Betelgeuse Supernova: Is the Star Ready to Explode?
There's talk that Betelgeuse's recent strange pulsating might be a signal that the star is about to blow apart and become a supernova. But is it true? We explain.
Quick Reference: Betelgeuse and a Future Supernova
- What it is: a red supergiant star marking the shoulder of Orion the Hunter, about 640 light-years away.
- How big: roughly 770 times the diameter of the Sun. Drop it where the Sun sits and it would swallow the orbit of Mars.
- Will it explode? Yes, eventually. Betelgeuse is primed to end as a supernova anytime within the next 100,000 years.
- Anytime soon? Most astronomers say no. The recent dimming was not a countdown to the blast.
- If it did blow: it would shine about as bright as the full Moon for a few months, then slowly fade, with no harm to Earth.
On clear winter nights, the brightest and grandest constellation of all stands upright over the Earth: Orion, the Mighty Hunter. Skywatchers with clear weather can pick him out high in the southern sky at around 8:30 p.m. local time. Three bright stars mark Orion’s belt, and above and below it sit two immense stars, blue-white Rigel and red-orange Betelgeuse.
That red-orange star has kept astronomers and headline writers talking for years. Every so often the news fills with talk that a Betelgeuse supernova is about to light up our sky. Here is what is actually going on, what the science says, and what you would really see if the old giant finally let go.
The Armpit of the Giant: Ready to Explode?
Betelgeuse, whose name is often translated as “The Armpit of the Giant,” is a gigantic bloated globe of cooler gas, measuring roughly 770 times the diameter of the Sun. If it were to replace the Sun in our solar system, it would extend well beyond the orbit of Mars.
Compared to its neighbor Rigel, Betelgeuse is near the end of its career. It sits 640 light-years away, and it does not shine with a steady light. It is a pulsating star, expanding and contracting in fits and starts, and those pulsations are so irregular that no one can predict exactly when it will swell or shrink.
Trying to describe Betelgeuse many years ago, Henry Neely, a lecturer at New York’s Hayden Planetarium, once said it is “like an old man with his strength almost entirely spent, panting in the asthmatic decrepitude of old age.” The image still fits. This is a star in its twilight, burning through the last of its fuel.
Stars make their energy by fusing hydrogen into helium deep within their cores. When a star builds up enough helium in its core, its energy output climbs and it swells into a red giant, or a supergiant, like Betelgeuse.
In such stars, the core forges successively heavier elements to push back against the constant crush of gravity. But once the core begins making iron, the star’s days are numbered. Building elements heavier than iron consumes energy instead of producing it, so the furnace stalls. NASA’s Imagine the Universe team walks through the full sequence of how a massive star ends in a supernova if you want the deeper physics.
Eventually the core can no longer hold up the star’s vast weight. It collapses in a fraction of a second, and the rebound triggers a cataclysmic supernova explosion.
Stellar Death Throes?
Betelgeuse is in its final stage and could explode anytime within the next 100,000 years. In cosmic terms that is soon. In human terms it could be tonight or it could be a thousand generations from now, and nobody can narrow it down further than that.
The star drew a wave of attention when it became noticeably dimmer. On its own, a dimming Betelgeuse is nothing strange, since it pulsates and has faded before. This particular fading, though, was unusual. Normally Betelgeuse shines at a magnitude of +0.69, ranking it 11th among the 21 brightest stars in the sky. Estimates showed it fading to around magnitude +1.7, which dropped it clean out of the Top 21. For a while, one of the sky’s landmark stars was noticeably dim.
The media pointed to this “bizarre fading” and floated the idea that it might be the signal that Betelgeuse was about to blow apart and become a supernova. It made for a dramatic headline. It was also, most likely, wrong.
Most reputable astronomers and astrophysicists did not buy that scenario. Their read was that the dimming came from other causes, such as an eruption of gas or dust or shifts in the star’s surface brightness, and that Betelgeuse would gradually climb back to normal. It did. So while we know the old giant is primed to blow apart someday, the scientific consensus is plain: not anytime soon.
What Made Betelgeuse Dim?
The fade that started the whole scare has a name now: the Great Dimming. From late 2019 into the spring of 2020, Betelgeuse lost so much of its shine that the change was easy to see with the naked eye, no telescope required. That is a rare thing for a star this famous, and it is what set the supernova rumor loose.
The answer turned out to be closer to weather than to doom. Astronomers using the Hubble Space Telescope traced the Great Dimming to a huge cloud of dust. The star belched out a bubble of hot gas, the gas cooled as it moved away, and the dust that formed briefly veiled part of the star from our view, like a curtain drawn across a lamp. You can read the researchers’ own account of what caused Betelgeuse’s mysterious dimming. By 2020 the star had recovered its usual brightness, right on cue.
Boom!
But just supposing. What would we see and feel from here on Earth if Betelgeuse were to finally pop?
The three brightest supernovae ever recorded lit our skies in the years 1006, 1054, and 1572. Each of them either matched or rivaled the planet Venus at its brightest. But each of those stars sat thousands of light-years away.
Betelgeuse is only 640 light-years away.
That nearness changes everything. If Betelgeuse exploded without warning, the doomed star would grow tremendously luminous, likely approaching the brilliance of the full Moon and casting distinct shadows on the ground. It would be easy to see even through the glare of the daytime sky.
For two or three months it would hold that dazzling brightness. Then, over the next two or three years, it would slowly fade from view. In the years after that, where Betelgeuse once hung, we might see only a diffuse patch of faint light. That would be the intensely hot, newly exposed core of the star, wrapped in an expanding cloud of gaseous debris. That would be the whole of it, with no other effect on our planet.
What If Betelgeuse Were Closer?
We are fortunate that Betelgeuse sits where it does, and not as close as, for example, Capella, a very bright yellowish star that rides high overhead during the mid-evening hours. Capella is only 43 light-years away.
What if Betelgeuse were parked that close to us? In his book What if the Moon Didn’t Exist? Voyages to Earths That Might Have Been (Harper Collins, 1993), author Neil F. Comins speculates on what might happen if a star 50 light-years away from Earth were to explode. It would brighten, he writes, to become:
..fifty times brighter than the Moon and only eight thousand times less bright than the Sun. The Earth would be subjected to blasts of electromagnetic rays, which would destroy the Earth’s ozone layer in a matter of days, and bombard the Earth with radiation (though both the ozone layer and radiation level would eventually return to normal).
Surviving plants and animals by the millions would undergo genetic mutations due to the radiation. While (humans) would be injured by the initial dose of gamma rays, the most effective screen against the radiation would be Earth itself. The rock would absorb virtually all of the ultraviolet rays, X rays, gamma rays, and most of the cosmic rays. The greatest number of people would survive if they all moved underground for a few years . . . this option would clearly be open to only a relative handful of people; neither the room nor the facilities underground exist for extended living by so many (billions) of people.
It is a sobering thought experiment. The comfort is in the distance. At 640 light-years, Betelgeuse is far enough that its eventual blast is a light show, not a threat. Even the gamma rays and cosmic rays from a star that remote would arrive too weak and too spread out to trouble life on Earth.
How to Spot Betelgeuse Yourself
You do not need a telescope to find Betelgeuse. On a clear winter evening, face south and look for Orion’s three-star belt. Betelgeuse is the bright red-orange star at the Hunter’s upper shoulder, and blue-white Rigel marks his lower foot. Hold the two side by side and the color difference is easy to see, one warm and ember-red, the other cool and icy.
Betelgeuse rides highest on winter nights and slips into the dawn sky by late spring, so a summer stargazer will do better watching for the Perseid meteor shower or checking our guide to the year’s meteor showers until Orion returns. When it does, keep an eye on Betelgeuse from one clear night to the next. Because it pulses, its brightness really does drift over weeks and months, and tracking it by eye is a fine backyard project. For the darkest viewing, plan around the full Moon dates and head out when the Moon is low or below the horizon.
Point your eyes at Orion’s shoulder on the next clear night and judge the old giant for yourself. It has been dying slowly for a very long time, and there is a good chance it will still be shining there long after all of us. Watch it while you can, and decide what the sight is worth to you.
Betelgeuse Supernova: Frequently Asked Questions
Is Betelgeuse going to explode soon?
Betelgeuse will end its life as a supernova, but most astronomers say it will not happen anytime soon. It could explode anytime within the next 100,000 years. That window is short by cosmic standards and enormous by human ones, so tonight is possible but not likely.
Why did Betelgeuse get so dim in 2019 and 2020?
The fade, now called the Great Dimming, was not a countdown to a blast. Astronomers using the Hubble Space Telescope traced it to a cloud of dust. Betelgeuse ejected a bubble of hot gas that cooled into dust and briefly veiled part of the star. By 2020 it had returned to its normal brightness.
How far away is Betelgeuse?
Betelgeuse sits about 640 light-years from Earth in the constellation Orion. That is far enough that even when it does explode, the supernova would be a spectacular light show with no physical harm to our planet.
How big is Betelgeuse compared to the Sun?
Betelgeuse is a red supergiant measuring roughly 770 times the diameter of the Sun. If it replaced the Sun at the center of our solar system, its surface would reach out well beyond the orbit of Mars.
What would a Betelgeuse supernova look like from Earth?
If Betelgeuse exploded, it would grow tremendously bright, likely rivaling the full Moon and casting shadows, and it would be visible even in daytime. It would hold that brightness for two or three months, then fade over two or three years, leaving a faint patch of glowing debris where the star used to be.
Where can I find Betelgeuse in the night sky?
Look for Orion on a clear winter evening, high in the southern sky around 8:30 p.m. local time. Find the three stars of Orion’s belt, then look up to the Hunter’s shoulder for the bright red-orange star. That is Betelgeuse, opposite the blue-white star Rigel at his foot.
Joe Rao
Joe Rao is an esteemed astronomer who writes for Space.com, Sky & Telescope, and Natural History Magazine. Mr. Rao is a regular contributor to the Farmers' Almanacand serves as an associate lecturer for the Hayden Planetarium in New York City.
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- Joe Rao
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