The stars at the bottom are the biggest currently known stars. 1 AU is the distance between the Sun and the Earth.
The biggest stars that could ever have formed are thought to have existed billions of years ago, just after the Big Bang. They were enormous, even more than 1000 times as massive as our Sun. In comparison, the biggest star known from our time is BAT99‑98, which is only 226 times as massive as our Sun.
These hypothetical stars were called quasi-stars or black hole stars because they had a black hole in the middle of them.
Because of their size, they are thought only to have existed for about 7 million years. They were so huge that they might have been progenitors of the supermassive black holes at the center of modern galaxies. They weren’t powered by nuclear fusion like regular stars; instead, their brightness came from the material falling into the black hole in their core. Because they were so massive when the black hole formed inside them, they didn’t explode like a supernova. Surrounding layers of the star absorbed the explosion and the newly formed black hole started consuming the star from within.
In our time, such huge stars cannot form anymore—there is too much contamination from elements heavier than hydrogen and helium. There were no such heavy elements in the first generation of stars just after the Big Bang. They appeared when the first stars formed, produced them via nuclear fusion, and spread them in supernovas.
These monsters had temperatures of about 10,000 K, and they might have been more than 7000 times as big as our Sun or 66 times as wide as the distance between the Sun and the Earth. During their short lives, they are thought to have cooled to temperatures of about 4,000 K, which caused them to dissipate and die, leaving behind just the black hole.
