Black holes tend to suck up surrounding matter in a process called ‘accretion’. Interestingly, BHs are completely ”black” only within the event horizon, from but outside the event horizon, light can escape. For these reasons, the event horizon has been defined as “the shell of points of no return”. The existence of even larger black holes, each with a mass equal to 10 billion Suns, can be inferred from the energetic effects on gas swirling at extremely high velocities around the centre of NGC 3842 and NGC 4889, galaxies near the Milky Way.The defining feature of a BH is the event horizon, the boundary from within which a particle cannot escape.Īt the boundary of the event horizon, the gravitational pull becomes so large that nothing can escape, including light (that is why BHs are black!). It was the first black hole to be imaged directly. That black hole has a mass equal to six and a half billion Suns but is only 38 billion km (24 billion miles) across. In 2017 the Event Horizon Telescope obtained an image of the supermassive black hole at the centre of the M87 galaxy. (For these observations, American astronomer Andrea Ghez and German astronomer Reinhard Genzel were awarded the 2020 Nobel Prize for Physics.) Supermassive black holes have been detected in other galaxies as well. Observations of stars orbiting the position of Sagittarius A* demonstrate the presence of a black hole with a mass equivalent to more than 4,000,000 Suns. One such supermassive black hole, Sagittarius A*, exists at the centre of the Milky Way Galaxy. Accordingly, the collapse of millions or billions of solar masses of interstellar gas under gravitational force into a large black hole would account for the enormous energy output of quasars and certain galactic systems. A mass of gas falling rapidly into a black hole is estimated to give off more than 100 times as much energy as is released by the identical amount of mass through nuclear fusion. Various astronomers have speculated that large volumes of interstellar gas collect and collapse into supermassive black holes at the centres of quasars and galaxies. ![]() Some black holes apparently have nonstellar origins. Discovered in 1971 in the constellation Cygnus, this binary consists of a blue supergiant and an invisible companion 14.8 times the mass of the Sun that revolve about one another in a period of 5.6 days. One of the component stars of the binary X-ray system Cygnus X-1 is a black hole. For example, if a black hole is a member of a binary star system, matter flowing into it from its companion becomes intensely heated and then radiates X-rays copiously before entering the event horizon of the black hole and disappearing forever. They can be “observed,” however, by the effects of their enormous gravitational fields on nearby matter. Stars with a smaller amount of mass evolve into less compressed bodies, either white dwarfs or neutron stars.īlack holes usually cannot be observed directly on account of both their small size and the fact that they emit no light. Only the most massive stars-those of more than three solar masses-become black holes at the end of their lives. ![]()
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