Astronomers cannot use visible light to peer directly at the core of our own Milky Way or any other galaxy. The view is obscured by dust, gas, and countless stars that form seemingly impenetrable sheets of brightness. X-rays and radio waves, however, can pierce those veils. Images of the cosmos taken at those wavelengths have convinced astronomers that black holes as massive as millions or billions of Suns lurk at the cores of many--and maybe most--galaxies in the cosmos.
The celestial beacons known as quasars are several hundred billion times brighter than normal stars, and they shine across distances of billions of light-years. Many quasars reside at the cores of apparently normal galaxies, such as the quasar PG 0052+251 (right), about 1.4 billion light-years away. However, intense x-rays and radio waves from quasars suggest that the cores of these galaxies are far from normal. Of all the models devised to account for the prodigious energies of quasars, the most successful calls for supermassive black holes that swallow up to 10 Suns' worth of mass each year; radiation streams from blazingly hot disks of gas spiraling into the holes.
One of the strongest sources of radio waves in the sky is M87 (opposite), a huge galaxy best known for a narrow jet that blasts into space for hundreds of thousands of light-years. Measurements of this jet show that its particles move at nearly the speed of light, accelerated by a powerful cosmic engine at the galaxy's core. Spectroscopic observations with the Hubble Space Telescope zeroed in on the whirling motions of gas and stars near the center of M87. The gravitational forces needed to create such motions imply that a concentration of matter with 3 billion times the mass of our Sun sits within the galaxy's innermost 10 light-years. That virtually rules out any explanation other than a black hole.