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Black holes are points in space with immense gravitational pull, formed by collapsing stars or galaxy cores. They can be characterized by mass, spin, and charge. Supermassive black holes exist in every galaxy, including the Milky Way. Intermediate and micro black holes are also theorized.
Black holes are points in space with so much gravitational pull that not even light can escape them. The “tipping point” around a black hole is referred to as its event horizon. Black holes form when a star with more than 20 solar masses collapses or when the entire core of a galaxy collapses. Black holes from collapsing galaxies release huge amounts of energy that can be observed from billions of light-years away. This energy comes from the friction of the infalling matter against itself: no radiation can emanate from the black hole itself.
Black holes are among the most intriguing astrophysical phenomena in the universe. They have captured the public imagination and dedicated study by many famous physicists, including Albert Einstein and Stephen Hawking. Black holes are interesting from the point of view of physicists because they can be comprehensively characterized by only three values: their mass, spin and charge. Slightly different theories are used to describe the behavior of rotating versus non-rotating, charged versus neutral black holes.
The biggest distinction between black holes isn’t their spin or charge, but their mass. Known black holes cluster around two mass levels: stellar-mass black holes, ranging from 1.5 to 14 solar masses, and supermassive black holes, with millions or billions of solar masses. Stellar black holes are created whenever a star with more than 20 solar masses runs out of all its nuclear fuel and collapses.
Supermassive black holes probably formed billions of years ago, early in the history of the universe, and still exist today. Every galaxy is currently thought to have a supermassive black hole at its center, and at least 30 are suspected to have been observed. Most astronomers believe that Sagittarius A*, a solar system-sized area containing 2.6 million solar masses, is the supermassive black hole of the Milky Way.
There are two further categories of black holes that can exist. The first is intermediate black holes, with masses ranging from 14 solar masses to millions. Ultra-luminous X-ray sources, long-lasting sources of X-rays in other galaxies, are perhaps intermediate-mass black holes in the process of accreting matter. If these are indeed intermediate-mass black holes, they have a mass close to one hundred thousand solar masses. Some astronomers have also observed an object near the center of our galaxy theorized to be an intermediate-mass black hole with 1,300 solar masses.
The second type of unconfirmed black hole would be micro black holes, which would be created in particle accelerators or by the Big Bang. None have yet been observed, although some astrophysicists suspect they exist and are designing telescopes to look for them
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