BACKGROUND: A team of astronomers led by the University of Michigan may know how black holes are lighting up the universe. New data from NASA's Chandra X-ray Observatory show for the first time that powerful magnetic fields are the key to these brilliant and startling light shows. By gaining deeper understanding of how black holes gather matter into themselves, astronomers also hope to learn more about other properties of black holes, including how they grow.
LIGHT FROM DARK: Black holes are the darkest objects in the universe. If a gas in a disk around a black hole loses energy, it will swirl toward the black hole, generating light along the way. In 1973, physicists suggested that magnetic fields could drive the generation of light by black holes. They would do this by generating friction in the gas and driving a wind from the disk that carries momentum outward. It is estimated that up to half of the total radiation in the universe since the Big Bang comes from material falling towards super-massive black holes.
WHAT CHANDRA FOUND: Chandra measures the amount of X-rays emitted at different energies, called X-ray spectroscopy. Spectroscopy is the study of light's "fingerprint," according to its color, which indicates its energy. Chemical elements each shine brightly at certain energies, so scientists can determine the chemical composition of an object. Chandra studied the X-ray spectra coming from a black hole system known as J1655 located in the Milky Way galaxy. The black hole was pulling material from a companion star into a disk, emitting the telltale radiation. The Michigan astronomers showed that the speed and density of the wind from the disk in J1655 corresponded to computer simulation predictions for winds driven by magnetic fields.
ABOUT BLACK HOLES: A black hole forms when a massive star has used up all its fuel. The reason the Sun and other stars emit light is because trillions of nuclear reactions are taking place at the cores. With core temperatures of millions of degrees, hydrogen atoms can convert into helium atoms, emitting radiation in the process. At some point, however, all the atoms are used up and no more nuclear fusion can take place. Without that outward counter-force to the pull of gravity, a star collapses inward, eventually reaching a point where the attractive gravitational force is so strong, not even light can escape. No one has ever observed the center of a black hole; until quite recently, such objects only existed in theory. But scientists surmise that a black hole has at its center an infinite density and an infinite gravitational field, as well as infinite entropy, which means no further change can take place. This is known as a "singularity." The event horizon of a black hole is not so much a physical surface as the theoretical point of no return for any object that gets caught in the black hole's powerful gravitational field.
The American Astronomical Society contributed to the information contained in the TV portion of this report.