The constellation of Orion the Hunter is one of the most dependable sights in the night sky. Even in the most light-polluted cities, the outline is readily visible. Orion has always looked a little cocky to me: the three stars of his belt cut an angled line, suggesting a hip thrust out to one side, while the bright stars that bejewel the corners of his shoulders and tunic make him look like he could be drawing his bow, which is what some mythological traditions say he’s doing—or he could be standing with his hands on hips, demanding to speak to the manager.
However, recent observations reveal that Orion may not be so stalwart after all: Betelgeuse, the bright star whose ruby-tinted glow marks Orion’s shoulder, is getting dimmer. This “fainting” (yes, cue your mental image of a giant star swooning on an equally large chaise longue) has some people speculating that we might be witnessing the final days before Betelgeuse burns out in a spectacular stellar death explosion, known as a supernova. Over the past few months, Betelgeuse has plunged in rank from being the 12th brightest star in the sky to being closer to the 20th, and astronomers all over the world are keenly watching to see what happens next.
Betelgeuse is a what’s known as a red supergiant, a gargantuan star with about 20 times the mass of the sun. Betelgeuse’s outer layers have swollen so much that if it were plopped down in our solar system in place of the sun, it would fill the space all the way out to Jupiter, swallowing up Mercury, Venus, Earth, and Mars. Massive stars like Betelgeuse are known to “live fast, die young”: At a mere 8 million years young, Betelgeuse is nearing the end of its days. (For comparison, our sun is a bit under 5 billion years old and hasn’t even reached midlife yet.) While supernovae happen all over the universe, it’s a rare treat to have one so close, and even better that at roughly 600 lightyears away, Betelgeuse is far enough that we’ll get to see the fireworks without also being zapped by high energy radiation from them. Betelgeuse’s distance also means that, whatever happens, we are currently observing its past—the dimming starlight we are seeing now on Earth has been traveling towards us for several hundred years. For all we know, Betelgeuse may have already exploded, but if it has, the light of its supernova hasn’t reached us yet. If Betelgeuse has already exploded and the light from the explosion arrives here (for example) tomorrow, then we’ll see the supernova tomorrow—but if Betelgeuse explodes tomorrow, Betelgeuse-time, Earth won’t see it for another 600 or so years.
Betelgeuse’s fainting is an exciting chance for naked-eye stargazers to see stellar evolution in action: If you go out and find the constellation of Orion, Betelgeuse is noticeably dimmer than before. The fact that Betelgeuse is ordinarily bright means that people have been studying it for a long time, and we rarely get such a detailed glimpse of what happens when stars die, let alone a star we know well. Betelgeuse is one of the only stars besides the sun whose surface has been imaged, revealing a face mottled by bright hot spots. Betelgeuse’s end, like that of all stars, will come as the result of a fuel crisis: Stars create energy via nuclear fusion, but over time run out of fresh fuel to fuse. In Betelgeuse’s case, it’s reached a stage where its puffed-up outer layers are unstable. Like a boiling pot left unattended, their churning dredges heavier elements from deep within the star, flecks of burnt stellar stew rising to the top.
Part of what has captured people’s imaginations about Betelgeuse isn’t just the possible supernova light show—it’s that it is very rare to see the night sky change in such a clear, easily observable way. The stars tempt us into thinking that they are static—many things in space change over timescales so cosmically long, no human can hope to see them. The idea that the stars were “fixed” has been around a long time, and throughout history, it’s even been considered sacrilege to say otherwise.
But while Betelgeuse’s recent behavior has attracted special attention, it’s actually changing all the time. Betelgeuse’s atmosphere pulses at multiple frequencies, making the star change in brightness over periods of hundreds of days to years. While Betelgeuse may mark Orion’s shoulder, its pulsing could just as easily be thought of as Orion’s heart, a blood-red giant beating in our night sky. But it hasn’t always been there, either. Betelgeuse was born around the time when the last common ancestors of humans and chimpanzees lived, so while it has evolved alongside humans as a species, no dinosaur ever saw its light.
Betelgeuse’s recent changes are a reminder that the universe is a dynamic place, both variable and transient: Every night, across the sky, stars are changing in brightness while planets whirl around them, comets and asteroids are whizzing through the solar system, and distant stars die in huge explosions, scattering stardust that will eventually become a new generation of stars. Modern telescopes, like the upcoming Vera Rubin Observatory, survey the sky over and over, collecting images to create cosmic movies that will reveal our changing sky like never before.
It’s unclear whether Betelgeuse is beginning its death rattle or just having a particularly big swing in its already variable brightness. If it does go supernova, it will be nearly as bright in the sky as the full moon and will glow for months before its embers fade. If you are in a dark enough place on Earth during that time, it might even be possible to see your shadow cast by its last light. This spectacular show would come with a cost, however. While the leftover supernova remnant would still be visible to telescopes, its glowing filaments gracing Orion’s shoulder like a corsage, humanity would lose the ability to see it with our bare eyes. Perhaps then, instead of wishing for its demise, we can all look to Betelgeuse to live on and remind us that in this universe, change and impermanence are the only constants.