The biggest black holes can be less dense than water
A black hole of billions of solar masses sounds like the densest thing imaginable. Yet divide its mass by the volume inside its event horizon and the average can fall below the density of water. At such a giant's edge, the tidal stretch on a person would feel no worse than standing on Earth.
The oddity comes from geometry. An event horizon's radius grows in step with mass, but volume grows with the cube of the radius, so average density drops as the square of the mass. A black hole of a billion Suns has a horizon radius of about 19 astronomical units, comparable to Uranus's orbit. Tidal forces weaken the same way, which is why falling into one of 10 million solar masses would not tear you apart at the boundary.
Supermassive black holes run from about 100,000 Suns to several billion, and almost every large galaxy seems to host one. Feeding on gas, they power quasars and active galactic nuclei. Some exceed five billion solar masses, informally ultramassive, including those in TON 618 and NGC 4889. A 2020 study floated stupendously large ones above 100 billion Suns, possibly including the heart of Phoenix A.
The trail began in 1963, when Maarten Schmidt found that radio source 3C 273 had strongly redshifted hydrogen lines, putting it billions of light-years away and shining like hundreds of galaxies from a region under a parsec wide. Fred Hoyle and William Fowler proposed giant stars, but Richard Feynman pointed out such stars would collapse. In 1964 Edwin Salpeter and Yakov Zeldovich suggested matter falling onto a huge compact object, and in 1969 Donald Lynden-Bell described the spiralling disc and guessed that quiet galactic cores were burnt-out quasars.
Evidence piled up. Sagittarius A* was found at the Milky Way's centre in February 1974. Hubble in 1994 clocked gas whirling around the core of Messier 87 at about 500 kilometres per second, and a 1995 study of Messier 106 left a single black hole as the only viable explanation. On 10 April 2019 the Event Horizon Telescope released the first image of a black hole, in M87, and in 2020 Andrea Ghez and Reinhard Genzel shared a Nobel Prize for discovering the compact giant at our galaxy's centre, seen as final confirmation that Sagittarius A* is one.
Source: Supermassive black hole
