We chat with astronomy graduate student Daisuke Taniguchi about his clever method of looking back in time at the peculiar behavior of the star Betelgeuse, which will (relatively) soon explode and disappear from our skies.

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ERIC: From the Museum of Science in Boston, this is Pulsar, a podcast where we hunt for answers to the brightest questions we've ever gotten from our visitors. I'm your host, Eric, and as I've mentioned in several past episodes, we get asked about stars exploding...a lot. This is known as a supernova. We've talked about why stars explode, whether planets orbiting exploding stars can survive such a cataclysmic event, and even our namesake, pulsars, are the remnants of stellar explosions. But another question we get quite frequently is, will any of the stars in our night sky explode anytime soon? Most of the stars that make up our constellations are just like the sun - they won't ever explode. Only the most massive stars in the universe go supernova. But the 10th brightest star visible from Earth is Betelgeuse, a red supergiant that will indeed end its life with a bang. It's not spelled the same as the 1988 Tim Burton movie, although it is the inspiration for the main character's name. And although we don't expect Betelgeuse the star to explode anytime soon on a human timescale, ut has made headlines in the last few years with its strange behavior, causing astronomers to wonder: is it about to reach the climactic end of its lifecycle? My guest today is Daisuke Taniguchi, a graduate student in astronomy at the University of Tokyo, who recently discovered a fascinating way to track the behavior of Betelgeuse back in time and solve the mystery of its perplexing behavior. Daisuke, thank you so much for joining me all the way from Japan. I know it's late evening there, and early morning here. Welcome to Pulsar.

DAISUKE: Thank you so much for inviting me.

ERIC: So let's start with some facts about the star Betelgeuse. How big is the star?

DAISUKE: The radius of Betelgeuse is about 800 times that of the sun. And this corresponds to the distance between the Sun and the Earth. So if there is a Betelgeuse at the position of the sun, the Earth is inside Betelgeuse. So Betelgeuse is a very, very large star.

ERIC: So enormous. But it's also a red star. And we usually say at the museum that the color of a star tells us its temperature. So how hot is Betelgeuse compared to the sun?

DAISUKE: Betelgeuse is a cool star, and its temperature is about 3,600 Kelvin.

ERIC: So a lot cooler than the sun.

DAISUKE: Yes, much cooler than the sun.

ERIC: And about how far away is it? Is it in our neighborhood?

DAISUKE: In fact, it is one of the nearest red supergiant stars, but the distance is not so near. Betelgeuse is located at around 500 light years away from the Earth. And this distance is similar to another red supergiant star Antares.

ERIC: So, it's pretty close for stars that explode, but it's not going to explode and affect us directly. We don't have to worry about it.

DAISUKE: I hope so!

ERIC: Alright, so can you talk about Betelgeuse's great dimming that started in 2019. Exactly what happened?

DAISUKE: Usually the brightness of Betelgeuse changes with time, but only a bit. And in contrast to this usual variability, Betelgeuse started to dim in late 2019. This drop in the brightness of Betelgeuse by 70% is called the great dimming of Betelgeuse.

ERIC: So 70%! That is a really big amount. So it's something that you could go out and see in the night sky and notice that that star is not as bright as it used to be.

DAISUKE: Yes, we can see that. Usually Betelgeuse and another star in the Orion constellation, Rigel, are almost a similar brightness. But during the great dimming Betelgeuse was much fainter than Rigel.

ERIC: So what did astronomers think could be the causes of this dimming?

DAISUKE: It has been well known that the great dimming of Betelgeuse is partly due to its drop in surface temperature. However, it is under debate what is the other part of the cause of the great dimming. Several scenarios have been proposed. And the first one is dust varying scenario. In this scenario, a dust cloud formed just above the surface of Betelgeuse. And this dust cloud veils the light from the surface of Betelgeuse. The next scenario is a temperature inhomogeneity scenario. It is known that there is a temperature inhomogeneity in the surface temperature of Betelgeuse. And in the second scenario, it was proposed that the surface inhomogeneity increased during the great dimming.

ERIC: So we think it definitely got cooler. And it could have possibly had some dust just above its surface to block it out. And it could have had this spot on its surface to make it seem even more dim. And it was really difficult to tell what those were because it's difficult to observe them from the Earth's surface. Because the light that would tell us about these clues, it gets absorbed by our atmosphere. And so we would need to have been looking at the star with telescopes in space. And we just weren't doing that. Or so we thought, until your team came up with a really clever way to kind of look back in time at Betelgeuse. So how did you come up with a way to answer this question of what happened?

DAISUKE: We first thought about the fact that the Himawari-8 satellite, which is a Japanese geostationary meteorological satellite, can be used as a space telescope to observe some stars. At this first stage, we did not think about Betelgeuse. Then we searched which stars can be observed with Himawari-8, and theorized that Betelgeuse can be observed with Himawari. Then we realize that this project seems very interesting. And we started to use Himawari-8 to investigate great dimming of Betelgeuse.

ERIC: I love that, because it's data from a satellite that was not meant to look at the stars at all. The stars just happened to be in the pictures. It was supposed to be used for weather, and it was. But since that data was available and can be shared, you were able to find a way to use it for astronomy, even though it's not a telescope. That's really amazing.

DAISUKE: Thank you so much. We are so proud of our new concept of using meteorological satellites as space telescopes.

ERIC: So using this data from weather satellite, what did that tell you about what caused the dimming?

DAISUKE: Previous works mainly used the data from optical or near-infrared light. Only with this data, it cannot be said whether the dust formation scenario contributed to the great dimming or not. And in contrast, we focus on the light from the mid-infrared wavelength range. In this wavelength range, around ten micrometer, we can see the light that is emitted by the circumstellar dust. And using this light from the circumstellar dust, we can determine the amount of dust around the surface of Betelgeuse. We made a catalog of the time variation of the amount of dust around Betelgeuse. And when looking at this data, we found that the amount of dust actually increased during the great diming phase. So the great dimming was partly contributed to by the dust formation scenario, together with the temperature decrease scenario.

ERIC: So you could say for sure that it was both?


ERIC: Not only did the solve this mystery that goes back a couple of years. But you said at the end of your paper that this information could help us understand how massive stars explode at the end of their lives in general.

DAISUKE: The dust formation and subsequent mass loss around the red supergiants affects their evolutionary paths to supernova and also the light curve of the supernovae after its explosion. Therefore, basic understanding of this process, including dust formation and the mass loss of red supergiants, we think is important for understanding supernovae.

ERIC: Knowing how that mass gets thrown off from the star and becomes a dust cloud, how that happens, when that happens, what timing, all of that goes into understanding the process of the end of a star's life. That's fascinating.

DAISUKE: Yes, that's right.

ERIC: When do you think Betelgeuse will explode?

DAISUKE: It's a very, very good question. And as far as I know, astronomers do not know when. And according to a paper published two years ago, it will take more than one million years for Betelgeuse to explode. But the theory always changes. So it is possible that tomorrow, Betelgeuse will explode.

ERIC: Right. And since it's over 500 light years away, it could have exploded hundreds of years ago and we would just find out about it when the light got here, right?

DAISUKE: Yes, that's right. If the Betlegeuse exploded five hundred years ago, then we will see the explosion of Betlegeuse tomorrow.

ERIC: Daisuke, thank you so much for joining me and telling me all about your research.

DAISUKE: Thank you so much.

ERIC: On your next visit to the Museum of Science, catch a show at the Charles Hayden Planetarium and ask the knowledgeable staff all about exploding stars. While you're at home, subscribe to our YouTube page where you can find videos covering the history of astronomy and the groundbreaking scientists like Annie Jump Cannon who furthered our understanding of stars and their life cycles. And to catch a glimpse of Betelgeuse from New England, look to the southern sky in winter for the three distinct stars of Orion's belt, and just above them you'll find the red supergiant. Until next time, keep asking questions.

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