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The first known black holes were the collapsed cores of supernova stars, but astronomers have since discovered signs of much larger black holes at the centers of galaxies. These galactic black holes contain millions of times as much mass as the Sun. Astronomers believe that huge black holes such as these provide the energy of mysterious objects called quasars. Quasars are very distant objects that are moving away from Earth at high speed. The first ones discovered were very powerful radio sources, but scientists have since discovered quasars that don’t strongly emit radio waves. Astronomers believe that almost every galaxy, whether spiral or elliptical, has a huge black hole at its center.

Astronomers look for galactic black holes by studying the movement of galaxies. By studying the spectrum of a galaxy, astronomers can tell if gas near the center of the galaxy is rotating rapidly. By measuring the speed of rotation and the distance from various points in the galaxy to the center of the galaxy, astronomers can determine the amount of mass in the center of the galaxy. Measurements of many galaxies show that gas near the center is moving so quickly that only a black hole could be dense enough to concentrate so much mass in such a small space. Astronomers suspect that a significant black hole occupies even the center of the Milky Way. The clear images from the Hubble Space Telescope have allowed measurements of motions closer to the centers of galaxies than previously possible, and have led to the confirmation in several cases that giant black holes are present.


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