Betelgeuse became a star in more ways than one recently when scientists and backyard astronomers noticed the red giant had dimmed noticeably over the past year. Among the speculations for what was causing the dimming was the obvious: Betelgeuse was reaching the end of its lifetime and would soon explode catastrophically into a supernova. But scientists at Lowell Observatory in Flagstaff, AZ and the University of Washington have a different idea based on their research. According to them, Betelgeuse isn’t going to explode—it’s just a little dusty.
Emily Levesque, a University of Washington associate professor of astronomy, and Philip Massey, an astronomer with Lowell Observatory, spoke about the dimming of Betelgeuse and agreed that one way to figure out what was going on with the giant star was to get a spectrum of the electromagnetic radiation it was emitting. By looking at the types of radiation the star lets off, scientists can calculate its temperature. They could then compare this reading to a reading that was done in 2004 to see if the star had cooled at all. That would allow them to connect its dimming to cooling temperatures on the star.
How would cooling darken the star? Stars have convection cells that circulate hot, less dense material from the interior of the star up to the surface, where it cools, becomes denser, and then flows back down to the interior. Our sun has these, but they are small (about Texas-sized) and numerous. Red supergiants, which are cooler than our sun and have weaker gravity, only have a few convection cells—three or four over the star’s surface—but they are MASSIVE. A hotter star like our sun would have material from its core coming up that is so hot it wouldn’t darken significantly as it cooled on the star’s surface. But if the star was cooler, that material would have low enough temperatures where it would appear darker on the star’s surface, and therefore make the star look dimmer.
Getting a usable spectrum was easier said than done. Betelgeuse, even though it’s dimmer than usual, is still one of the brightest stars in the sky. It can be hard to get a detailed spectrum of a bright star because radiation levels are so high. It’s difficult to see the differences between one type of radiation and another. Massey solved this problem by using a type of filter to selectively allow certain types of radiation to come through to be read. Then they could search the spectrum for telltale signs of titanium dioxide, a molecule that often accumulates in the upper layers of the “cooler” (for stars) red giants. This would help the scientists to calculate the average surface temperature of Betelgeuse.
After examining these telltale signs of titanium dioxide and calculating the star’s temperature via a reading taken on February 14, 2020, Massey and Levesque compared their answer to the temperature that was calculated in 2004. They found the star is only 50-100 degrees Celsius cooler now than it was then. In the terms of star temperatures, that’s barely a difference, and certainly, the temperature is higher than what would cause a loss of light. According to these temperatures, Betelgeuse is no closer to going supernova than it was before it started dimming, and its dimming is not due to the convection cells described above.
So if the change in brightness isn’t because of cooling, what is causing it?
“We knew the answer had to be dust,” says Massey.
Dust isn’t an unusual occurrence around red supergiant stars.
“We see this all the time in red supergiants, and it’s a normal part of their life cycle,” says Levesque. “Red supergiants will occasionally shed material from their surfaces, which will condense around the star as dust. As it cools and dissipates, the dust grains will absorb some of the light heading toward us and block our view.”
Betelgeuse started to dim around October of 2019, but in fact, recently, the star has begun to brighten very slightly once more.
Why is all of this important? Well, like many other things in science, studying red giant stars and learning about their regular patterns of behavior can help us predict things for the future.
“The more we can learn about their normal behavior,” says Massey, “the better we can understand them and recognize when something truly unique, like a supernova, might happen.”
Betelgeuse will still explode into a supernova someday when its core collapses, but that day is still estimated to be about 100,000 years away. When it does, it’s bound to be a hell of a light show, shining as bright as the quarter moon in the sky, but concentrated in a smaller point. It will be visible during the day (for about a year), and at night, it will cast shadows. But we won’t have to worry about much beyond that. The star is far enough away that other impacts won’t really affect Earth or life upon it.
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