Stephenson 2-18 has recently become the biggest star in the known Universe.

[TheWorldNewsOrg]

It's named after the astronomer Charles Stephenson:

Stephenson 2-18 is a member of the open cluster Stephenson 2, which occupies an area of 1.8’ of the sky but is not visible in amateur telescopes. The cluster cannot be detected in visible light at all because it is heavily obscured by dust, but it can be seen in infrared light.

It lies in the region of the sky between Alpha and Beta Scuti. The open cluster Stephenson 2 is one of the most massive open clusters in the Milky Way. It was first noticed by American astronomer Charles Bruce Stephenson, who reported the discovery in June 1990.

Stephenson 2-18 (St2-18, also known as Stephenson 2-18 and RSGC2-18) is a red supergiant (RSG) star. It is close to, and a possible member of, the open cluster Stephenson 2. It is about 6,000 parsecs (20,000 ly) away from Earth in the constellation of Scutum and is one of the largest known stars. It is also one of the most luminous of its type. It has a radius of 2,150 times that of the Sun, and has a volume 10 billion times that of the Sun. This star has a spectral type of M6, which is pretty unusual for a supergiant star and corresponds to its cool temperature of 3,200 Kelvin. But there is also a hypothetical Quasi-star... Quasi-star, also known as black hole star, is hypothetically star are believed to have only existed of beginning of the universe when most a the material in the galaxy was hydrogen and helium. A Quasi-star could be as large as 10 billion kilometers or roughly large than 7.000 times the radius of the sun.

[TheWorldNewsOrg]

https://en.m.wikipedia.org/wiki/List_of_most_massive_stars

Under “Uncertainties and caveats” there’s a solid explanation of how a star bigger than ~150 solar masses would break our current understanding of how stars are created and evolve