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Clues from Distant Betelgeuse

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A few weeks ago I wrote about the “Betelgeuse problem” which asks the question: why is it dimming? A lot of people are really in love with the idea that it’s going to blow up soon. I have to admit, it’s a fascinating thought, that we would get to see a giant star blow itself to smithereens from a safe distance of about 700 hundred light-years. But, it’s probably not what’s going to happen. At least, not just yet.

Taking a Direct Look at Betelgeuse

Figuring out exactly what IS happening with this star is complex. We know it’s an old star. Those aged, dying supergiants get a little eccentric in their final epochs of life. Astronomers have a general idea of what’s going on inside Betelgeuse, but that doesn’t explain why its appearance is changing. It could be due to some kind of stellar activity affecting its appearance and apparent size and shape. Could the surface be cooling for some reason? If so, what would explain that?

Another idea is that the star is ejecting dust away from itself. It ends up as clouds in the line of sight between us and the star. Such clouds could affect the star’s appearance pretty drastically. They would also absorb some of the light, which makes it look dimmer.

To figure this out, we need direct images of the star. So, astronomers used the Very Large Telescope in Chile to look directly at the star. They deployed special instruments to get a direct view of its surface in visible light as well as infrared light. Of course, Betelgeuse, like the Sun, is a massive ball of superhot gas. So, there’s not really a surface like we have on Earth. On the Sun, the “surface” is the “photosphere”. We can “see” all the way through the solar atmosphere to that region, and below that, we can’t see because it’s opaque to visible light.

December’s View of Betelgeuse

The “surface” on Betelgeuse is a similar region. The VLT showed astronomers the surface, and also let them study the material surrounding the star. That cloud of “star stuff” has been ejected from Betelgeuse in its later life. The December images of Betelgeuse show it not as a rounded sphere of hot gas, but a somewhat squished-looking thing. The infrared images of the dust look even stranger, showing clumps of star-stuff nearby the star.

The red supergiant star Betelgeuse, in the constellation of Orion, has been undergoing unprecedented dimming. This stunning image of the star’s surface, taken with the SPHERE instrument on ESO’s Very Large Telescope late last year, is among the first observations to come out of an observing campaign aimed at understanding why the star is becoming fainter. When compared with the image taken in January 2019, it shows how much the star has faded and how its apparent shape has changed. Credit:
ESO/M. Montargès et al.)

That image of dust around Betelgeuse may be providing an important clue to understanding the dimming we’re seeing at the star. It could well be that those dust clouds are playing a role in blocking light from the star. That could be why it appears to be getting dimmer.

This image, obtained with the VISIR instrument on ESO’s Very Large Telescope, shows the infrared light being emitted by the dust surrounding Betelgeuse in December 2019. The clouds of dust, which resemble flames in this dramatic image, are formed when the star sheds its material back into space. The black disc obscures the star’s center and much of its surroundings, which are very bright and must be masked to allow the fainter dust plumes to be seen. The orange dot in the middle is the SPHERE image of Betelgeuse’s surface, which has a size close to that of Jupiter’s orbit. Credit: ESO/P. Kervella/M. Montargès et al., Acknowledgement: Eric Pantin)

Betelgeuse’s Future

The ejection of dust from older (and dying) stars, called “mass loss”, is one part of Betelgeuse’s aging process. It’s actually part of what many stars experience. The Sun itself will go through a period of mass loss as it ages, and will gently blow its atmosphere out to space. In the case of Betelgeuse, it may go through several epochs of mass loss. Eventually, it could end up with several surrounding shells of gas and dust. When it does go through the final collapse (in perhaps 100,000 to a million years), the outbursting material from the torn-apart star will encounter those shells. It’ll heat them up, cause them to sparkle and glow, and make the death of Betelgeuse quite spectacular.

But, for now, if the images from VLT are to be believed, we’re still in for some episodes of mass loss. They’ll continue as Betelgeuse makes its way through old age. It’s actually pretty cool that astronomers can capture views of the star at this point. What they see gives a much better idea of the things that happen to supergiant stars before they explode as Type IIa supernovae and light up our night sky.

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The Death March of Betelgeuse

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A Cloudy Precursor to a Violent Stellar End

You can’t see it in the evening skies right now, but the bright, old star Betelgeuse that makes up one of the shoulders of Orion, the Hunter (visible beginning late in the year), is giving up more of its secrets even as it continues down the road of old age and eventual disruption by a supernova explosion. Betelgeuse is a red supergiant star.  It’s so big that if you placed it in our solar system in place of the Sun, its “surface” would be out at the orbit of Jupiter.  But, Betelgeuse’s influences stretches far out beyond that.  Why?  You have to understand something about this big old star. It’s big. It’s old. And when big, old stars get older, they shed much of their material out to space in an intense stellar wind. In the final step of aging, such stars can lose as much as one solar mass (that is, the amount of mass it takes to make the Sun) in just about 10,000 years.

For Betelgeuse, scientists describe this mass loss as two processes:  the first occurred when huge plumes of gas began to snake their way out from the star into nearby space; the second one involves giant bubbles in the star’s atmosphere. Those bubbles move up and down through the atmosphere quite vigorously, similar to boiling water in a pan.

This picture of the dramatic nebula around the bright red supergiant star Betelgeuse was created from images taken with the VISIR infrared camera on ESO’s Very Large Telescope (VLT). This structure, resembling flames emanating from the star, forms because the behemoth is shedding its material into space. The earlier NACO observations of the plumes are reproduced in the central disc. The small red circle in the middle has a diameter about four and half times that of the Earth’s orbit and represents the location of Betelgeuse’s visible surface. The black disc corresponds to a very bright part of the image that was masked to allow the fainter nebula to be seen. Courtesy ESO/P. Kervella.

How do we know that this is what Betelgeuse is doing?  For one thing, astronomers have been able to image the plumes of material blowing away from the star. They used an instrument called VISIR (an infrared-sensitive camera) attached to the European Southern Observatory’s Very Large Telescope in Chile to measure the extent of the clouds of material coming off Betelgeuse. They found an interesting structure to the clouds (see picture at left). It almost looks like flames licking out from the star.  They’re not fire, but warm streams of  “star stuff” blowing away from Betelgeuse.

The astronomers’ observations show the plumes that are close to the star are probably connected to structures in the outer nebula now imaged in the infrared with VISIR. The nebula cannot be seen in visible light, as the very bright Betelgeuse completely outshines it.

Notice that the clouds of material are irregularly shaped, not symmetrical. This also tells astronomers that Betelgeuse hasn’t been losing its material at the same rate in all directions. In other words, the loss is not symmetrical.  This is indirect evidence that the bubbles in the atmosphere and their plumes are responsible for the nebula’s appearance.

So, what is this material that’s flowing away from Betelgeuse?

Based on the observations, it’s most likely that this stellar stuff is composed of silicate and alumina dust. This is the same material that forms most of the crust of the Earth and other rocky planets.

This is kind of interesting. Think about it.  It means that at some time in the distant past, the silicates that make up Earth were formed by a massive (and now extinct) star similar to Betelgeuse. It’s interesting to see evidence for that now, but in a star that is at least several hundred light-years away from us (possibly farther).

Now, you’re probably wondering when Betelgeuse will finally go supernova.  A good question. In cosmic timekeeping, it could be anytime, meaning anytime in the next million years. Stars die on lengthy timelines.  And, its distance will keep us from knowing that it happened until a few hundred years after the initial explosion. So, if Betelgeuse is, oh, say 500 light-years away (and we don’t know for sure how far away it is, so I’m using that number as an example), and it blows up tomorrow, we won’t see that flash in our skies until the year 2511.  We’ll probably see an influx of neutrinos before that, emanating from the direction of Betelgeuse. Eventually, sky observers will see it start to get very large and bright in the sky, and once the initial flash dies out, they’d start to see a colorful, glowing nebula where Betelgeuse used to be.  It would be a bright source in radio and x-rays, a new “thing” to study in the annals of violent star death.

For now, however, astronomers are marking the progression of Betelgeuse’s “change of life” events by observing it as much as they can, in as many regimes of light as they can. Those continue to tell the story of this star’s inevitable death march.