The biggest stars in the universe

They’re ridiculously big and stupidly hot, but what else do we know about supergiant stars?


An artist's impression of the red supergiant star VX Sagittarii. Credit: Tyrogthekreeper

Put the Sun next to a supergiant star and you’ll have a hard time finding it. Supergiant stars are the largest stars in the universe. They can be thousands of times bigger than our Sun and have a mass up to 100 times greater. The largest known supergiant star, VY Canis Majoris, is up to 2,100 times the size of the Sun (based on upper estimates). If it were put in the position of the Sun, it would extend out to the orbit of Saturn.

Betelgeuse is 370 times bigger than the Sun. Credit: ESO

Supergiants come in a variety of sizes and temperatures, but they are generally classed as being either red or blue. Red supergiants have a mass at least eight times that of our Sun, and are generally old stars that were once similar in size to the Sun. They form when a star more than 10 times the mass of our Sun runs out of its hydrogen fuel in its core, preventing fusion from occurring there. It subsequently begins collapsing but, as it does so, the hydrogen in its outer shells begins fusion of its own. At this point the entire star experiences fusion and begins to burn through the rest of its hydrogen at an astounding rate. In fact, they can burn all of their remaining hydrogen in just a few million years, compared to the several billion year lifetime of stars like our Sun. During this time they will shine at least 100,000 times brighter than the Sun. At the end of their life, red supergiant stars often explode as a supernova, producing either a neutron star or a black hole in the process. The nearby red supergiant Betelgeuse, which is 1,000 times the mass of the Sun, is only 8.5 million years old but it is expected to go supernova within the next 1,000 years.

Blue supergiants are considerably hotter than red supergiants, but generally much smaller, only about 25 times the size of the Sun. Like red supergiants they have very short lifetimes of only a few million years. They usually form when a star more than 10 times the mass of the Sun heading towards its own demise enters a slow burning phase. However, red supergiants can also turn blue if their own rate of nuclear fusion begins to slow down. In fact, a star can actually continually switch between being a red and blue supergiant over its lifetime. Between the two extremes it becomes a yellow supergiant such as the north star, Polaris. However, stars of this nature generally spend the majority of their time as red supergiants rather than blue or yellow.

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