Why Betelgeuse, A Star In Orion, Is Getting Dimmer Okay, it wouldn't technically be an explosion. And if it's "about" to happen, it already happened. About 650 years ago. We'll explain, with astronomer Emily Levesque, who studies massive stars at the University of Washington. Follow Short Wave's Emily Kwong on Twitter @emilykwong1234. Email the show at shortwave@npr.org.
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A Star In Orion Is Dimming. Is It About To Explode?

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A Star In Orion Is Dimming. Is It About To Explode?

A Star In Orion Is Dimming. Is It About To Explode?

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EMILY KWONG, HOST:

One of the brightest stars in the night sky is named Betelgeuse. It's about 650 light years away, which is pretty close in outer space terms. And if you've gazed up into the night sky and seen the constellation Orion, you've seen Betelgeuse before.

EMILY LEVESQUE: So if you were to look up at it, you would want to start by finding the three stars that make a nice, little line that we call Orion's belt. And then Betelgeuse is, as you're looking at it, the shoulder of Orion on the left.

KWONG: Emily Levesque is an astronomer at the University of Washington who studies stars like Betelgeuse, which is known as a red super giant - super giant because this star is enormous, much bigger than our sun.

LEVESQUE: If you were to put Betelgeuse where our sun is, it would swallow up all of the planets out past Mars. And because it's so massive, it means that it goes through a very different sort of life experience than our Sun will.

KWONG: Which brings us to why we're talking about Betelgeuse right now. In recent weeks, astronomers have noticed that Betelgeuse no longer appears to be one of the brightest stars in the night sky.

LEVESQUE: There were sort of quick reports put out from people who monitor and observe Betelgeuse very frequently saying, you know, it's getting dimmer and dimmer. It's starting to get closest to the dimmest we've seen. There's also big, dedicated networks of amateur astronomers that keep very close track of the brightness of stars like Betelgeuse. And they started noticing the same thing.

KWONG: When we called Emily, she was preparing for this big astronomy conference in Hawaii. And she thought there would be a lot of buzz there about the dimming of Betelgeuse.

LEVESQUE: Yeah, Betelgeuse is going to be a big topic, I'm sure.

KWONG: Especially for Emily. She told us Orion is her favorite constellation. But actually, you don't have to be a pro astronomer to see what's happening with Betelgeuse. You can look for yourself. At one point, Betelgeuse was one of the brightest stars in Orion but now not so much.

LEVESQUE: So if you were to go up and look at it tonight, it's dimmer than the star in Orion's right knee, which is Rigel. And it's about equal in brightness to Orion's other shoulder, which is a star named Bellatrix. So the fact that we can see with our eyes that it got noticeably dimmer really caught a lot of people's attention and then spiraled as just a wow, this is a really interesting and compelling thing changing sort of on our timescale in the night sky.

KWONG: So what's going on? As we'll explain with help from Emily Levesque, scientists have a few theories for why Betelgeuse appears so dim. And the most dramatic explanation is that this star could be about to die - what's known as going supernova.

LEVESQUE: It would look pretty epic.

KWONG: I'm Emily Kwong, filling in for Maddie Sofia today. This is SHORT WAVE, the daily science podcast from NPR.

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KWONG: So here's the thing. There's been quite a bit of speculation that the reason Betelgeuse is getting dimmer is that it's about to go supernova. That's the big, explody end to the lifespan of a massive star. And while dimming can mean that's about to happen - for reasons that we'll get into - it's not the most likely scenario for Betelgeuse. But first, we had to clear up something with astronomer Emily Levesque.

Is this very cool star named after the 10th-highest-grossing film of 1988?

LEVESQUE: I'm pretty sure that it's the other way around. The spelling is different.

KWONG: Right.

LEVESQUE: And you'll sometimes hear astronomers pronounce it a little bit differently. We'll say Betelgeuse instead of Beetlejuice three times. But it's actually derived from a Arabic name. And there's, I think, some disagreement on what exactly it means. But it either means, you know, the arm of Orion or the hand of Orion or the hand of the Hunter because the total constellation is - looks like a person hunting.

KWONG: It's the only - can I be honest? - it's the only constellation I can ever successfully identify...

LEVESQUE: (Laughter).

KWONG: ...'Cause that belt - it's very telling.

LEVESQUE: So I'll admit it's the easiest constellation for me to identify, too. This is one of those well-kept secrets of astronomy. A lot of us are embarrassingly bad at finding things in the night sky because we're used to looking at things that are so dim that you can't see them with your own eyes. And our telescopes have amazing computers that can help us find things. So we'll occasionally go out and look up and do just what a lot of people do and find, like, that familiar, little line of three or another easy constellation to get our bearings.

KWONG: So let's talk about how astronomers such as yourself, people who really study these stars, have noticed something different about how Betelgeuse looks in the night sky. How does it look different?

LEVESQUE: So I will say we've been monitoring the brightness of Betelgeuse for decades. And we have been measuring its brightness very frequently. And we've seen its brightness change with time. So we've watched it get brighter and dimmer. This just caught people's attention because it was close to the dimmest that Betelgeuse has ever been.

KWONG: And what could dimming like this indicate?

LEVESQUE: So our guess right now is that what we're seeing is a combination of a few behaviors that we see in red super giants and that we've seen before in Betelgeuse that just happened to be coinciding. So we know that stars like Betelgeuse have big sort of boiling, convective (ph) cells near their surfaces. So you'll sort of get a bright, hot spot and a slightly dimmer, cool spot. And it's entirely possible that this dimming is due in part to those convective cells. We also know that stars like Betelgeuse will actually shed off some mass from their outer layer. So they'll sort of puff off the outermost layers of the star. When that mass hits the interstellar medium, it'll condense into what we call dust. And dust in space kind of does what dust does here. It blocks light and gets in the way and can be a little bit of a nuisance, but it would make the star look a little bit dimmer if it then had a little veil of dust around it. We also know that stars like this can pulsate a little bit. So their outer layers will sort of squeeze in and puff out just due to instabilities in those layers. And that'll also affect how bright the star looks. So I think the current guess is that we're seeing a couple different behaviors in Betelgeuse that on their own aren't too strange, that just happened to be coinciding to make the star look especially dim.

KWONG: So just as a thought experiment, say Betelgeuse is going to go supernova. How would we know, and what would it look like?

LEVESQUE: So first of all, the light that we're seeing from Betelgeuse was emitted by the Star about 650 years ago. Betelgeuse is 650 light years from Earth. So when the light emerges, it comes toward us as fast as it can, but it's moving at the speed of light. So looking at Betelgeuse is a little bit like looking back in time to what the star was actually doing 650 years ago. In terms of whether we will see Betelgeuse go supernova in our lifetimes, Betelgeuse and other massive stars like this kind of follow a live fast, die young philosophy. So they live about 10 million with an M years. Betelgeuse in particular, we know, is moving into a later stage of its life because it is so big and so red. But that could mean that we still have 100,000 years before it dies and produces a supernova. If it did, though - if, say, we all went outside tomorrow and we were seeing the light arrive from Betelgeuse dying as a supernova 650 years minus a day ago.

KWONG: Sometime in the Middle Ages, let's say.

LEVESQUE: Yeah.

KWONG: OK.

LEVESQUE: It would look pretty epic. We have some records of other supernovae that have happened in the Milky Way. And their appearance is incredibly dramatic. What we would see is Betelgeuse getting brighter and brighter because we'd be seeing the incredibly bright signature of the supernova explosion. It would actually get so bright that if Betelgeuse was up during the day, we'd be able to see it during the daytime alongside the sun. And it would last for weeks. And I think that if Betelgeuse were to go supernova tomorrow, and we saw it at night, it would be comparable in brightness, I think, to the full moon.

KWONG: Wow.

LEVESQUE: We'd be able to see our shadows based on the light from the supernova.

KWONG: OK. So what would it look like for Betelgeuse? Is it more of a going - collapsing inward on itself? Or is it more of a exploding outward with star debris scattering across the universe? What does it look like for Betelgeuse?

LEVESQUE: It's a good question. And we think that it's a bit of both.

KWONG: OK.

LEVESQUE: The initial disruption comes when the core of the star collapses. And depending on the type of star and how much mass is in that core, it'll collapse into a neutron star or a black hole. After that collapse, all the outer layers of the star come falling in toward the core and then bounce back off in a sort of rebound shock. And that shock is what we see as a supernova and what we would call a supernova because we see this outward blast of material, you know, new gas slamming into the interstellar medium and getting really bright. And it looks to us like an explosion. But it originally did start as a collapse. It's why I try to avoid saying that a star exploded as a supernova because it's not - to be the pedantic scientist - it's not quite the first thing that happened in the star.

KWONG: Right. But it's a bit more rock 'n' roll as a turn of phrase (laughter).

LEVESQUE: Oh, yeah.

KWONG: How would you - so this star is a part of your favorite constellation, Orion. And how would you feel if, indeed, it were going supernova?

LEVESQUE: I would be psyched.

KWONG: (Laughter).

LEVESQUE: And I think some people expect that we would be very sad. But it's a very exciting transition to watch. And this would be one of the best-studied stars we have available to us producing a supernova, which right now is a process that we are still trying to understand. We'd still be able to see the supernova as it happened and then faded away. These stars also leave behind what we call supernova remnants. So they're these beautiful, multicolored gas clouds that show us the dissipating material from the star. So it would be this amazing font of data and new ways to understand stars. So I think it would be incredibly exciting.

KWONG: Emily Levesque at the University of Washington. This episode was produced by Brent Baughman and edited by Viet Le, with special help from Geoff Brumfiel and fact checking by Emily Vaughn. I'm Emily Kwong. We're back tomorrow with a new episode of SHORT WAVE from NPR.

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