The biggest known star may be 2,150 times the Sun's radius
Even at light speed, circling it once would take 8.7 hours, and set where the Sun sits, it would reach Saturn.
Kurzgesagt – In a Nutshell · The Largest Star in the Universe – Size ComparisonThe largest known star may measure about 2,150 times the Sun's radius.
That star is Stephenson 2-18, a red hypergiant, which is what astronomers call a massive star that has burned through its main fuel and swollen enormously as it nears death. While typical stars of its type are about 1,500 times the Sun's radius, the largest rough estimate puts this one at 2,150 times the Sun's radius, putting out nearly 500,000 times as much energy as the Sun.
The scale is hard to grasp. Traveling at the speed of light, it would take 8.7 hours to make one circuit. A ride on the speediest airplane on the planet would need about 500 years for the same loop. Placed where the Sun sits, the star would stretch out to fill the orbit of Saturn.
And the claim of being the largest known star is a rough, uncertain estimate. These stars are extremely bright, very far away, and blowing themselves apart, so small measurement errors mean huge uncertainty about their true size.
Questions people ask
Where is Stephenson 2-18?
It sits in a crowded cluster of stars called Stephenson 2, in the constellation Scutum, about 19,000 light-years from Earth. That great distance is one reason its exact size is so hard to pin down.
If it is so huge, is it also the heaviest star?
No. It probably started life only a few dozen times the mass of the Sun and has since lost about half of that. Its huge size comes from swelling late in life, not from starting with vastly more material.
What will happen to it?
It will keep shedding material, shrink, and heat up as heavier elements build in its core. Eventually the core will collapse and the star will explode as a supernova. The gas thrown off will later help form a new generation of stars.