A quasi-star (also called black hole star) is a hypothetical type of extremely massive star that may have existed very 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 central black hole.
A quasi-star is predicted to form when the core of a large protostar collapses into a black hole during its formation and the outer layers of the star are massive enough to absorb the resulting burst of energy without being blown away (as they are with modern supernovae). Such a star would have to be at least one thousand times the mass of the Sun. Stars this large could only form early in the history of the Universe before the hydrogen and helium were contaminated by heavier elements; see Population III stars.
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 additional stellar material. This energy would counteract the force of the gravity, creating an equilibrium similar to the one that supports modern fusion-based stars. A quasi-star is predicted to have had a maximum lifespan of about one million years, after which the core black hole would have grown to about ten thousand solar masses. These intermediate mass black holes have been suggested as the origin of the modern era's supermassive black holes. Quasi-stars are predicted to have surface temperatures comparable to that of the Sun, but, with diameters of approximately ten billion kilometers or over seven thousand times the diameter of the Sun, each one would produce as much light as a small galaxy.