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Starts With A Bang

This Is Why Betelgeuse (Probably) Isn’t About To Explode

Despite its changing shape and its unprecedented dimming, a supernova this year, decade, or even century is highly unlikely.

For months, skywatchers have enjoyed a view of the night sky that’s wholly unfamiliar to anyone alive today: a sky where Betelgeuse, one of the 10 brightest stars in the sky, has dimmed to around #25, about the same as the stars in Orion’s belt. The dimming appears to have stabilized for now; Betelgeuse has remained constant since late January. This is the faintest Betelgeuse, one of the closest red supergiants to Earth, has ever appeared in our lifetimes.

Just a few days ago, the first image of the star’s shape since the dimming began was released, showing significant changes since it was last observed in shape, brightness, and its luminous extent. Speculation about Betelgeuse’s near-term fate, which is expected to eventually end in a supernova, are running rampant, with everyone wanting to know if and when it will explode. But the answer is probably “not anytime soon,” and astronomers know this to be the case. Here’s the reason why.

The Sun, today, is very small compared to giants, but will grow to the size of Arcturus in its red giant phase, some 250 times its current size. A monstrous supergiant like Antares or Betelgeuse will be forever beyond our Sun’s reach, as we will never begin fusing carbon in the core: the necessary step for growth to this size. (ENGLISH WIKIPEDIA AUTHOR SAKURAMBO)

Despite being a red supergiant star, Betelgeuse has a lot in common with our own Sun. They both possess a core region where nuclear fusion occurs. They both have large radiative zones, where the light and energy generated from the core’s fusion reactions propagate outward, taking a “random walk” path to get there. And they both have diffuse photospheres, where the light, having propagated to the star’s outermost layers, finally gets released into the Universe.

Like all stars, the Sun and Betelgeuse both transport energy in a complex fashion, driven by large-scale electric currents, magnetic fields, moving particles, and thermally driven convection. The science of magnetohydrodynamics dictates how this all plays out in detail, with processes that occur in the core eventually propagating to the photosphere and influencing what we wind up observing.

This cutaway showcases the various regions of the surface and interior of the Sun, including the core, which is where nuclear fusion occurs. As time goes on, the helium-containing region in the core expands and the maximum temperature increases, causing the Sun’s energy output to increase. When our Sun runs out of hydrogen fuel in the core, it will contract and heat up to a sufficient degree that helium fusion can begin. (WIKIMEDIA COMMONS USER KELVINSONG)

However, what happens in the core takes a very long time to make its way to the photosphere. When light elements fuse into heavy ones in a star’s core, energy is produced. Those energetic quanta collide with other particles in the star’s interior, exchanging energy and causing any photons to bounce around in random directions. Given the sheer number of particles inside a star and a star’s typical size, it takes an enormous length of time for any change in a star’s core to propagate to what we loosely think of as the star’s surface.

In our own Sun, that timescale is between 100,000 and 200,000 years: the typical amount of time it takes the energy produced in a fusion reaction to reach the photosphere. The light that we see from our own Sun might take a little more than 8 minutes to reach our eyes from the moment it leaves the photosphere, but it takes over 100,000 years for the light produced in the core to even reach the photosphere in the first place.

This illustration shows how a photon produced in the core of the Sun randomly collides with other particles in the Sun before eventually reaching the photosphere, where it has a chance of being released to the outer Universe. Inside our Sun, it takes a little more than 100,000 years, on average, for such a photon to escape from the Sun. (RICHARD POGGE / HARRIET HUNT)

With a star like Betelgeuse, which is significantly more massive (about 20 times our Sun’s mass) but also much larger (about the size of Jupiter’s orbit), this is still a major problem. It’s absolutely true that the outer layers of red supergiant stars do vary, consisting of a number of large convective cells that go down a significant depth into the star’s interior. Inside a star like this, energy is exchanged and transported between the various layers in a complex and intricate manner, with energy from the core still taking a very long time to propagate to the star’s outer layers.

It’s not as bad for a star like Betelgeuse as ~100,000 years, which is much lower in density (and hence has a lower rate of particle interactions) than our Sun, but it still takes somewhere on the order of millennia for changes occurring in its core to propagate to its photosphere.

This simulation of a red supergiant’s surface, sped up to display an entire year of evolution in just a few seconds, shows how a “normal” red supergiant evolves during a relatively quiet period with no perceptible changes to its interior processes. The enormity of its surface and the volatility of the tenuous outer layers leads to tremendous variability on short but irregular timescales. (BERND FREYTAG WITH SUSANNE HÖFNER & SOFIE LILJEGREN)

That said, we absolutely do see that Betelgeuse, like all red supergiants that we’ve either observed or simulated, is extremely variable in shape, size, brightness, and brightness distribution. The changes that occur in its outermost layers occur on timescales of a few months, rather than thousands of years. Even violent events, such as the ejection of matter (which is what’s likely substantially dimming the star), are unrelated to what’s occurring in the core.

Which is a bit unfortunate, because all we can observe is what’s happening at that outermost layer of Betelgeuse: the photosphere. We can make observations of what happens outside of that photosphere as well, where we use multiwavelength observations to find and map out an enormous amount of extended matter that was doubtlessly ejected over centuries and millennia past.

Betelgeuse has given off large amounts of gas and dust over its history, filling the interstellar medium surrounding it with matter, which gets illuminated in infrared light. This image was taken in December of 2019, based on data obtained with the VISIR instrument aboard the ESO’s Very Large Telescope. (ESO/P. KERVELLA/M. MONTARGÈS ET AL., ACKNOWLEDGEMENT: ERIC PANTIN)

But none of that gives any indication of what’s actually occurring in the core, which is the information we actually need to determine when or whether Betelgeuse is most likely to go supernova. Inside a red supergiant like this, we fully expect that the star has moved on to the stage where it’s fusing carbon in its core. In order to go supernova, however, it needs to proceed from:

  • fusing carbon in its inner core,
  • to fusing neon in its inner core,
  • to fusing oxygen in its inner core,
  • to fusing silicon in its inner core.

Only when the silicon in the core has been exhausted, leaving an “ash” remnant of iron, nickel and cobalt (which will not fuse and release energy under these conditions), will the supernova actually occur.

The electromagnetic output (left) and the spectrum of neutrino/antineutrino energies (right) produced as a very massive star comparable to Betelgeuse evolves through carbon, neon, oxygen, and silicon-burning on its way to core-collapse. Note how the electromagnetic signal barely varies at all, while the neutrino signal crosses a critical threshold on the way towards core-collapse. (A. ODRZYWOLEK (2015))

Unfortunately, our understanding of stars is good enough that we are absolutely certain that none of the surface changes we’ll see are any indication of what’s occurring in the star’s core. The only observable signal we’ll have isn’t visual at all, but rather comes in the form of neutrinos. While transitioning between carbon, neon, oxygen and silicon-burning offer no change in the luminosity or temperature of the star, the neutrino spectrum does change in both energy-per-neutrino and the total flux of neutrinos.

Even from Earth, some 640 light-years away from Betelgeuse, the final stages of this transition will be noticeable. During silicon-burning, there will come a stage where the energy that each (anti-)neutrino is produced with is sufficient that it will trigger an inverse beta decay reaction in our terrestrial detectors, producing scores of these interactions during the final few hours of the star’s life.

An enormous chamber containing a total of 260,000 tons of water will be surrounded with photomultiplier tubes capable of catching the light generated from neutrino interactions with the particles inside the to-be-completed Hyper-Kamiokande detector, which will become the world’s largest water-based neutrino detector upon completion. (UNITED STATES GOVERNMENT/FLICKR)

The neutrino signal that we’ll receive from Betelgeuse just before it goes supernova is, as far as we can tell, the only early-warning system we have with any sort of physical merit. What we’ve been seeing with our own eyes and telescopes is fascinating, but merely evidence of variability in supergiant stars, something that’s been known and studied very well for hundreds of years. What’s occurring with Betelgeuse isn’t even that unusual; it’s only remarkable because Betelgeuse is so close and so familiar.

Of all the stars we’ve ever observed where a supernova did eventually occur, we have never yet uncovered a correlation between a dimming event such as this and a supernova. As much as we might hope for such a spectacular, rare, and nearby event like a supernova to occur in our lifetime, the recent faintness of Betelgeuse doesn’t point in that direction at all.

This graph from the AAVSO tracks the brightness of Betelgeuse over a six-month period that begins in August of 2019 and ends in February of 2020. The brightness of Betelgeuse has dimmed tremendously from its original, normal value, but has stabilized over the past few weeks, and is expected to brighten again. (AMERICAN ASSOCIATION OF VARIABLE STAR OBSERVERS / E. SIEGEL)

What we’re seeing today, based on the changing brightness of Betelgeuse, is consistent with something much more mundane than, “it’s about to go supernova.” Instead, what appears to be going on is simply a large ejection event, where matter from Betelgeuse’s outer layers — originating perhaps a billion kilometers from the core — is spit out from the star’s interior.

Once this material makes in beyond the photosphere, it expands and cools, where it begins absorbing and obscuring portions of the starlight. The fact that one portion of Betelgeuse appears fainter than the remainder is suggestive of where this ejection event has occurred. If this is what’s truly going on, we can fully expect that, over the coming months, the brightness will remain stable, followed by a gradual re-brightening to its original status. By 2022 or 2023 at the latest, Betelgeuse should be back in the top 10 for brightest stars in the sky.

The constellation Orion as it would appear if Betelgeuse went supernova in the very near future. The star would shine approximately as brightly as the full Moon, but all the light would be concentrated to a point, rather than extended over approximately half a degree. (WIKIMEDIA COMMONS USER HENRYKUS / CELESTIA)

Even though it’s unlikely that Betelgeuse is about to explode, we must keep in mind that this is both a possibility and an inevitability. When that finally does occur, it will become the most widely-viewed astronomical event in human history, visible to everyone on Earth over the course of a year or more at a time where more humans exist on Earth than ever before. It’s going to happen eventually, but probably not for somewhere around 100,000 years.

Travel the Universe with astrophysicist Ethan Siegel. Subscribers will get the newsletter every Saturday. All aboard!

While you should absolutely go out and enjoy this unprecedentedly dim sight, as Betelgeuse is only ~36% as bright as it was a year ago, you must keep in mind that its current brightness variations are due to processes in its outermost layers alone, and have nothing to do with its core. Betelgeuse might go supernova at any time, but if it does, its correlation with this recent dimming event will be due to pure coincidence. What happens in the core doesn’t make it to the surface fast enough to give us any real, meaningful clues.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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