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Quasi-Stars and Supermassive Black Hole Formation

Some of the largest stars in our universe are around 2100 times the size of our Sun. The largest of these stars being Stephenson 2-18 which stretches out millions of miles in diameter. However, even Stephenson 2-18 is dwarfed by hypothetical stars called Quasi-Stars. Quasi-Stars are enormous stars that are theorized to be 7000 times the size of our Sun. Within these stars are black holes that power the reactions that keep the star from collapsing in on itself. However, these reactions do not last forever, as the black hole is continuously consuming the star from the inside. It is theorized that these stars could be the creators of supermassive black holes.

Quasi Stars compared to some of the largest stars known to have existed.

Other Theories of Supermassive Black Hole Formation

Quasi-Stars are still hypothetical structures and it would be extremely difficult and improbable for scientists to find evidence of their existence. Evidence for the existence of Quasi-Stars would be almost impossible for scientists to find, this is because the lifespan of Quasi-Stars is extremely short in the timeline of the universe. To maintain a Quasi-Star the black hole must consume around 1 solar mass of in-falling gas to maintain the pressure required to keep the star from collapsing in on itself. At this rate the black hole within the Quasi-Star would consume all the hydrogen available in about 7 million years, a minuscule amount of time on the universe's timescale. Light from these stars would have faded out long ago before the formation of our own solar system. However, other theories surround the formation of supermassive black holes. One of these theories is the Pop III theory. Pop III was a group of very large stars that formed from the gaseous remnants of the Big Bang. These stars would have been larger than any other star that we know of, including Stephenson 2-18. These stars would have been large enough to provide enough gas when they exploded in a supernova to begin the formation of supermassive black holes early in the history of the universe. Black holes formed from Pop III stars would have had ample time to grow in size to become supermassive black holes.

Interior of a Quasi Star

Quasi-Stars Forming Supermassive Black Holes

Quasi-Stars are theorized to be powered by an interior black hole. This is theoretical possible because in-falling gas from the star in to the black hole traps its own radiation within the star creating a radiation pressure-supported star. Black holes created by Quasi-Stars would have started off with much larger initial masses compared to Pop III black holes. This is due to the star having a much more gas-rich environment early on in its lifespan facilitating the growth of larger black holes. However, these stars would have had much less time to grow in to supermassive black holes compared to Pop III black holes as Quasi-Stars formed from the collapse of the interior layers of protostars, which formed from gas leftover from the Big Bang. There are some problems with this theory however, for example one would expect fragmentation to occur after the star collapsed in on itself. Fragmentation is the breakup of gas leftover from supernova in to smaller clouds. However, Schleicher et al. estimate that stars with masses between 10^7 and 10^8 solar masses could collapse in to large black hole without fragmentation, Quasi-Stars would fall under this category of star. Schleicher et al. adds that fragmentation could be prevented if molecular hydrogen could be disassociated efficiently enough to allow for gas to quickly collapse in to a black hole, however this would also require the star to have strong UV field. Black holes made by Quasi-Stars could occur in galaxies where their is a large abundance of gas and dust for the black hole to feed on. It is also estimated that these galaxies are frequent enough to account for the large amount of supermassive black holes that have been observed.