Contact Binary

A contact binary star occurs when two stars are close enough that their outer layers are in contact with each other. Often this can occur when one member of a close binary system enters a red giant stage and swells to the point where it is in contact with the companion star. Such systems can create novae, and perhaps supernovae. But recently we’ve discovered¹ a contact binary consisting of two O-type stars.

Currently the two stars orbit each other about once a day, as seen in the artist video above. But their eventual fate is still unknown. Stars of this size typically have a cosmically short lifetime of about 5 million years. It’s possible that they merge into a single, fast-rotating star. Such a supermassive could end its life with a long duration gamma ray burst. But it’s also possible that the stars remain a stable binary throughout their lifetime. If that’s the case, then each would end their lives as supernovae, and produce a black hole. This could create a close black hole binary.

Such a black hole binary would be a boon for astronomers trying to detect gravitational waves, since close binary black holes would create the strongest consistent signal.