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The smoking gun (in a sense, literally) may have been found for the reason why Betelgeuse, everybody's favorite red supergiant star, dimmed so dramatically earlier this year: Dust. A lot of it.
Since 2020 has already been like 75 years along, here's a refresher: Betelgeuse is one of the brightest stars in the sky, located in the constellation of Orion (his right armpit; the name literally means "The Armpit of Orion" or "The Armpit of the Giant"). It's a red supergiant, a colossal star with 15 times the Sun's mass and a diameter of a staggering billion kilometers or more.
It undergoes regular variations in light, brightening and dimming over time. There are a couple of cycles on top of one another; one takes about 420 days (plus or minus), and the other about 2000 days. These variations usually change its brightness by a small amount; you'd really have to pay attention to notice under normal circumstances.
But in late 2019 Betelgeuse took a nosedive, and by mid February 2020 its brightness had dropped by a stunning 70%, so obvious you could just go outside and see it.
Astronomers knew this didn't mean an explosion was imminent — a supernova is when the star's core collapses, but Betelgeuse is so swollenly huge that its core and outer atmosphere are essentially independent of one another — though of course speculation online ran rampant. But still, this was a big mystery. What could cause such a large drop in brightness?
Betelgeuse is so big that observations show it as a disk, and in early 2020 the southern half of the star could be seen to be physically dimmer! One idea is that a huge starspot was covering its surface. Like sunspots, these are cooler, darker regions on a star, and red supergiants are known to be plagued by them. Some of the evidence supported this idea, some didn't.
Another idea is that Betelgeuse blarted out a huge cloud of dust. Red supergiants do this too. Stars like this physically pulsate, with their upper layers expanding and contracting by large amounts (this is what causes the 420-day period; when the star is bigger it emits more light, and when it's smaller it emits less).
So which is it? The mystery now looks pretty conclusively solved: New Hubble observations of the bloated supergiant show strong evidence that its southern hemisphere underwent significant brightening in the ultraviolet right before the sudden dimming event. This is what you expect from a pulse moving upward through the star, the same kind of thing that generates the enormous eructations of dust.
The observations used the Space Telescope Imaging Spectrograph (yay, STIS!), which can see out into the ultraviolet. It takes spectra, where the light is broken down into its constituent colors, hundreds or even thousands of them. This isolates the signatures of various elements in the star's atmosphere that give diagnostics of the conditions there.
STIS observed Betelgeuse in January and March of 2019, which was during the star's normal behavior, then again several times in late 2019 and early 2020, during what's called The Great Dimming. Betelgeuse is so big that the camera could look at just the southern hemisphere, where the dimming later occurred, and that's where it found the UV shenanigans.
The STIS observations also indicate an increase in the density of the gas in the star's upper atmosphere at the same time as the UV brightening. So what it saw was that gas getting warmer and denser: Just what you'd expect from a pulsation wave moving through it.
The authors speculate that convection — hot gas rising in the star's atmosphere — may have triggered a more significant upward flow (a plume), and it coincided with the normal expansion part of Betelgeuse's periodic cycle. This created a massive pulse in the atmosphere, heating and compressing it as it rose. Once it reached the very upper limits of the star's surface (such as it is; the star is so bloated that it just kinda slowly fades away as you move away from it without an actual surface), the gas cooled, formed dust, became opaque, and blocked the light coming from the star below it.
Dust in space comes in two types; silicaceous (rocky) grains, and grains of complex carbon chains called — get this — polycyclic aromatic hydrocarbons. That's basically soot. If this was made in the atmosphere of Betelgeuse and is helping cause the dimming, it's literally like smoke, hence the phrase smoking gun. On the other hand, some work indicates the grains made were bigger than usual, which would imply they were silicates, and my ordnance-based metaphorical phrase falls short. Unless it was a pellet gun. Hmmm.
Anyway, this does seem to all add up to Betelgeuse sneezing/belching/flatulating out a ginormous cloud of dust, big enough and dense enough to block most of the light from the star. Again, given that Betelgeuse is a billion kilometers wide, that's a huge load of dust.
Can this happen again? It seems unlikely, since such a deep dip in light has never been seen before (big drops have been seen, just none like this one). However, some observations have shown that it may already be dimming again! Maybe there was another dust eruption, smaller this time. Unfortunately, this happened while Betelgeuse was too close to the Sun in the sky to observe from Earth (the dimming was seen by solar observatories currently in a different part of Earth's orbit, with a very different view).
We may know better soon enough; Orion is just now starting to rise early enough in the dawn to get observations again. Stay tuned! Orion's armpit may just be getting started.