[Borodog]

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Binary stars orbit each other around a common center of mass. Is it possible that the combined mass of a binary star system be such that an event horizon is created?

Stu

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We don't know yet. If it can happen at all, it would involve the collision of two neutron stars. People (Google Doug Swesty) are simulating such collisions, but it is unclear yet whether such a collision can form an event horizon, or just a larger neutron star. It seems at first blush that a larger neutron star is not allowed by the Chandrasekhar limit, but because of the tremendous angular momentum incolved (the neutron stars can be orbiting at of order 60,000 RPM during the collision), a black hole might not form.

In any event, these sorts of collisions, are almost certainly responsible for one class of the most luminous events in the universe, gamma-ray bursts (the majority of them though are due to core-collapse of massive stars into black holes).