Hi

Scientific American had an article discussing all the different results of stellar collisions. Within the Galactic disk the odds of any two stars colliding is very low - on average once every hundred aeons (aeon=1 billion years.) But within globular clusters the odds are very high and many "weird" stars have been observed that are the remains of stellar collisions.

Main sequence stars, when they collide, merge to produce a higher mass star that burns brighter than the original components.

A main sequence and a white dwarf produce a supernova because of mass accretion onto the dwarf causing it to exceed the Chandrasekhar mass limit and collapse into a neutron star - producing a supernova detonation wave.

Two white dwarfs, two neutron stars or dwarf-neutron star combinations will produce gamma-ray burst producing "hypernova". The end result is probably a black-hole.

A red-giant and a main sequence star will strip the red giant and reveal the helium or carbon burning core of the red giant, dispersing the diffuse outer envelope. The main sequence star might gain a bit of mass from the envelope, but would be otherwise unaffected - unless it collides with the dense core. Then a supernova is very likely.

Two brown-dwarfs colliding might produce a new low-mass star, if they're heavy enough. As the average fraction of heavy elements increases in the Interstellar Medium - what stars are ultimately made from - then the fusion ignition mass for a red-dwarf actually decreases. Currently it's about 0.08 solar masses - 80 times Jupiter's mass - but eventually it will drop to about half that. Such "stars" will glow feebly compared to the Sun, barely reaching the boiling point of water in temperature, but will sustain a fusion burn for many trillions of years.

Another possibility, which we don't yet see, involves the remnants of low mass stars colliding. For example below about 0.3 solar masses a star can't burn helium after the Main Sequence ends, so the star dies as a helium dwarf. If two collide with enough energy and mass then they will burn helium. A bit heavier and they will burn carbon. In either case they will burn brighter and hotter than would be expected for their mass - about 10 times and 1000 times brighter than the Sun, respectively.

Another side effect of stellar collision is the formation of a new set of planets from any ejected mass. Some very oddly composed planets will be found around such stars - imagine gas giants composed of carbon monoxide, with diamond cores!

Adam