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Normal Galaxies
Chandra image of
NGC 3079
(NASA/CXC/SAO)
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The X-ray images of elliptical galaxies reveal that they are filled with multimillion degree gas, heated presumably by supernova explosions. Most of the gas in spiral galaxies is in the form of cool, dusty clouds. In both elliptical and spiral galaxies, X-ray images give us portraits of the end phases of stellar evolution - regions where supernovas have heated gas to millions of degrees, and objects where gravity has tightened its grip to form neutron stars and black holes.
Chandra image of
NGC 4261
(NASA/CXC/SAO)
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The most extreme examples of gravity's force are found deep in the centers of most galaxies where supermassive black holes lurk. These gravitational monsters can contain masses ranging from a few million to a few billion Suns. In normal galaxies, the supermassive black hole mainly makes its presence known through its gravitational force on the motions of stars, and by X-rays produced when gas is heated as it falls toward the black hole. But when supermassive black holes are surrounded by large supplies of dust and gas, the acceleration and heating of this gas as it is pulled into the black hole can produce stupendous amounts of energy at X-ray and other wavelengths and transform the appearance of the entire galaxy. Such galaxies are called active galaxies or quasars.
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