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Once belonging to the realm of theoretical speculation, black holes have moved from science fiction to science fact. The evidence for their existence is so overwhelming that very few research astronomers doubt their existence.
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| Artist's concept of Cygnus X-1. |
The first strong case for a black hole dates back to the early 1970s, when a small X-ray satellite named Uhuru picked up X-ray emission from Cygnus X-1, the brightest X-ray source in the constellation Cygnus. Optical observations of the source reveal a blue supergiant star that is orbiting a massive but unseen companion. Astronomers quickly recognized that they had found a strong black hole candidate. As some of the supergiant's wind is captured by the black hole's gravity, it spirals inward and heats up to temperatures hot enough to emit X-rays. Since the early 1970s, evidence continues to mount that Cygnus X-1 harbors a black hole of approximately 20 solar masses, and dozens of similar systems have since been found in our galaxy. The evidence is so compelling that famed physicist Stephen Hawking has conceded a bet about the nature of Cygnus X-1 made with Caltech physicist Kip Thorne.
Evidence for supermassive black holes actually surfaced in 1918, when astronomer Heber Curtis first noticed a straight ray emanating from M87, a giant elliptical galaxy that anchors the Virgo Cluster. Since then, astronomers have found hundreds of other galaxies with such structures (now known as jets), along with thousands of other galaxies that have extremely bright nuclei. Astronomers have come to realize that these extraordinary phenomena are powered by black holes weighing millions or even billions of solar masses.
Better yet, when astronomers started zooming into the cores of nearby large galaxies (such as M87) with the Hubble Space Telescope and other instruments, they found stars and gas whipping around at speeds exceeding a million miles per hour. Incontrovertible evidence for the existence of a supermassive black hole at the center of our own Galaxy came from long-term studies of stars at the very center. An immensely massive object must be accelerating the gas to such high velocities. However, there is nowhere near enough light emanating from the centers of these galaxies to explain these mass concentrations as stars or even large star clusters. Most astronomers agree that the simplest and most straightforward explanation is that supermassive black holes are accelerating stars and gas to these high speeds. The incredible luminosity of active galactic nuclei is also most easily explained as matter spiraling into supermassive black holes.
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