Definition
Betelgeuse is a huge red star that glows in the constellation Orion in the winter night sky. It is a red supergiant in the last stage of its life, expected to explode as a supernova someday. It is hundreds of millions of times larger than the Sun by volume, and it is a special object because we can watch the final stage of a star's life with our own eyes.
Orion's Red Shoulder: Just How Big Is It?
Look up at the winter night sky and you'll spot Orion, shaped like an hourglass. The star that shines unusually red and bright at its upper-left shoulder is Betelgeuse.
What would happen if we put this star in the middle of our solar system? The Sun would vanish inside it, and so would Mercury, Venus, Earth, and even the orbit of Mars. Its surface is estimated to reach past Mars's orbit and into the inner part of Jupiter's orbit. By volume, you could fit hundreds of millions of Suns inside it. It boasts a truly mind-boggling size.
It is so big because the fuel in its core has run low, and its outer layers have puffed up like a balloon. As the star grew old, its temperature dropped and it turned red, and its body swelled. It became a red supergiant (an old star far bigger than a red giant).
A Ticking Time Bomb: The Star's Final Moments
Throughout its life, a star makes energy by fusing elements in its core. A young star burns hydrogen. An old giant like Betelgeuse holds out by burning helium, then carbon, then oxygen in turn. In the end, iron, which can't produce any more energy through fusion, piles up in the core. Then the balance of forces that held the star up collapses completely.
The core falls inward in an instant and sends out a huge shock wave. This is a supernova, one of the biggest explosions in the universe. However, a 2020 study concluded that Betelgeuse is still in the stage of burning helium in its core.
To be more precise, "soon" in astronomy doesn't mean what it means to us. That study estimated that roughly 100,000 years remain before the explosion, and some researchers think the time left is even shorter. Next to the universe's history of billions of years, 100,000 years is a blink of an eye. That's why it's called "soon."
Will the Explosion Put Earth in Danger?
Many people worry that Earth would be devastated if Betelgeuse blew up. Luckily, Betelgeuse is about 550 to 720 light-years away (a distance that takes light hundreds of years to cross). That's not close enough for its shock wave or radiation to directly harm Earth.
Instead, we'd get the most spectacular sky show in human history. When the supernova happens, it would brighten to about the brightness of a half moon, shining so brightly that it would be visible to the naked eye even in the daytime. At night, its light would be bright enough to cast shadows for several months, then slowly fade.
Once the explosion is over, a cloud of gas (a nebula), the star's remains, would spread out where Betelgeuse was, and a small neutron star is expected to be left at the center. Orion's red shoulder would vanish from the night sky for good, and a whole new view of the universe would open up.
Common misconceptions
If Betelgeuse explodes as a supernova, it will cause deadly radiation damage on Earth.
Betelgeuse is about 550 to 720 light-years from Earth. A supernova is thought to need to be within a few dozen light-years to seriously harm life on Earth, so from this distance it would look like nothing more than a sky show.
Where you meet it
In the winter night sky, you can find Betelgeuse with your naked eye, twinkling red at the upper left of Orion.
In late 2019, Betelgeuse suddenly dimmed to less than half its usual brightness, and astronomers around the world wondered whether it was a sign of an explosion.
In one sentence
Betelgeuse is an old star so huge that it would swallow the orbit of Mars if placed where the Sun is, and it is expected to explode as a supernova someday.