Stephenon 2 DFK 1 (also known as Stephenson 2-18) isn’t the largest known star

This star is well-known due to it’s estimated radius (2,150 solar radii, 10 astronomical units, 1.5 billion kilometers). However, the estimated parameters are rather unlikely. The estimated luminosity used in this radius estimate is 437,000 solar (more recent and more reliable estimates give it an even higher luminosity of 630,000 solar), placing it above the theoretical red supergiant luminosity limit (320,000 – 400,000 solar luminosities depending on spectral type), called the Humphreys-Davidson limit. The most luminous red supergiant in the galaxy is Stephenson 2 DFK 49 (from the same cluster), with an estimated luminosity of 390,000 solar. RSGC1-F01 has a similar luminosity of 380,000 solar. This is also true for other galaxies (where the most luminous red supergiants have luminosities of 340,000-390,000 solar, with the exception of the Triangulum Galaxy for some reason). Stellar evolution models also predict that cool red supergiants shouldn’t have luminosities that are this high. The estimated temperature (3200 K) is below the Hayashi Limit (~3500 K), meaning the star, if it has those properties, it isn’t in hydrostatic equilibrium. Note that the M6 star NML Cygni has a similar temperature (3300 K) and I’m not sure what’s going on with it. Wing (2009) also pointed out it probably shouldn’t be in hydrostatic equilibrium. This star also has a much lower luminosity (229,000 solar).

Also, Stephenson 2 DFK 1 is probably not a member of its cluster (this has been pointed out a few times in different papers) which would make it distance unknown. Unknown distance means unknown luminosity and therefore unknown radius. The stellar evolution models predict that the largest stars should have a radius of around ~1,500 solar radii.

The largest known star is probably WOH G64, located in the Large Magellanic Cloud around 163,000 light years away. The radius is 1,540 solar radii and the temperature is 3,400 K (Levesque, 2009). UY Scuti has an estimated radius of 1,708 solar radii, but this is rather unreliable (outdated and inaccurate distance estimate).