Our star, the Sun is a giant among stars, it is larger than 70% of observed stars. The Sun is still only a dwarf star compared to some really giant stars out there such as Antares and Betelgeuse for example. The real monsters of this galaxy are UY Scuti and AH Scorpii. Take UY Scuti, it is estimated at about 1,708 times the size of our Sun and is currently largest known star. However all the stars mentioned are tiny compared to Quasi-stars.
Quasi-stars may be confused with quasar but they are both complete different. Quasars or to give them their fullname QUASi-StAllar Radio object are known to exist in the far reaches of the universe whereas Quasi-stars are theoretical. Quasars are galaxies with an active nucleus whereas as Quasi-stars are stars. Whilst Quasi-stars are massive, they are still too small to rival a galaxy for size, they do have the same amount of mass as a small galaxy.
Quasi-stars are believed to have only existed at the beginning of the universe when most of the material in the galaxy was Hydrogen and Helium. Any other materials would have proved a problem for the Quasi-star.
A Quasi-star could be as large as 10 billion kilometers or roughy larger than 7,000 times the radius of the Sun. 10 billion kilometers is equivalent to roughly 67 A.U. To put that in context, Pluto is about 40 A.U. from the Sun. One A.U. or Astronomical Unit is the distance from the Sun to the Earth.
The New Horizons probe took 10 years to get from Earth to Pluto, it would roughly take another 5 years to reach the outside of the Quasi-star if it was trying to escape one. The New Horizons probe is the fastest probe that we have sent out into the far reaches out of Solar System. Even with the speed it is going, New Horizons will not encounter another solar system once it leaves our for hundreds of thousands of years.
The larger the size of a star is, the shorter the lifespan. The Sun has a lifespan of about 10 billion years of which it is roughly halfway through. A red dwarf star such as Proxima Centauri can expect to live for over eight trillion years as it is more efficient at using its fuel. Quora
A Quasi-Star is believed to only live for about seven million years. Quasi-stars like other giant stars are not very efficient in using their fuel, they live fast, die young to use the old expression.
A Quasi-star would have been formed when a large protostar collapsed in on itself. Unlike normal stars, a quasi-star is not powered by nuclear fusion. A Quasi-star is powered by a black hole that is at the centre. When the protostar collapses, the outward explosion would have been contained by the outer layers. There would not be a supernova explosion which most stars do when they come to the end of their lives.
Quasi-stars are believed to be the origins points of Supermassive Black Hole, those centres of many galaxies. The Quasi-star eventually collapsed in on itself and created the centre of a galaxy. Quasi-stars are just one possible source of the creation of Supermassive Black Hole, the other is a supermassive star which like Quasi-Stars don't exist current. When we talk Supermassive star, we are talking stars that are larger than UY Scuti. Arxiv
The temperatures of a Quasi-star is around 4,000 Kelvin which to put in context might seem hot. The Sun's temperature is a mere 5,778 Kelvin so Quasi-stars for all their size are actually cooler than the Sun. The hottest known star is Eta Carinae.