Stellar Black Holes

The Milky Way contains about a hundred million black holes, which were formed by the collapse of very massive stars. Each of these stellar black holes weighs about 10 times as much as our Sun. A very few of these black holes are closely orbited by an ordinary star that is slowly bleeding matter onto the black hole. As this gas falls toward the black hole, it is heated by strong gravity and friction. Near the black hole, the gas reaches a typical temperature of 10 million degrees. These black-hole X-ray sources are easily observed throughout the Milky Way, and in nearby galaxies as well, using orbiting X-ray observatories.

Remarkably, any black hole is fully described by just two numbers that specify its mass and how fast it is spinning. We know of nothing else this simple except for an elementary particle such as an electron. Scientists at CfA have measured both of these fundamental parameters -- mass and spin -- for over a dozen stellar black holes, while studying all aspects of these black holes and their host systems.

Despite their ubiquity in the universe, black holes remain deeply mysterious. We require a theory of quantum gravity that will unite Einstein's 1916 theory of relativity with the 1926 theory of quantum mechanics. No such theory exists despite decades of theoretical effort by string theorists and others. The establishment of a theory of quantum gravity will be a crowning achievement of physics on par with the accomplishments of Newton, Einstein and other giants.