Stellar Black Holes
A dying star that has a mass greater than 20 times the mass of our Sun may produce a black hole at the end of its life. Once all the fuel in a star is exhausted, the thermonuclear forces that have held the star together
disappear and the core contracts due to the strong gravity. In the most massive stars this collapse culminates in a supernova, an explosion that sends the outer part of the star streaming into space. At the same time the
gravitational force in the star’s inner regions causes the core to collapse into a single point of infinite density — a black hole.
Stellar-mass black holes are often paired with a “companion star” in a binary system. Such a system is made up of two objects: an ordinary Sun-like star and a dense “compact object” that is either a neutron star or a black hole. The black hole’s gravity pulls matter off the companion star Charles Bailyn and into itself, generating heat and light that are detectable in the X-ray spectrum.
Searching for Black Holes
Yale astronomer Charles Bailyn was among the first scientists to positively identify black holes. Although the presence of black holes was predicted in 1939 by Einstein’s general theory of relativity, it was not until 1986 that the first black hole was confirmed. Bailyn was part of the team that detected a second black hole in 1992. Today there are about two dozen confirmed stellar black holes, around half of which were discovered by Bailyn’s team. It is very likely that there are thousands more throughout the Milky Way Galaxy.
The presence of a black hole is confirmed when an object’s mass is too great for it to be a neutron star, or anything else. The orbit of a companion star can tell us the mass of a compact object — the more massive the compact object, the faster its companion star must move to maintain its orbit. We measure this motion by observing the star’s Doppler shift, an effect that makes the light of an approaching object look bluer and a receding object look redder. The Doppler shift of a companion star orbiting a black hole changes from red to blue and back again.
Once we know the mass of the black hole, we can understand the details of the X-ray emissions from the material it is pulling in. Bailyn and his collaborators at the Massachusetts Institute of Technology and the Smithsonian Astrophysical Observatory have been studying these X-rays to measure the spin of a black hole. A spinning black hole twists the spacetime in its vicinity, changing the trajectory of the matter falling into it. The X-rays can tell us about the behavior of this infalling matter, enabling astronomers to determine how fast the black hole is spinning.
Image Middle Far Left
Image Bottom Far Left
This Chandra X-ray Observatory image is a spectrum of the black hole in the binary system XTE J1118+480. It is similar to the colorful spectrum of sunlight produced by a prism.
NASA/CfA/J.McClintock & M.Garcia
Image Bottom Center Left
An illustration of the binary system GRO J1655-40. This black hole, with a mass seven times that of the sun, is pulling matter from its companion star that is about twice as massive as the sun. The X-ray spectrum (see inset) indicates that turbulent winds of multimillion degree gas are swirling around the black hole. While much of the hot gas is spiraling inward toward the black hole, about 30% is blowing away. Powerful magnetic fields, likely carried by the gas flowing from the companion start, drive winds from the gaseous disk that carry momentum outward away from the black hole.
NASA/CXC/M.Weiss; X-ray Spectrum: NASA/CXC/U.Michigan/J.Miller et al.
Image Bottom Center Right
The Doppler Shift
The light from the companion star looks redder as it moves away from the viewer and bluer as it approaches the viewer. Astronomers can calculate how fast the star is orbiting the black hole by observing this shift in color.
Image Bottom Far Right
This illustration depicts a non-spinning (left) and spinning (right) black hole. Spinning black holes drag space with them as they spin, making it possible for particles to orbit nearer to the black hole. A possible explanation for the differences in spin among stellar black holes is that they are born spinning at different rates. Another is that the gas flowing into the black hole speeds it up.
NASA/CXC/SAO/J.Miller et al.