A black hole cannot be viewed directly because light cannot escape it. It can, however, be identified by its effect on the matter around it. Matter swirling around a black hole heats up and emits radiation that can be detected. Around a stellar black hole, this matter is composed of gas. Around a supermassive black hole in the center of a galaxy, the swirling disk is made of not only gas but also stars. An instrument aboard the Hubble Space Telescope, called the Space Telescope Imaging Spectrograph (STIS), was installed in February 1997. STIS is the space telescope’s main "black hole hunter." A spectrograph uses prisms or diffraction gratings to split the incoming light into its rainbow pattern. The position and strength of the line in a spectrum gives scientists valuable information. STIS spans ultraviolet, visible, and near-infrared wavelengths. The instrument can take a spectrum of many places at once across the center of a galaxy. Each spectrum tells scientists how fast the stars and gas are swirling at that location. With that information, the central mass that the stars are orbiting can be calculated. The faster the stars go, the more massive the central object must be.
STIS found, for instance, the signature of a supermassive black hole in the center of the galaxy M84. The spectra showed a rotation velocity of 400 km/s (248 miles per second), equivalent to 1.4 million km (.86 million miles) every hour. Earth orbits our Sun at 30 km/s (18.6 miles per second). If Earth moved as fast as 400 km/s (248 miles per second), our year would be only 27 days long.
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