Can a Single Star Collapse into a Supermassive Black Hole?

Common misconception aside, a single star cannot directly collapse into a supermassive black hole (SMBH). While the process of star formation and black hole formation are interrelated, the mechanisms and scales involved differ significantly. This article explores the different types of black holes, their formation processes, and the reasons a star of stellar magnitude cannot become a supermassive black hole.

Understanding Stellar and Supermassive Black Holes

Stellar Black Holes

Stellar black holes form from the remnants of massive stars, typically more than 20 times the mass of the Sun. When these stars exhaust their nuclear fuel, they undergo a supernova explosion, leaving behind a core that collapses under its own gravity, transforming into a black hole. The mass of such black holes is usually between 3 and several tens of solar masses.

Supermassive Black Holes (SMBHs)

The exact formation processes of SMBHs are still subjects of extensive research but are believed to involve the following:

Merging of smaller black holes: Over time, stellar black holes can merge, gradually increasing the mass and thus the black hole's size. Accretion of mass: A black hole can grow by attracting matter from its surroundings or by merging with other massive objects, including stars and gas clouds. Direct collapse of massive gas clouds: In the early universe, large gas clouds might collapse directly into SMBHs without first forming stars.

It is also thought that SMBHs can continue to grow through hierarchical mergers of whole galaxies, where the central black holes of merging galaxies combine.

Empirical Limits and Extreme Cases

The mass of stars has an empirical limit around 150 solar masses (M☉). Notably, the most massive known star, R136a1, has an initial mass of 325 M☉. Despite defying this limit, even such a star cannot collapse directly into an intermediate mass black hole or a supermassive black hole.

The smallest known supermassive black hole is located in the galaxy RGG 118, with a mass of 50,000 M☉. This is significantly less than the mass required for a direct stellar collapse into a supermassive black hole, making it clear that no single star can achieve this transformation.

Intermediate Mass Black Holes

Between stellar mass and supermassive black holes lies the category of intermediate mass black holes (IMBHs), which have masses ranging from 100 to 1,000,000 M☉. However, R136a1, with its 315 M☉, is unlikely to evolve into an IMBH due to the mass loss during its supernova explosion.

A star with an initial mass of 15–20 M☉ typically collapses into a stellar mass black hole with a final mass of 3 M☉. These black holes can grow further through accretion and mergers, forming intermediate mass black holes.

Conclusion and Further Observations

To summarize, while a single massive star can collapse into a stellar black hole, the pathways to forming supermassive black holes are more complex and involve the merging and accretion of multiple stellar black holes over billions of years. The growth of supermassive black holes to the size of 50 billion M☉ is achieved through both conventional accretion disc methods and mergers of galaxies.

The remarkable size range and formation processes of black holes, from the smallest stellar black holes to the largest supermassive black holes, underline the intricate and dynamic nature of the universe we observe.