Everybody’s always worried about Betelgeuse in Orion blowing up as a supernova. There’s a good chance that may happen one day, but no need to panic. The star’s too far away to trouble earthlings with its future fireworks. Opposite Betelgeuse and below the winsome triad of stars that form Orion’s Belt, another potential supernova star sparks and sputters on winter nights – Rigel. The name comes from ancient Arabic and refers to the foot or leg of Orion.
Like Betelgeuse, Rigel (RYE-jel) is also a supergiant star but one of a different color and temperature. Astronomers classify it as a blue supergiant with a surface temperature of over 20,000 degrees, twice that of the sun and 3 1/2 times hotter than Betelgeuse.
At a distance of 860 light years, Rigel is big enough and close enough to have its diameter measured directly. As you might guess, it’s huge – 74 times the size of our sun.
Placed where the sun is now, this stellar beast would extend nearly to the orbit of Mercury. From Earth, Rigel would span 35 degrees of sky and shine at a blinding -38 magnitude. We’re talking a powerful sunburn in a minute or two.
Great distance tames Rigel’s true ferocity as a young, energy gobbling star into a pretty blue-white twinkle reminiscent of sunlight on snowflakes. Rigel shines at 0.1 magnitude or about as bright as Capella in Auriga and Vega in Lyra.
Being extremely hot, blue supergiants burn up their energy stores quickly. At the tender age of 10 million years (young for a star), Rigel has already depleted its core of hydrogen fuel and has moved on to burning hydrogen in a surrounding shell.
Helium “ash” created from hydrogen burning will one day ignite and serve as fuel as will progressively heavier elements like oxygen, neon and silicon over time. Rigel will puff up and redden just like Betelgeuse in those far-off days.
Unfortunately, supergiant stars reach the end of the line once all the remaining silicon fuel has undergone nuclear fusion to create a core of iron. Iron requires more energy to fuse than the energy it releases, so it won’t burn like the other elemental fuels. With no burning to push back against the crushing force of gravity in so large a star, the core collapses and sends out shock waves that rip it apart in a supernova explosion.
When will this happen? Probably millions of years down the road. Since Rigel’s 300 light years farther from Earth than Betelgeuse, we needn’t worry about it either. Instead, our future descendants should prepare for a wondrous light show. Jim Kaler, professor emeritus at the University of Illinois, estimates that Rigel will become as bright as the half-moon when it finally blows up. Picture all that light concentrated in a tiny point of light. We’d easily see our shadows at night by supernova light!
If you have a small telescope 4.5 inches or larger, point it at Rigel some night. It’s one of the finest, if challenging, double stars in the sky. The 7th magnitude companion peaks out from under the glare of the main star a very short distance (9 arc seconds) to its south. On a night with steady air and good seeing, this pair is a beautiful sight.