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, between 13,7 to 10 billion years ago. Unlike modern stars, which are powered by nuclear fusion in their cores, a quasi-star's energy would come from its material falling into its central black hole core. Quasi-Stars are very massive, weighing in to up to 10,000 solar masses. They can only have formed early in the history of the Universe because the Hydrogen and Helium should not have been contaminated by heavier elements, like Oxygen. A quasi-star is believed to have formed when the core of a large protostar collapses into a black hole during its formation due to the high mass and the outer layers of the star are massive enough to absorb the resulting burst of energy without being blown away. Quasi-Stars could only live up to 7 million years, but with diameters up to 7,187 times greater than the Sun, or equivalent to 10 billion kilometers, they could produce as much light as a small galaxy.