A commonly stated proposition is that "black holes have no hair," meaning that they have no observable external characteristics that can be used to determine what they are like inside. Black holes have only three measurable characteristics: mass, rotation?, and electric charge. Any black hole is completely defined by these three parameters.
Nevertheless, there is now a great deal of observational evidence for the existence of two types of black holes: those with masses of a typical star (4-15 times the mass of our Sun), and those with masses of a typical galaxy. This evidence comes not from seeing the black holes directly, but by observing the behavior of stars and other material near them!
In the case of a stellar size black hole, matter can be drawn in from a companion star producing an [accretion disk]? and large amounts of X-rays.
Galaxy-mass black holes are found in Active Galactic Nuclei (AGN). They are thought to have the mass of about 10 to 100 billion Suns! The mass of one of these supermassive black holes has recently been measured using radio astronomy. X-ray observations of iron in the accretion disks may actually be showing the effects of massive black holes as well.
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