Home / Opinion / Columns /  Opinion | Spot the difference, on the Betelgeuse Star

If you take an interest in astronomy, you will quickly rack up a long list of phenomena whose effect ranges from puzzling to jaw-dropping. Mine, for example, comprised black holes and how galaxies move, pulsars and gravitational lensing... and, for a long time, sunspots.

Intriguing indeed, sunspots. I always thought the Sun was this gigantic roiling ball of fire, which, in essence, it is. So, how does a fireball like that have little black spots here and there? Black spots that move across the face of the Sun? This was such a strange idea that I couldn’t quite believe it for a long time; or at least, I thought, only astronomers using some exotic equipment could observe these spots, certainly not we ordinary folks. But one day I managed to project the image of the Sun onto a blank sheet of paper. There on the great round disk, to my delight and astonishment, were three or four little black dots.

But if seeing like that was believing, there was still the conundrum of what causes these spots. It’s some kind of magnetic field activity that we don’t yet fully understand. But what we do know is why they look black, and that’s a marvel in itself. For the truth is that sunspots are not intrinsically black, but they look that way because they are much cooler than the rest of the sun, the material that surrounds them.

Think of this for a moment, because it has some bearing on what comes later in this column. As the minerals and gases in the Sun burn, an intense amount of light is generated, which is why the Sun is so bright. But the burning also produces an intense heat, which is why the Sun is also so hot. How hot? On its surface, the temperature is over 5,500°C. I find it near-impossible to comprehend just how hot that is, except to assume that pretty much anything that you might drop onto the Sun will be instantly incinerated.

So, on that searing surface, what does it mean to have some “cooler" spots? The word is deceptive, because it brings to mind crisp mornings at hill stations when you shiver a little and pull your jacket closer. On the Sun, the word is relative. It’s only compared to the surrounding inferno that sunspots are “cooler"— for they are unimaginably hot anyway: 3,700°C. And, being nearly 2,000°C cooler, they generate less light than the surrounding inferno. But again, this is a relative statement. One estimate is that if you were to extract a sunspot and position it somewhere in our night sky, it would shine as brightly as the full moon: not the Sun, of course, but still pretty bright.

Yet in place on the Sun, these burning, shining magnetic phenomena look black.

One more thing to note about sunspots. They are indeed just “little black dots" on the Sun’s surface, like pimples or blackheads on your face. But again—you guessed it—this is relative. They seem “little" only because the Sun itself is so huge. Sunspots can be as large as 50,000 km across. Four Planet Earths would comfortably fit inside one.

Hold on to all that for a few moments.

Some weeks ago, I wrote here about the star Betelgeuse, in the constellation Orion. Distinctly red, and the 10th-brightest star in the night sky, Betelgeuse is a familiar favourite among stargazers.

But in October 2019, it caused an outbreak of consternation and sorrow, because it suddenly became a whole lot dimmer. By February, its brightness had dropped by nearly two-thirds and it wasn’t even among the 20 brightest visible stars. Was Betelgeuse dying? In fact, given how far from us it is, had it died several hundred years ago and we were only now seeing its death throes? Any other explanation?

There was plenty of speculation, plenty of theories. Two of those got a lot of attention.

One, it was indeed on its deathbed and would soon turn into a supernova. Betelgeuse is a red supergiant star, and we know it is about 10 million years old, which is a typical lifespan for a red supergiant. We also know that such stars typically end their lives in a gigantic explosion. So perhaps we were indeed seeing Betelgeuse readying to go supernova. Except that astronomers estimate that the star might have another 100,000 years left to live; that is, it could be that long before it explodes. But more to the point, in February, Betelgeuse started getting brighter again, and by April 2020, it was back to its familiar brightness. So the deathbed explanation... well, it died.

Two, there’s a cloud of dust obscuring the star. Red supergiants are known for their “pulsations"—meaning they regularly throw off their outer layers. This material is ejected out into space. Since it is no longer aflame, as it was while on the star, it cools and forms a cloud of fine rubble that astronomers refer to as “dust". Had Betelgeuse gone through one of these convulsions, leaving a gigantic cloud of dust between it and us on Planet Earth that obscured our view of the star?

A team of astronomers decided to test this hypothesis, using the Atacama Pathfinder Experiment (APEX) telescope in Chile and the James Clerk Maxwell Telescope (JCMT) in Hawaii. These telescopes are fitted with cameras that detect radiation in the “submillimetre" range; meaning the wavelength of this radiation is just under a millimetre.

This is about a thousand times longer than the wavelength of visible light.

Why choose to observe Betelgeuse at submillimetre wavelengths? Well, imagine that when you’re out on your morning walk, you have to cross a broad stretch of rubble. The stones are like small footballs and are shaky, so that on each step you take, you risk stumbling and falling. How should you negotiate this obstacle without breaking your bones? If you take several tiny mincing steps, there’s a good chance you’ll fall at some point on the way across. But if you instead take a few long strides, you’re much more likely to make it across without a fall. That’s the way to consider these differing wavelengths. Where visible light is blocked by a cloud of dust, longer waves will make it through intact. The astronomers write that the dust is “optically thin" at these longer wavelengths, and that using these wavelengths to observe “avoid[s] the effects of extinction along the line of sight".

What does this mean for our observation of Betelgeuse as it dimmed? We know that the star appeared dimmer to optical telescopes and the naked eye, meaning visible light from the star was diminished. The hypothesis, remember, is that a dust cloud between the star and us is responsible for this. Now, when we “look" at the star at submillimetre wavelengths, if its brightness does not diminish at those wavelengths, that’s evidence to support the hypothesis. Because the submillimetre waves will get through the cloud.

Only, the team found something else: Betelgeuse was dimming even at the longer wavelengths. The star’s brightness at submillimetre wavelengths, they wrote, “is lower during the recent [visible] minimum than it was before". This “argues against models in which the recent optical dimming is caused by the formation of a new cloud of dust along the line of sight to the star". (Betelgeuse fainter in the sub-millimetre too: an analysis of JCMT and APEX monitoring during the recent optical minimum, Thavisha E. Dharmawardena et al, arXiv, 14 June 2020). As one of the authors told the press: “What surprised us was that Betelgeuse turned 20% darker even in the submillimetre wave range." That is: no cloud of dust.

So what did cause Betelgeuse to dim? Well, remember sunspots? This column began with sunspots. The team looked at high-resolution photographs of the star before and during its dimming and found areas on its surface of “varying brightness". Put that together with the submillimetre observations, and the clear inference is sunspots on Betelgeuse. Betelgeuse-spots, actually. Huge Betelgeuse-spots. As another of the authors commented: “(This) is a supergiant star growing a super-sized star spot."

Just like our own sunspots, these are “cooler" than their surroundings, too. At some 3,500-4,000°C, Betelgeuse is already considerably cooler than the Sun. The astronomers estimated that the dimming we observed starting last October can be explained by spots about 400°C cooler still, that cover 50% of Betelgeuse’s surface; or spots about 300°C cooler that cover 70% of its surface. Given Betelgeuse’s size, we know these are not the little dots our Sun’s spots are. Instead, these are like gargantuan blankets that cover an enormous fraction of Betelgeuse, visibly dimming it. (And, as it turns out, not-so-visibly dimming it as well).

All of which might remind you that our more familiar sunspots also dim our Sun, even if you’ve not noticed it. “Dim", you see, is also a relative word.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.

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