A quasi-star (also called black hole star) is a hypothetical type of extremely massive and luminous star that may have existed early in the history of the Universe. Unlike modern stars, which are powered by nuclear fusion in their cores, a quasi-star's energy would come from material falling into a black hole at its core.
A quasi-star would have resulted from the core of a large protostar collapsing into a black hole, where the outer layers of the protostar are massive enough to absorb the resulting burst of energy without being blown away or falling into the black hole, as occurs with modern supernova. The star would have been almost 10 million solar masses at this point. Quasi-stars may have also formed from dark matter halos (up to 100 million solar masses, for reference, some small galaxies have only 5 million solar masses) drawing in enormous amounts of gas via gravity, which can produce supermassive stars with tens of thousands of solar masses. Formation of quasi-stars could only happen early in the development of the Universe, before hydrogen and helium were contaminated by heavier elements; thus, they may have been very massive Population III stars. Such stars would dwarf VY Canis Majoris and Stephenson 2 DFK 1, both among the largest known modern stars, in size.
Once the black hole had formed at the core of the protostar, it would continue generating a large amount of radiant energy from the infall of stellar material. This constant outburst of energy would counteract the force of gravity, creating an equilibrium similar to the one that supports modern fusion-based stars. Quasi-stars would have had a short maximum lifespan, approximately 7 million years, during which the core black hole would have grown to about 1,000–10,000 solar masses (2×1033–2×1034 kg). These intermediate-mass black holes have been suggested as the progenitors of modern supermassive black holes such as the one in the center of the Galaxy.
Quasi-stars are predicted to have had surface temperatures higher than 10,000 K (9,700 °C). At these temperatures, and with a radius of approximately 800 thousand times that of the Sun (as large as the Solar System), each one would be about as luminous as a small galaxy. As a quasi-star cools over time, its outer envelope would become transparent, until further cooling to a limiting temperature of 4,000 K (3,730 °C) and growing to a size of 30 solar systems in terms of length. This limiting temperature would mark the end of the quasi-star's life since there is no hydrostatic equilibrium at or below this limiting temperature. The object would then quickly dissipate, leaving behind the intermediate mass black hole.