cool to watch. $\endgroup$ – zeta-band Dec 1 '17 at 0:11
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    $\begingroup$ Blue straggler stars are often explained as the result of stellar mergers. Stellar mergers are fairly common in dense clusters: en.wikipedia.org/wiki/Stellar_collision $\endgroup$ – Anders Sandberg Dec 1 '17 at 2:46

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