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Home >Science >News >Opinion | Notes on the fainting of a red supergiant star

Last night, I dreamt I was on my terrace again. Though I haven’t really needed to dream that, because I’ve been climbing up there regularly in these corona weeks, especially at dusk or later. There’s a simple reason: With so many fewer cars on our streets, the air is noticeably cleaner and this makes for a gorgeous night sky. And the dream served as a hint to write, for a change, about something other than corona numbers, but that is still, in some ways, corona-wrought.

Thus the night sky.

Caveat: The word “gorgeous" needs some perspective. One recent night, I counted the stars I could see. You regular admirers of night skies are already shaking your heads in pity, I know, for which of you would even begin to count those thousands of little twinklers? But in Bombay, it’s an exercise that can be attempted. You’ll laugh out loud at my count that night: 26, and that includes the planet Venus. Yet in this city, that does qualify as “gorgeous", because that’s about 15 more stars than I would have seen on any given pre-corona night. That’s the measure of what Bombay’s bright lights and its polluted air have done to the night sky. So, if I can see 26 stars, for this city that is a red-letter day. Or night.

Venus has been particularly easy to see: Easily the brightest object in the late evening sky apart from the Moon and, on some nights, prettily aligned with the crescent Moon. We peered at it once through a telescope and were delighted to find it was a crescent, too, nicely matching the nearby Moon. Yes, being a planet that we can see only because it reflects light from the Sun, Venus has its phases just like the Moon does.

Apart from those two, Orion the Hunter has also been a friendly presence above in the early night hours. Orion is, of course, arguably the most familiar and recognizable constellation out there. There’s the large sort-of rectangle that forms his body and the three stars that define his belt. He has a sword too, that hangs faintly from the belt. One of the stars you think you see in that sword is not actually a star. It’s the Orion Nebula, a vast cloud of dust and gas inside which, like some cosmic witch working on her brew, stars are being moulded and formed.

But it’s one of the stars that form the rectangle that this essay is about: Betelgeuse, on Orion’s right shoulder, his second-brightest star and the tenth-brightest in the sky.

Betelgeuse has long fascinated stargazers, because of its colour: a distinct red-orange. This is because it is a “red supergiant"—among the largest and intrinsically brightest stars there are. How intrinsically bright? The astronomical term for that is “luminosity", and Betelgeuse’s luminosity is about 100,000 times that of the Sun. How large? Well, if you replaced our Sun with Betelgeuse, Mercury, Venus, Earth, Mars and possibly even Jupiter would vanish beneath its surface.

The size comparison, incidentally, has always delighted me, because it offers some real perspective on our place in this universe. It goes like this: Our planet seems enormous to us, but it is a mere pebble compared to Jupiter: You could fit over a thousand Earths into Jupiter. But wait, you could fit a thousand Jupiters into the Sun. Then again, you could fit—wait for it—about 700 million Suns into Betelgeuse. Now we’re talking: some real size here!

Of course, there are stars in our own galaxy that would dwarf even Betelgeuse. Of course too, when we start considering galaxies themselves and then the distances between them, Betelgeuse becomes vanishingly tiny. But if it is difficult to comprehend all this, for me Betelgeuse has always been a marker of the kinds of distances and scales I need to start understanding if the universe is to start making sense. That’s what a vision of a star that swallows up most of the planets in our solar system will do.

Size, though, is only one of the interesting things about Betelgeuse. It is also much cooler than the Sun. Though you should remember that “cooler" is a relative term. The surface temperature on Betelgeuse is about 4,000°C, a few degrees warmer than you and I would be comfortable with. Compare that to the 6,000 degrees on our Sun, though, and “cooler" fits well. All of which might make you wonder: If it is so much cooler than the Sun, why is it so much brighter? Precisely because it is so enormous compared to the Sun. Think of a 20 watt LED bulb — bright enough to light up a room, certainly. But compare it to a 10 watt bulb that’s the size of the Wankhede Stadium. Which of those light sources would strike you as more luminous? Well, that’s the way to think of the Sun and Betelgeuse.

One more thing about this intriguing star, before I come to why Betelgeuse has been in the news of late. Red supergiants tend to be young stars with a short life, and Betelgeuse fits that description. Yet again though, remember that “young" and “short" are relative terms. Betelgeuse is about 10 million years old—born not much earlier, certainly, than when silent films became talkies—and again, compare to the Sun, which is about 4.6 billion years old. Nearly 500 times older than Betelgeuse. Betelgeuse won’t last much longer, either; there’s no chance of it getting to be billions of years old. Supergiants evolve quickly because of their size, but more about that another time.

Betelgeuse has always been an attraction to amateur and professional astronomers, as well as ordinary folks who simply enjoy the night sky. But last October, it began attracting much more attention, including news reports. Why? Because out of the blue that month, this red shoulder of Orion began to grow visibly dimmer. In two months, it was no longer the 10th-brightest in the sky. In a bulletin— appropriately titled Updates on the ‘Fainting’ of Betelgeuse —the Villanova University professor Edward Guinan pointed out that it now wasn’t even among the brightest 20 stars in the sky. By February this year, it was only a third as bright as it had always been and stargazers around the world were almost in mourning. Was a beloved constellation now going to look totally different? Were these Betelgeuse’s death throes we were witnessing?

Well, possibly yes, possibly no. For one thing, in late February Betelgeuse started getting brighter again. So perhaps there is another explanation for its dimming than an approaching death. Betelgeuse’s brightness has varied in the past, if not quite as dramatically, and there are other ways supergiants’ can dim and brighten again.

But even so, astronomers are certain that Betelgeuse is nearing its end anyway. Nearly 10 million years is a good long life for a red supergiant. All stars eventually burn through the material in their core, but massive ones do so faster. When there’s little or nothing there left to burn, Betelgeuse’s own gravity will cause it to collapse and eventually explode, turning into what’s called a “supernova". This is what astronomers thought might be happening as Betelgeuse dimmed, and so they looked forward to seeing that supernova. It promises to be quite a show, actually. Betelgeuse is close enough to the Earth that every stargazer on the planet will be able to see the explosion without a telescope. In fact, the star will glow as brightly as the Moon for months, actually casting shadows, visible even in the daytime.

A delicious prospect, right? But before you get excited and station yourself under the night sky, waiting for celestial fireworks, I’m going to end with two more things we know about Betelgeuse.

First, it is about 700 light years away. That’s a nearly incomprehensible distance. In cosmic terms, though, Betelgeuse is practically in our pockets, if you consider that our Milky Way galaxy is 100,000 light years across. Still, 700 years is how long light from Betelgeuse will take to reach us.

Second, it’s not just distance that we have to think about differently. Phrases like “nearing its end" and “death throes" take on new meaning when we’re talking about stars.

Yes, Betelgeuse is near the end of its life. Astronomers believe that end will come sometime in the next 100,000 years.

So, if Betelgeuse has indeed gone supernova, sure we might see the show tomorrow or next week. But we might have to wait 700 years. We might have to wait 100,000 years.

Whichever it is, please go out and look up at spectacular Orion now—while corona gives us clear skies, before Betelgeuse vanishes forever.

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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