Why Black Holes May Have a Growth Limit of 50 Billion Suns

A black hole may hit a growth limit around 50 billion Suns because its feeding disk can start forming stars instead of falling inward.

Astronomy

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Black holes can become unimaginably massive, but there is a proposed limit on how far the usual process of feeding on gas can take them. A commonly cited model puts the limit for a luminous, gas-fed black hole at about 50 billion times the mass of the Sun under typical conditions. The reason is surprisingly indirect: as a black hole grows, its accretion disk can become so large that parts of the disk become unstable and begin forming stars. That changes the way matter is supplied to the black hole, making continued rapid growth much harder. The number is therefore a limit on a particular growth process, not a simple rule saying that no black hole can ever exceed 50 billion solar masses. The underlying research also allows substantially higher masses under extreme conditions, showing why the popular figure should be treated as a theoretical estimate rather than an absolute cosmic speed limit. Observed ultramassive black holes also have uncertain mass estimates, so comparisons between the largest known objects and theoretical limits require care.

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