Supergiants
A supergiant is one of the largest and brightest types of star in the universe. Supergiants are absolute monsters: some are so big that if you placed one where our Sun is, it would stretch all the way out past the orbit of Jupiter and swallow up the inner Solar System. They are also among the shortest-lived stars in the universe: a supergiant burns through its fuel so fast that it only lasts a few million years before exploding as a spectacular supernova.
- Mass8 to 40+ solar massesAmong the most massive stars known
- DiameterUp to approx. 2,000 SunsSome so big they would swallow Jupiter
- Lifespanapprox. 10 million yearsBurn fast, die young
- Brightest knownR136a1About 4 million times brighter than the Sun
- Famous red supergiantBetelgeuseIn Orion, will explode within 100,000 years
- Famous blue supergiantRigelAlso in Orion, 120,000 times brighter than the Sun
What is a supergiant?
A supergiant is a star that started life as one of the most massive stars in the universe, usually at least 8 times the mass of our Sun. Once such a star runs out of hydrogen to burn in its core, it expands to enormous size. The star's outer layers puff up by a factor of hundreds or thousands, and what was already a huge star turns into a true monster.
Astronomers split supergiants by their colour into two main groups:
- Red supergiants are cooler, between about 3,500 and 4,500 °C at the surface. They include Betelgeuse, Antares and UY Scuti.
- Blue supergiants are hotter, between about 10,000 and 50,000 °C. They include Rigel, Deneb and Eta Carinae.
Famous supergiants in the night sky
Several of the brightest stars in our sky are supergiants. Even though they sit hundreds or thousands of light years away, their enormous brightness lets us see them clearly with the naked eye.
- Betelgeuse (in Orion): a red supergiant about 640 light years away. Diameter roughly 700 times the Sun. If placed where the Sun is, it would reach almost out to Jupiter.
- Rigel (also in Orion): a blue supergiant about 860 light years away. Pumps out approx. 120,000 times the Sun's light.
- Antares (in Scorpius): a red supergiant about 550 light years away, roughly 700 times the Sun's diameter. Its name means "rival of Mars" because both look reddish.
- Deneb (in Cygnus): a blue-white supergiant approximately 2,600 light years away, pumping out 200,000 times the Sun's light.
- UY Scuti (in Scutum): one of the biggest stars known, around 1,700 times the diameter of the Sun. Too faint to see without binoculars at 5,200 light years away.
Why supergiants live such short lives
It seems strange that the biggest stars die the youngest, but it makes sense once you think about it. A supergiant has a huge mass, so its core is crushed by enormous gravity, which makes it hotter than the core of a smaller star. The hotter the core, the faster fusion runs. A supergiant burns through hydrogen at a colossal rate, in some cases millions of times faster than the Sun does.
Even though a supergiant has more fuel than a smaller star, it uses it up so fast that it only lives for 10 to 50 million years. Compare that with our Sun's 10-billion-year lifespan, or a red dwarf's 10-trillion-year lifespan. In cosmic terms, supergiants are flash-in-the-pan stars: brilliant for a short while, then gone.
How supergiants die
When a supergiant finally runs out of fusable elements in its core, it dies in the most spectacular way the universe knows: a supernova. The core collapses in a fraction of a second, and the outer layers of the star are blasted into space at thousands of kilometres per second. For a few weeks, the exploding star can shine brighter than the entire galaxy of billions of stars around it.
What is left behind depends on the original mass. A "lighter" supergiant of 8 to 25 solar masses leaves a neutron star. A heavier one leaves a black hole. Either way, the supernova explosion enriches the surrounding gas with all the heavy elements the star fused over its lifetime: carbon, oxygen, silicon, iron and so on. That recycled gas eventually becomes the raw material for the next generation of stars and planets.
The very brightest stars
Some supergiants are not just big but unbelievably bright. The current record-holder is R136a1, a giant blue star in the Large Magellanic Cloud. R136a1 is around 4 million times brighter than our Sun and has roughly 200 times the Sun's mass. It sits near the top of what physics allows: any more massive and the star would blow itself apart by the force of its own radiation. R136a1 is shedding mass so fast that it loses the equivalent of an entire Earth every month.
Deeper dive: how big can a star get?
There is an upper limit to how massive a star can be. As a star gets bigger, the radiation pouring out of its core pushes harder and harder on its outer layers. At a certain mass, that outward push of light becomes stronger than the inward pull of gravity, and the star starts blowing itself apart faster than it can pull in new gas. This upper limit is called the Eddington limit.
For decades, astronomers thought no star could be heavier than about 150 times the Sun. Then in 2010, the star R136a1 was measured at around 200 solar masses, which is still being debated. The star is throwing off mass at such an extreme rate that it must have been even heavier when it first formed. Most very massive stars exist in tight binary or triple systems, and computer simulations suggest that one way to break the limit is for two large stars to merge into one even larger star.
There is also a limit to how large (in diameter) a star can be. Beyond about 1,500 to 2,000 times the radius of the Sun, the outer layers of the star are so loosely held that they become unstable and shed back into space. So the largest supergiants like Stephenson 2-18 and UY Scuti are probably close to the natural ceiling that physics allows.
For other stars, see red dwarfs at the small end, white dwarfs in the middle, or neutron stars for what supergiants leave behind. The whole story is in life cycle of a star.