The biggest stars in the Universe are the red supergiant stars. And we’re talking really, really big. The largest known red supergiant is thought to be VY Canis Majoris, measuring about 1800 times the size of the Sun. Imagine if the Sun extended out to the orbit of Saturn. Let’s take a look at where red supergiant stars come from.
Red supergiants are similar to red giants. They form when a star runs out of hydrogen fuel in their core, begins collapsing, and then outer shells of hydrogen around the core get hot enough to begin fusion. While a red giant might form when a star with the mass of our Sun runs out of fuel, a red supergiant occurs when a star with more than 10 solar masses begins this phase.
The five largest known supergiants in the galaxy are red supergiants: VY Canis Majoris, Mu Cephei, KW Sagitarii, V354 Cephei, and KY Cygni. Each of these stars has a radius larger than 1500 times the size of the Sun. In comparison, regular red giant is only 200 to 800 times the size of the Sun.
Red supergiant stars don’t last long; typically only a few hundred thousand years, maybe up to a million. Within this period, the core of the red supergiant continues to fuse heavier and heavier elements. This process stops when iron builds up in the core of the star. Iron is the equivalent of ash when it comes to nuclear fusion. The process of fusing iron actually requires more energy than it releases.
At this point, many red supergiants will detonate as Type II supernovae.