Extreme Stellar Black Hole

Credit: NASA/CXC/M. Weiss

An artist's representation of M33 X-7: a binary system in the nearby galaxy M33, containing a massive blue star feeding material to a black hole surrounded by a small accretion disk.

Stellar black holes form when stars with masses around 20 times that of the sun collapse under the weight of their own gravity at the ends of their lives. Most stellar black holes weigh in at around 10 solar masses when the smoke blows away.

The black hole in M33 X-7 located 2.7 million light-years from Earth, is also the most distant stellar black hole ever observed.

The findings, detailed in the Oct. 17 issue of the journal Nature, could help improve formation models of "binary" systems containing a black hole and a star. It could also help explain one of the brightest star explosions ever observed.

The blackhole orbits a companion star in the spiral galaxy Messier 33. The companion star of M33 X-7 passes directly in front of the black hole as seen from Earth once every three days, completely eclipsing its X-ray emissions. It is the only known binary system in which this occurs, and it was this unusual arrangement that allowed astronomers to calculate the pair's masses very precisely.

The tight orbits of the black hole and star suggests the system underwent a violent stage of star evolution called the common-envelope phase, in which a dying star swells so much it sucks the companion inside its gas envelope.

The result is either a merger between the two stars or the formation of a tight binary in which one star is stripped of its outer layers. The latter scenario may be what happens in the case of M33 X-7, and the stripped star explodes as a supernova before imploding to form a black hole.

However, something unusual must have happened to M33 X-7 during this phase to create such a massive black hole. The black hole must have lost a large amount of mass for the two objects to be so close, but on the other hand, it must have retained enough mass to form such a heavy black hole.

M33 X-7 might thus provide both the upper and lower limits on the amount of mass loss and orbital tightening that can occur in the common envelope.

While the estimated 16 solar masses of the black hole in M33 X-7 is hefty for a stellar black hole, it is miniscule compared with the black holes thought to lie in the heart of many large galaxies.

Such "supermassive" black holes have masses millions to billions times that of our sun, but they are thought to form by mechanisms different from the stellar variety.

M33 X-7: Heaviest Stellar Black Hole Discovered in Nearby Galaxy from Chandra
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