Other meanings of black hole
Astrophysics
A supermassive black hole (SMBH) is a type of black hole with a mass ranging from hundreds of thousands to billions of times the mass of the Sun. These cosmic giants reside at the centers of most massive galaxies, including the Milky Way, and their formation and evolution are intimately linked to the galaxies they inhabit.
Supermassive black holes are the largest known type of black hole, with masses ranging from about 105 to 1010 solar masses1. They are characterized by an event horizon, a boundary beyond which nothing can escape, and their size scales linearly with mass: the Schwarzschild radius of a non-rotating SMBH is about 3 km per solar mass, so a 109 solar-mass black hole has a radius of about 3 billion km, roughly the orbit of Uranus2.
Unlike stellar-mass black holes, which form from the collapse of massive stars, SMBHs are thought to grow through a combination of accretion of gas and mergers with other black holes. Their presence is inferred from the gravitational influence on surrounding stars and gas, as well as from the energetic phenomena they power, such as active galactic nuclei (AGN) and quasars3.
The most compelling evidence for SMBHs comes from the orbits of stars near the galactic center. For example, the star S2 orbits the Milky Way's central object, Sagittarius A*, with a period of about 16 years, and its orbit indicates a mass of about 4.3 million solar masses within a very small volume4.
In 2019, the Event Horizon Telescope (EHT) collaboration released the first direct image of a black hole's shadow, capturing the SMBH at the center of the galaxy M87 (M87*). This image provided visual confirmation of the existence of SMBHs and tested general relativity in the strong-field regime5. In 2022, the EHT released an image of Sagittarius A*, confirming it as a black hole6.
The formation of SMBHs remains an active area of research. One leading theory is that they originate from 'seeds' formed by the collapse of massive population III stars in the early universe, which then grow by accreting gas and merging with other black holes1. Another possibility is the direct collapse of massive gas clouds, producing seeds of thousands of solar masses7.
Observations of quasars at high redshift (z > 6) show that SMBHs of a billion solar masses existed when the universe was less than a billion years old, posing constraints on growth models8. The growth of SMBHs is also linked to galaxy evolution through feedback processes, where energy released by accretion heats and expels gas, regulating star formation3.
Supermassive black holes are found at the centers of most massive galaxies, including the Milky Way. There is a well-established correlation between the mass of the central SMBH and the velocity dispersion of the host galaxy's bulge, known as the M-sigma relation9. This suggests that SMBHs and their host galaxies evolve together, though the exact mechanisms are still debated.
Some galaxies, such as the dwarf galaxy RGG 118, host SMBHs with masses as low as 50,000 solar masses, while others, like the quasar TON 618, may harbor black holes of up to 66 billion solar masses10. The most massive SMBHs are found in brightest cluster galaxies, such as Holm 15A, with an estimated mass of 40 billion solar masses.
Beyond the well-known examples, SMBHs exhibit a range of surprising behaviors. For instance, some SMBHs are 'overmassive' relative to their host galaxy's bulge, challenging the simple M-sigma relation11. Others are 'wandering' black holes, displaced from galactic centers due to galaxy mergers or gravitational recoil12.
In 2020, a team discovered a SMBH in a dwarf galaxy that is actively accreting matter, providing clues about how SMBHs form in low-mass galaxies13. Additionally, the detection of gravitational waves from merging SMBHs, though not yet achieved, is a major goal for future missions like LISA14.
Some SMBHs produce relativistic jets that extend for millions of light-years, influencing the intergalactic medium. The physics of jet launching and collimation remains an active field of study15.
This article focuses on supermassive black holes, the largest class of black holes, which are central to our understanding of galaxy evolution and extreme gravity.
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