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A black hole is a region of space with such high mass that gravity allows nothing to escape, including electromagnetic radiation (EMR). General relativity's consequence that gravity bends light suggests that enough mass within a small enough area could bend nearby light to the point where it cannot escape. Such an "object" could have a very small mass if sufficiently concentrated (in which case the volume of from which no escape is possible would also be very small). The surface-shape delineating the volume from which light cannot escape is termed its event horizon. Since black holes lose their ability to interact via EMR, seeing into them is impossible and they can be fully characterized from our point of view by just a few aspects (the no-hair theorem): their mass, rotation (angular momentum), and electric charge. The event horizon forms a sphere if the black hole has no rotation, and forms an oblated sphere according to the amount of rotation. A common classification of black holes is by mass:
Gravitational wave detections have produced unexpected evidence of black holes between 50 and 100 solar masses: these have been referred to as intermediate mass black holes, or it may be that stellar-mass black holes may range to larger than previously thought, and the nomenclature may end up adjusted. Selection bias undoubtedly plays some role in the early detection of large examples.
Stellar black holes are thought to form from stars that have burned off sufficient fuel that their fusion no longer keeps them hot enough to maintain the pressure to counteract gravity, allowing the mass to collapse (gravitational collapse). Very small black holes are also termed mini black holes or micro black holes, and in addition to primordial black hole theories, some theories suggest cosmic rays or high-energy accelerators might create them. The term Planck hole represents the smallest possible black hole according to quantum theory. It would be smaller than an atom but would have the mass of "a flea's egg", about 1/50000 gram. Hawking radiation, a theorized quantum phenomenon that would slowly dissolve black holes would instantly dissolve one that small. Quantum fluctuations may be able to create such a black hole (a virtual black hole) for such a brief period.
Though black holes emit no EMR, material surrounding them can shine, a major contributor being the Kelvin-Helmholtz mechanism heating any surrounding material that is falling into their gravity well, and quasars, which are presumed to be SMBHs undergoing considerable such accretion, are among the brightest objects of the universe. A black hole without such accretion is very challenging to detect, and it is unclear how many such "dark" black holes exist. A primary method of detecting such hidden black holes is through the orbits of detectable bodies that are orbiting them.
Models for the formation of the largest black holes (SMBHs) that have been detected are a challenge (some are from so long ago that the universe has allowed them very little time to reach their observed size), but there is no theoretical upper limit to the size of a black hole ignoring a means of forming it: conceivably the universe is a black hole in the process of formation, though current observations of the universe's expansion complicate such a theory.
Rotating black holes are more complicated, and virtually all black holes manufactured by astronomical events (e.g., core collapse supernovae) would rotate on creation, at least a little. Rotation can be slowed (presumably even reversed, or can be increased) by accretion, and other methods of slowing have been theorized (Penrose process and Blandford-Znajek mechanism). Models suggest a non-rotating black hole includes a point singularity (point where the equations governing normal space and mass reach zeros and infinities) at the center, but some models suggest a rotating black hole would have a ring-shaped singularity. Also, rotating black holes exhibit significant frame dragging, a rotation of space around them, and with sufficient black hole rotation, a layer of this dragged space can be traveling faster than light as compared to the surrounding space further out. This "faster than light" region is termed the ergosphere. (Such a region has been described as one in which a particle cannot stand still, and as far as I can understand, that's to say it is impossible for it to move fast enough, given the dragged frame it inhabits, to appear to be standing still in relation to the surrounding space further out.) The outer boundary of the ergosphere is termed the static limit or static surface.
The term black hole candidate (BHC) is used for observed astronomical objects under consideration as black holes, often X-ray sources.