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Black Holes Encyclopedia

M33 X-7

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Stellar mass


In M33, the Triangulum galaxy

Finder Chart


2.7 million light-years (0.84 megaparsecs)


About 15.7 times the mass of the Sun


Diameter roughly 60 miles (95 km)

Discovery Methods


M33 X-7

A binary star system that once consisted of two of the brightest stars in our region of the universe soon will consist of two of the darkest: a pair of massive black holes.

The system is aleady half-way to that fate. It consists of a supergiant star that is about 70 times as massive as the Sun, and a black hole about 16 times as massive as the Sun. At its discovery in late 2007, it was the most massive "stellar" black hole yet found.

The system is known as M33 X-7 -- the seventh X-ray source discovered in M33, the spiral Triangulum galaxy. It is about 2.7 million light-years from Earth.

Astronomers discovered the system because it glows brightly in X-rays. The black hole's powerful gravity pulls gas from the surface of the supergiant star. The gas spirals around the black hole, forming a superhot accretion disk, which produces enormous amounts of X-rays and other forms of energy.

Astronomers measured the masses of the two stars thanks to a lucky coincidence. The system aligns so that the two stars periodically eclipse each other as seen from Earth. The eclipses provide precise timing of the orbit, as well as the masses of the two stars.

The black hole in M33 X-7 formed when a supergiant star ended its life. As the star reached its final stages it could no longer produce energy in its core, so the core collapsed to form the black hole. The outer layers exploded as a supernova. The same fate awaits the other star in the system, probably within a few hundred thousand years. (In fact, the star probably exploded as a supernova long ago, but it is so far away that the light of the explosion has not yet reached Earth.)

The two stars are so close together that they orbit each other once every 3.5 days. Originally, though, they must have been much farther apart. Over time, they moved closer together.

When the second star collapses and explodes, the system could undergo one of several fates. The explosion could "kick" the supergiant star away from the current black hole at millions of miles an hour, so the stars would go their separate ways. Or, with a smaller combined mass, the two stars could remain gravitationally bound but it in a larger orbit. Or they could be pushed closer together, so they eventually merge to form an even bigger black hole. In any case, the system is guaranteed to produce a lot of fireworks in the next few million years.


Hefty Black Hole Discovered in Neighbor Galaxy (Chandra X-ray Center)
A black hole in a neighboring galaxy is the heftiest member of its class -- black holes that formed from the collapse of giant stars. It's about twice as massive as any other black hole of that type yet discovered. What's more, the star that gave birth to the black hole would have been one of the most massive in the universe, and its death could have been extraordinarily bright and violent.



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This document was last modified: January 24, 2011.


M33 X-7 Accretion Disk
Artist's Rendering

M33 X-7
Ground-Based Photo

Bright Glow
Space-Based Photo


No animations available for this black hole.