The rate of light variations of the source dubbed a quasi-stellar object, or quasar, suggested the emitting region had a diameter of one parsec or less. Hubble's law showed that the object was located several billion light-years away, and thus must be emitting the energy equivalent of hundreds of galaxies. It was determined to be hydrogen emission lines that had been red shifted, indicating the object was moving away from the Earth. Initially this was thought to be a star, but the spectrum proved puzzling.
The story of how supermassive black holes were found began with the investigation by Maarten Schmidt of the radio source 3C 273 in 1963. The Schwarzschild radius of the event horizon of a (nonrotating) supermassive black hole of ~1 billion M ☉ is comparable to the semi-major axis of the orbit of planet Uranus, which is 19ĪU. Since the volume of a spherical object (such as the event horizon of a non-rotating black hole) is directly proportional to the cube of the radius, the density of a black hole is inversely proportional to the square of the mass, and thus higher mass black holes have lower average density. This is because the Schwarzschild radius is directly proportional to its mass.
In addition, it is somewhat counterintuitive to note that the average density of a SMBH within its event horizon (defined as the mass of the black hole divided by the volume of space within its Schwarzschild radius) can be less than the density of water. Unlike with stellar mass black holes, one would not experience significant tidal force until very deep into the black hole. The tidal force on a body at a black hole's event horizon is inversely proportional to the square of the black hole's mass: a person at the event horizon of a 10 million M ☉ black hole experiences about the same tidal force between their head and feet as a person on the surface of the earth. First, the tidal forces in the vicinity of the event horizon are significantly weaker for supermassive black holes. Supermassive black holes have physical properties that clearly distinguish them from lower-mass classifications. Some studies have suggested that the maximum mass that a black hole can reach, while being luminous accretors, is of the order of ~50 billion M ☉. Although they noted there is currently no evidence that stupendously large black holes are real, they noted that supermassive black holes almost that size do exist. Even larger ones have been dubbed stupendously large black holes (SLAB) with masses greater than 100 billion M ☉. Most of these (such as TON 618) are associated with exceptionally energetic quasars. Some astronomers have begun labeling black holes of at least 10 billion M ☉ as ultramassive black holes. Supermassive black holes are classically defined as black holes with a mass above 0.1 million to 1 million M ☉. Accretion of interstellar gas onto supermassive black holes is the process responsible for powering active galactic nuclei and quasars. The Milky Way has a supermassive black hole in its Galactic Center, which corresponds to the location of Sagittarius A*. Observational evidence indicates that almost every large galaxy has a supermassive black hole at the galaxy's center. Black holes are a class of astronomical objects that have undergone gravitational collapse, leaving behind spheroidal regions of space from which nothing can escape, not even light. The image was released in 2019 by the Event Horizon Telescope Collaboration.Ī supermassive black hole ( SMBH or sometimes SBH) is the largest type of black hole, with mass on the order of millions to billions of times the mass of the Sun ( M ☉). The dark center is the event horizon and its shadow. It shows radio-wave emission from a heated accretion ring orbiting the object at a mean separation of 350 AU, or ten times larger than the orbit of Neptune around the Sun. This is the first direct image of a supermassive black hole, located at the galactic core of Messier 87.