What would happen if there was a collision between two white dwarfs?
What would happen if there was a collision between 2 white dwarfs (or any other super compact objects), especially if their combined mass would not create a black hole? Would the two be destroyed? If they are destroyed, what would be left?
Your question sparked quite a discussion among the graduate students in the lunch room today! The reason is because white dwarfs have degenerate, crystalline material in them, which may or may not behave like you would expect "normal" matter to. No one could come up with an answer, but it was agreed that it would be difficult to speculate what would happen without detailed simulations. It appears that this has been done by a group affiliated with the Armagh Observatory.
It appears as though the merger of two white dwarfs does not produce a black hole or a supernova, but a larger star that has a high concentration of elements heavier than hydrogen in it (we think that white dwarfs themselves are made mostly of helium, carbon and oxygen). After an initial explosion, the matter is strewn into a disk whose size depends on the initial angular momentum of the system, and then collapses into a star. Keep in mind, however, that these stars are not like the ones we usually see in the sky; they do not burn hydrogen to helium to counteract gravity but rather shine because of more complex, perhaps poorly understood mechanisms.
A final caveat: the authors admit that their simulation has left out some fundamental physics - the details have yet to be worked out for white dwarfs of different masses and compositions. Nonetheless, it provides the first indication of what might come of the collision of two white dwarfs.
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