Getting to Know Orion: Betelgeuse

Welcome to part 2 of my miniseries exploring the many fascinating astronomical aspects of the winter constellation Orion (part 1, all about Orion’s famous belt, is here). This series was always going to include an entry on arguably the constellation’s most well-known single star, Betelgeuse, and given that it showed up in the news this week it seemed like a good idea to cover it next.

Betelgeuse is not only the most famous star in the constellation Orion, it’s one of the most famous stars in the whole sky. If you ask a random person on the street to name a star and they are able to do so, there’s a decent chance the name they’re going to pull out is “Betelgeuse”. But just what makes this star so famous? Or perhaps the term I should be using is infamous? Let’s find out!

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What’s in a Name?

I’d be willing to bet a fair amount of money that the reason so many people mentally glom on to Betelgeuse as the one star whose name they’ll remember is because it shares that name with a popular movie character. If you’ve never seen the classic 1988 horror-comedy “Beetlejuice”, you’re missing out on a very weird but good time.

While the film title makes use of a phonetic spelling of the main character’s name, the actual spelling of the name of Michael Keating’s “ghost with the most” in the film is Betelgeuse, like the star. That name derives from an Arabic phrase meaning “the arm (or hand) of al-Jawza”, though I’ve seen it translated far more roughly (and humorously) as “armpit of the great one”.

Stars are also known by more functional names known as Bayer designations that give stars a letter from the Greek alphabet according to their brightness, followed by a Latinized version of their constellation’s name. So the brightest star in Orion would be dubbed Alpha Orionis. This is, in fact, the Bayer designation assigned to Betelgeuse. But wait, plot twist! Rigel, aka Beta Orionis, is actually the constellation’s brightest star.

I couldn’t find a verifiable source giving a reason why Johann Bayer, the originator of this system, chose to snub Rigel in this fashion, but despite it being a measured fact that Rigel is the brighter of the two, Betelgeuse has kept its Alpha designation, perhaps largely out of a sense of tradition. If I were Rigel, I’d be feeling some kind of salty about that.

A Honkin’ Giant Dying Star

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If you’ve ever seen Orion in the sky, you’ll know that even among the notoriously bright stars of this constellation Betelgeuse sticks out. It is Orion’s westward shoulder (so the one on the left to an Earthbound viewer) and it’s distinctly reddish in color. That’s because Betelgeuse is in the end phase of its life, having swollen up its outer layers. 

When this happens a star’s outermost layers, now far removed from the broiling hot core, cool off and become red (I know we like to refer to super-hot things on Earth as being red hot and we use red to mean hot on faucets, but red stars are the cooler ones, with surface temps of  only a few thousand degrees). The star also becomes ridiculously, stupidly gigantic in the process.

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Now you’re going to ask me how big Betelgeuse is. The answer is we don’t quite know, because we don’t quite know how far away it is (more on that in a moment). But an accurate, if imprecise, answer would be ridiculously honkin’ big. It’s probably at least 700 million miles (1.2 billion km) in diameter. That means if you stuck it in our solar system in the Sun’s place its outer edge would be around Jupiter’s orbit (so long inner solar system, it was nice knowing you).

A star reaches this state when it is about to die. Betelgeuse is probably only about 10 million years old or so, but it started out big and big stars don’t live long (by comparison our middle-aged Sun is about 5 billion years old). This monster is running out of fuel. When it does it’s going to blow in a spectacular supernova. It is among the nearest stars to us that will do so. How near? Well, I did promise I’d come back around to that, didn’t I.

Going the Distance

We don’t know how far away Betelgeuse is. Not for sure anyway. For a long time we thought it was about 430 light years away based on measurements from the Hipparcos satellite which launched in 1989 and was designed to make precise measurements of relatively nearby stars.

Only it turns out that Hipparcos had troubles with variable stars, which are stars that for a variety of reasons regularly change in brightness (I went into more detail on them in this post). And guess what! Betelgeuse is a variable star. When the errors from Hipparcos were corrected for, a new distance of 643 (+/- 46) light years became the norm.

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But in 2017 a new study combined Hipparcos data with data from radio telescopes to come up with a new number. This latest measurement puts Betelgeuse at somewhere around 724 light years away, but as close as 613 light years or as far as 881. This star refuses to be pinned down.

Whatever the exact measurement is, it’s close enough that when it blows itself to smithereens it’s going to light up the sky. Imagine all the brightness of a Full Moon, only emanating from a single point. It will be so bright that it will be visible even if it happens while Betelgeuse is in Earth’s daytime sky. For two months or so it will be impossible to miss. When it happens of course. It’s hard to tell where exactly Betelgeuse is in the timeline of star death, but it’s thought that it may only be in the early late stage of its life, with thousands of years still to go before the big blow. 

Betelgeuse is also close enough and big enough that it is among the very few stars whose surfaces we’ve been able to directly image. So you’d think, even if we can’t quite tell the distance, that we’d know all about what’s going on immediately around this star, right? Ha, yeah right.

The Great Dimming

If that were the case then we would have been able to quickly diagnose what the heck happened to Betelgeuse in late 2019 and early 2020. The star suddenly went very dim. I’ve already said Betelgeuse is a variable star, but that’s not what this was. Variable stars change their brightness with quite a bit of regularity. In December 2019 Betelgeuse suddenly started getting waaay fainter. This event, known as the Great Dimming, peaked with the star dimmed by 60%.

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That’s not normal and there were all sorts of explanations getting thrown around for what was going on (many of which revolved around the idea that Betelgeuse was preparing to explode). Some of these theories were highly entertaining for me. I especially enjoyed the one that proposed that Betelgeuse was bent out of shape by a wandering black hole. 

It took months but the star did eventually return to its previous brightness, and in the end (years later) the cause was determined to be far less exotic than a black hole: Betelgeuse burped. Literally. It ejected a gas bubble that, as it cooled, condensed into a dust cloud between us and the star, dimming its light from our viewpoint. That’s one heck of a belch.

Imaginary (or Not) Friends

So back to that whole “variable star” thing. Betelgeuse actually appears to have multiple different cycles of variability running at the same time, because it absolutely cannot do things the easy way. It’s got its major, primary variability cycle, which lasts around 400 days, and then a much longer secondary cycle lasting over 2,000 days.

That’s not necessarily unusual, but when you get a long secondary cycle like that it’s usually because the variable has a companion star going around it. But if a star as easy to see and widely-studied as Betelgeuse had a companion, we would have known about it a long time ago, right?

Ha. Yeah right. If you’ve learned nothing else about this star you’ve learned that it doesn’t like to give up its secrets easily.

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First proposed in September 2024 and then confirmed in 2025, it turns out Betelgeuse was hiding a friend from us this entire time. The companion star, first given the unofficial moniker of Betelbuddy (which definitely should have stuck) and then the more recent designation of Siwarha (which means “her bracelet”, a reference to Betelgeuse’s original meaning), is no tiny spark either.

Siwarha turns out to be sizeable, about 1.5 times the mass of the Sun! Yes, we managed to lose a star even bigger than our Sun in Betelgeuse’s glare. That’s how honkin’ huge and bright the dang thing is. Only last week a study using years of measurements from Hubble and a suite of ground-based observatories were able to detect the wake that Siwarha leaves in Betelgeuse’s outer atmosphere as it orbits the megastar. 

Big, Bright Unknowns

So to sum up: Betelgeuse is a star so bright in our sky that you can see it in the middle of a city and so near and large that we can make out individual features on its surface. Yet we don’t know quite how far away it is, how big it is, or how long until it explodes. We’ve known about its sizeable companion star for less than a year and it took us several years to figure out the thing belched out a dust cloud.

Which is to say that I doubt this star is done giving up secrets and blowing our minds. Ah well, at least you can always just look at it in the sky and appreciate its color and beauty. Unless it decides to burp again. You never know what this star is going to pull next!