What is a black hole?
A black hole is an object that is so compact (in other words, has enough mass in a small enough volume) that its gravitational force is strong enough to prevent light oranything else from escaping.
The name "black hole" was introduced by John Archibald Wheeler in 1967. It stuck, and has even become a common term for any type of mysterious bottomless pit.Physicists and mathematicians have found that space and time near black holes have many unusual properties. Because of this, black holes have become a favorite topic for science fiction writers. However, blackholes are not fiction. They form whenever massive but otherwise normal stars die. We cannot see black holes, but we can detect material falling into black holes and being attracted by black holes. Inthis way, astronomers have identified and measured the mass of many black holes in the Universe through careful observations of the sky. We now know that our Universe is quite literally filled withbillions of black holes.
What types of black holes are there?
Astronomers can measure the mass of black holes by studying the material that orbits around them. So far, we have found two typesof black holes: stellar-mass (just a few times heavier than Sun) or supermassive (about as heavy as a small galaxy). But black holes might exist in other mass ranges as well. For example, recentobservations suggest there may be black holes with masses between stellar-mass and supermassive black holes.
Black holes can spin around an axis, although the rotation speed cannot exceed some limit.Astronomers think that many black hole in the Universe probably do spin, because the objects from which black holes form (stars for example) generally rotate as well. Observations are starting to shedsome light on this issue, but no consensus has so far emerged. Black holes could also be electrically charged. However, they would then rapidly neutralize that charge by attracting and swallowing...
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