By Ron Cowen
For an object that wields such a powerful and complex influence, a black hole is amazingly simple. Although its gravity is strong enough to trap light and slow time until each second lasts an eternity, the basic astrophysical properties of a black hole can be described by just two properties: mass and spin rate.
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Over the past 15 years, astronomers have made great progress in measuring the mass of big black holes, which may lurk at the center of every large galaxy. Astronomers have found that these black holes weigh millions to billions times the mass of the sun.