A black hole is a region of spacetime where gravity is so strong that nothing – no particles or even electromagnetic radiation such as light – can escape from it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of no escape is called the event horizon. Although it has a great effect on the fate and circumstances of an object crossing it, it has no locally detectable features according to general relativity. In many ways, a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is of the order of billionths of a kelvin for stellar black holes, making it essentially impossible to observe directly.

Objects whose gravitational fields are too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. In 1916, Karl Schwarzschild found the first modern solution of general relativity that would characterize a black hole. David Finkelstein, in 1958, first published the interpretation of "black hole" as a region of space from which nothing can escape. Black holes were long considered a mathematical curiosity; it was not until the 1960s that theoretical work showed they were a generic prediction of general relativity. The discovery of neutron stars by Jocelyn Bell Burnell in 1967 sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality. The first black hole known was Cygnus X-1, identified by several researchers independently in 1971

Black holes of stellar mass form when massive stars collapse at the end of their life cycle. After a black hole has formed, it can grow by absorbing mass from its surroundings. Supermassive black holes of millions of solar masses (M☉) may form by absorbing other stars and merging with other black holes. There is consensus that supermassive black holes exist in the centres of most galaxies.

The presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as visible light. Any matter that falls onto a black hole can form an external accretion disk heated by friction, forming quasars, some of the brightest objects in the universe. Stars passing too close to a supermassive black hole can be shredded into streamers that shine very brightly before being "swallowed." If other stars are orbiting a black hole, their orbits can determine the black hole's mass and location. Such observations can be used to exclude possible alternatives such as neutron stars. In this way, astronomers have identified numerous stellar black hole candidates in binary systems and established that the radio source known as Sagittarius A*, at the core of the Milky Way galaxy, contains a supermassive black hole of about 4.3 million solar masses.

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  • emizeko [they/them]
    ·
    2 years ago

    4.3 million solar masses

    absolutely unimaginable amount of matter

    • vertexarray [any]
      ·
      2 years ago

      those quarks are gettin' cozy in there, that's for sure

      • emizeko [they/them]
        ·
        2 years ago

        As of 2020, S4714 is the current record holder of closest approach to Sagittarius A*, at about 12.6 AU (1.88 billion km), almost as close as Saturn gets to the Sun, traveling at about 8% of the speed of light.

        imagine hauling ass in orbit around a 4.3m solar mass black hole at 8% c

        • vertexarray [any]
          ·
          2 years ago

          my buddy tried to get there in a camaro and he turned his engine block into a glob of plasma on the interstate

      • keepcarrot [she/her]
        ·
        2 years ago

        Imagine you're falling backwards into a super-massive blackhole. Spaghettification wouldn't occur, the differences in forces wouldn't be that extreme. Instead, you'd see the night sky squish in towards the directly above you. A bright ring of stars, previously what had been the night sky, would go from 180 degrees of your vision to eventually a single point. This is similar to what you'd see if you were accelerating towards the speed of light. The time part of "the ring" would also squish; you'd see the end of the active universe as everything squished into that point.

        Marvelous thought to think about while dropping acid and doing whip-its.

          • keepcarrot [she/her]
            ·
            2 years ago

            One of my RPG settings (which was set in a fictional world war one), the entire universe was the echo of some stellar empire that was disappearing into a 6D black hole and being smeared over its surface, which explained universal expansion, where "the big bang" came from etc.