Welcome back to my blog "Mystery of Galaxy". There has been a lot of buzz lately about the star Betelgeuse and whether or not it's about to go supernova. Well the short answer is yes, but probably not or up to a hundred thousand years or so.
So what is it that has everybody so excited?
Well there's quite a bit going on with a star and we're going to talk about that in depth in today's article. Now Betelgeuse has been acting up lately or more accurately it's been acting down if anything. Betelgeuse normally has a visual magnitude of 0.5 although it does vary by less than one magnitude because it is a variable star. Then starting around Thanksgiving it began to drop below its typical low brightness. On December 8th 2019, Ed Guinan from Villanova University sent an astronomical telegram stating that Betelgeuse had faded to magnitude 1.1 -that at the time was a record low but then on December 23rd Guinan sent another telegram stating that Betelgeuse had dropped to magnitude 1.3. And as of this filming it's dropped even more all the way down to about magnitude 1.5! Now putting all of this another way Betelgeuse has lost at least half of its typical brightness over the course of just a couple of months! Now that's a rapid drop-off in light and now it's the faintest that's been recorded using modern photometric techniques. Naturally this got some people wondering if this was somehow the calm before the supernova storm. However it's much more likely that Betelgeuse probably won't explode for another 10,000 to perhaps 100,000 years. Now that is quite soon, astronomically speaking, but we're certainly not talking about anything happening within our lifetimes. Although we'd love to be wrong about this.
The closest supernova in living memory was SN 1987A which erupted on February 24th1987.
But that supernova was in the Large Magellanic Cloud which is a satellite galaxy to the Milky Way. Even though it's a 168,000light-years away, the supernova could be seen without a telescope. However you did have to know about where in the sky to look because it wasn't an unusually bright star in the sky at the time. Still, astronomers quickly got to work observing it and they've been studying it ever since. But the last supernova to go off in our galaxy was Kepler's supernova in 1604. The closest supernova ever recorded was in 1054 AD. It was recorded by chinese astronomers as a brilliant guest star which stayed bright for about a full year. It was so bright there are records of people seeing it in the daytime and reading by it at night. That supernova created the Crab Nebula and today it is the closest and most well studied supernova remnant. By measuring its rate of expansion, astronomers were able to work out its distance. It turns out that this supernova went off about 6,500 light-years away.
Betelgeuse on the other hand is only 640 light-years away! That's a factor of 10 times closer than the Crab, so yeah, we'd really love to be wrong in our estimates so that we could study a supernova up close. Now Betelgeuse is pretty much at the end of its life. Its core has long exhausted its supply of hydrogen fuel and has since expanded and cooled off, hence the name a "red super giant". And by supergiant I mean SUPER GIANT. The star's radius is 900 times larger than our Sun and takes up more than 700 million times its volume. If we could somehow move Betelgeuse to where the Sun is now, it would engulf the inner planets, much of the asteroid belt, and quite possibly even Jupiter! Even though its surface temperature is much cooler than the Sun, its sheer size makes it a whopping 100,000 times more luminous. All of this makes Betelgeuse the tenth brightest star in the night sky...at least it was before it started fading. In fact,you can go out and see this for yourself. Normally Betelgeuse is roughly as bright as Rigel in the constellation Orion. But now it appears to be closer in brightness to Bellatrix.
Red supergiant stars are notoriously variable; their stable days of fusing hydrogen into helium on the main sequence are long behind them. Back then, they propped themselves up by balancing their gravity with the radiation pressure exerted by their cores. This is hydrostatic equilibrium and it's the key to how stars work. But once the star exhausted its supply of hydrogen fuel, the core contracted and got hotter and that extra heat expanded the outer layers. As they expanded, those outer layers cooled off and the star became a red supergiant. But even though the core eventually started fusing helium into carbon, the outer layers didn't just stop expanding to reach a new equilibrium with gravity. Instead their momentum caused them to overshoot and the pressure underneath rapidly dropped off. Gravity took hold again and brought the outer layers back in. The pressure became even higher and pushed the outer layers back out. Once again, they overshot and the process has been repeating itself more or less ever since. The star became a pulsating variable. Now that's a gross oversimplification of how stars pulsate.
The reality is that stars undergo different types of pulsations for different types of reasons over the course of its evolution. Sometimes their pulsations have a steady, predictable cadence. These regularly pulsating supergiant's are often classified as Cepheid variables. Their pulsation periods correspond to their luminosities so we can use them as standard candles to measure their distances. But when the star evolves past the Cepheid stage, their pulsations become less stable and more erratic and they often will develop more than one pulsation cycle. In Betelgeuse's case, there's at least three pulsation cycles going on. There is a dominant period of420 days but that's plus or minus 15 days. Then there's a longer-term period that ranges anywhere from 5 to 6 years. And then there's even a shorter term period ranging from 100 to 180 days. And by the way, those are just the pulsation periods that we already know about.
We're still trying to figure out what causes all these different pulsations in red super giants, but one of the causes probably has to do with their surfaces. Stars typically have large convection cells covering their surfaces .The Sun has convection cells called granules. At the center of each granule, hot gases rise from the interior to the surface and radiate energy. Then it cools off and falls back down into the lower layers along the edges. The Sun has roughly a million of these cells, but red supergiant's are a whole other beast. Instead of a million smaller cells that cover our Sun, red supergiant's only have just a few gigantic convection cells that are thousands of times bigger. These cells can span up to 60% of the star surface. That means red super giants really don't look like stars so much as they appear as roiling angry clouds. We can see this pretty well in this image of Antares,
another nearby red supergiant. Notice that it has some hot and dark spots along the surface; it almost looks like the star is boiling. Betelgeuse really shows off its blobby shape with a huge hot spot in the upper left...at least that hot spot was there, it's possible that Betelgeuse has developed some exceptionally dark regions. These blobby convection cells throw out enormous amounts of gas. Betelgeuse is losing about a Sun's worth of mass every 10,000 years. Now that's a high rate of mass loss! In fact, we can see that the star has enshrouded itself in a nebula of its own making. Enormous plumes of gas extend out beyond Neptune's orbit, and that gas can cooldown and condense into dust. It's possible that maybe these dust clouds have gotten dense enough to block the light from the star ,making Betelgeuse appear dimmer than it really is.
But another idea has to do with Betelgeuse's magnetic field. It appears to be stronger than a typical red supergiant. That may be due to its convection, but it could also be due to a potentially faster rotation. In either case, strong, chaotic magnetic fields would create enormous star spots. They're kind of like sunspots but in this case they would cover as much as half of Betelgeuse's surface. So that means that Betelgeuse is probably dimming due to a combination of some of these mechanisms. Unfortunately we really don't know much about the detailed physics of red super giants because there really aren't that many of them that are nearby enough to study in great detail. You see, most supergiant stars have companions so we can watch them orbit the supergiant and use their orbits to effectively weigh the star. But that's not the case with Betelgeuse, so instead we estimate its mass by measuring its luminosity and temperature and plotting that on the Hertzsprung-Russell diagram. We can then compare its properties to similar stars whose masses we already know. Now there are loads of uncertainties in these luminosity and temperature measurements. I mean, we are dealing with a pulsating boiling star after all. But most estimates put Betelgeuse in the 15 to 20 solar mass range, so while that's not super-accurate it's enough to tell us that Betelgeuse probably started out life as a B type star and when it finally does go supernova, it will likely leave behind a neutron star. But what does all this weird dimming have to do with Betelgeuse going supernova? Well nothing really. The outer layers of the star are very unstable but that really doesn't have anything to do with what's going on in the core, and it's the core that dictates when the star is going to die. But how long it takes to go through each stage of fusion depends on the mass of the star; the more massive it, is the shorter the stages. Well we don't have an exact mass of Betelgeuse, but even if we did we'd love to have some way of knowing what stage it's actually in because from there we could estimate how much time it really does have left. For example if it's still burning helium in its core, we are probably talking about 100,000 to maybe even a million years before it eventually explodes. But if it's advanced to the next stage and it's burning carbon, then it may have only a hundred years left. Unfortunately the only way to tell what's really going on in the core would be from its neutrino emissions but Betelgeuse is 640 light years away and neutrinos are extremely difficult to detect; there are just too few of the marriving at Earth to be distinguished from the billions upon trillions of neutrinos coming from the Sun every second. However it's not like we wouldn't have any warning at all before Betelgeuse does the ultimate firework. During the final collapse of the star's core anywhere, from 10 to the 57 to 10 to the 58 neutrinos are released in a powerful burst. These neutrinos escape a star a few hours before the shockwave rips it apart in a supernova. That would be more than enough neutrinos to set off every neutrino detector on Earth, so if you were to get a neutrino alert on your phone you might want to head outside in an hour and watch Betelgeuse explode. It's estimated that Betelgeuse will probably be at least as bright as a full moon and possibly brighter. Even if it went off in the summertime when the Sun is nearby in the sky, we would still be able to see it during the day. And you'd have plenty of time to catch the supernova at night in the autumn and winter because it should remain fairly bright for up to a year. We have a supernova progenitor star that is not too close as to be dangerous but not so far away that we can't study it in great detail when it finally does go off. It will be the first star to have been comprehensively studied before, during, and after its supernova. In the mean time Betelgeuse, is still the faintest that's been in recent history and quite possibly much longer. Although nobody has a crystal ball, it's probably going to return to brightness at some point. The only thing is we just don't know when that will be which makes the star so insanely cool and mysterious. That's all, thank you.
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Betelgeuse captured by ALMASource: ALMA / CC BY |
So what is it that has everybody so excited?
Well there's quite a bit going on with a star and we're going to talk about that in depth in today's article. Now Betelgeuse has been acting up lately or more accurately it's been acting down if anything. Betelgeuse normally has a visual magnitude of 0.5 although it does vary by less than one magnitude because it is a variable star. Then starting around Thanksgiving it began to drop below its typical low brightness. On December 8th 2019, Ed Guinan from Villanova University sent an astronomical telegram stating that Betelgeuse had faded to magnitude 1.1 -that at the time was a record low but then on December 23rd Guinan sent another telegram stating that Betelgeuse had dropped to magnitude 1.3. And as of this filming it's dropped even more all the way down to about magnitude 1.5! Now putting all of this another way Betelgeuse has lost at least half of its typical brightness over the course of just a couple of months! Now that's a rapid drop-off in light and now it's the faintest that's been recorded using modern photometric techniques. Naturally this got some people wondering if this was somehow the calm before the supernova storm. However it's much more likely that Betelgeuse probably won't explode for another 10,000 to perhaps 100,000 years. Now that is quite soon, astronomically speaking, but we're certainly not talking about anything happening within our lifetimes. Although we'd love to be wrong about this.
The closest supernova in living memory was SN 1987A which erupted on February 24th1987.
 |
| SN 1987A supernovaSource: ESO/L. Calçada / CC BY |
But that supernova was in the Large Magellanic Cloud which is a satellite galaxy to the Milky Way. Even though it's a 168,000light-years away, the supernova could be seen without a telescope. However you did have to know about where in the sky to look because it wasn't an unusually bright star in the sky at the time. Still, astronomers quickly got to work observing it and they've been studying it ever since. But the last supernova to go off in our galaxy was Kepler's supernova in 1604. The closest supernova ever recorded was in 1054 AD. It was recorded by chinese astronomers as a brilliant guest star which stayed bright for about a full year. It was so bright there are records of people seeing it in the daytime and reading by it at night. That supernova created the Crab Nebula and today it is the closest and most well studied supernova remnant. By measuring its rate of expansion, astronomers were able to work out its distance. It turns out that this supernova went off about 6,500 light-years away.
Betelgeuse on the other hand is only 640 light-years away! That's a factor of 10 times closer than the Crab, so yeah, we'd really love to be wrong in our estimates so that we could study a supernova up close. Now Betelgeuse is pretty much at the end of its life. Its core has long exhausted its supply of hydrogen fuel and has since expanded and cooled off, hence the name a "red super giant". And by supergiant I mean SUPER GIANT. The star's radius is 900 times larger than our Sun and takes up more than 700 million times its volume. If we could somehow move Betelgeuse to where the Sun is now, it would engulf the inner planets, much of the asteroid belt, and quite possibly even Jupiter! Even though its surface temperature is much cooler than the Sun, its sheer size makes it a whopping 100,000 times more luminous. All of this makes Betelgeuse the tenth brightest star in the night sky...at least it was before it started fading. In fact,you can go out and see this for yourself. Normally Betelgeuse is roughly as bright as Rigel in the constellation Orion. But now it appears to be closer in brightness to Bellatrix.
Red supergiant stars are notoriously variable; their stable days of fusing hydrogen into helium on the main sequence are long behind them. Back then, they propped themselves up by balancing their gravity with the radiation pressure exerted by their cores. This is hydrostatic equilibrium and it's the key to how stars work. But once the star exhausted its supply of hydrogen fuel, the core contracted and got hotter and that extra heat expanded the outer layers. As they expanded, those outer layers cooled off and the star became a red supergiant. But even though the core eventually started fusing helium into carbon, the outer layers didn't just stop expanding to reach a new equilibrium with gravity. Instead their momentum caused them to overshoot and the pressure underneath rapidly dropped off. Gravity took hold again and brought the outer layers back in. The pressure became even higher and pushed the outer layers back out. Once again, they overshot and the process has been repeating itself more or less ever since. The star became a pulsating variable. Now that's a gross oversimplification of how stars pulsate.
The reality is that stars undergo different types of pulsations for different types of reasons over the course of its evolution. Sometimes their pulsations have a steady, predictable cadence. These regularly pulsating supergiant's are often classified as Cepheid variables. Their pulsation periods correspond to their luminosities so we can use them as standard candles to measure their distances. But when the star evolves past the Cepheid stage, their pulsations become less stable and more erratic and they often will develop more than one pulsation cycle. In Betelgeuse's case, there's at least three pulsation cycles going on. There is a dominant period of420 days but that's plus or minus 15 days. Then there's a longer-term period that ranges anywhere from 5 to 6 years. And then there's even a shorter term period ranging from 100 to 180 days. And by the way, those are just the pulsation periods that we already know about.
We're still trying to figure out what causes all these different pulsations in red super giants, but one of the causes probably has to do with their surfaces. Stars typically have large convection cells covering their surfaces .The Sun has convection cells called granules. At the center of each granule, hot gases rise from the interior to the surface and radiate energy. Then it cools off and falls back down into the lower layers along the edges. The Sun has roughly a million of these cells, but red supergiant's are a whole other beast. Instead of a million smaller cells that cover our Sun, red supergiant's only have just a few gigantic convection cells that are thousands of times bigger. These cells can span up to 60% of the star surface. That means red super giants really don't look like stars so much as they appear as roiling angry clouds. We can see this pretty well in this image of Antares,
 |
| Source: rainfall / CC BY-SA |
another nearby red supergiant. Notice that it has some hot and dark spots along the surface; it almost looks like the star is boiling. Betelgeuse really shows off its blobby shape with a huge hot spot in the upper left...at least that hot spot was there, it's possible that Betelgeuse has developed some exceptionally dark regions. These blobby convection cells throw out enormous amounts of gas. Betelgeuse is losing about a Sun's worth of mass every 10,000 years. Now that's a high rate of mass loss! In fact, we can see that the star has enshrouded itself in a nebula of its own making. Enormous plumes of gas extend out beyond Neptune's orbit, and that gas can cooldown and condense into dust. It's possible that maybe these dust clouds have gotten dense enough to block the light from the star ,making Betelgeuse appear dimmer than it really is.
But another idea has to do with Betelgeuse's magnetic field. It appears to be stronger than a typical red supergiant. That may be due to its convection, but it could also be due to a potentially faster rotation. In either case, strong, chaotic magnetic fields would create enormous star spots. They're kind of like sunspots but in this case they would cover as much as half of Betelgeuse's surface. So that means that Betelgeuse is probably dimming due to a combination of some of these mechanisms. Unfortunately we really don't know much about the detailed physics of red super giants because there really aren't that many of them that are nearby enough to study in great detail. You see, most supergiant stars have companions so we can watch them orbit the supergiant and use their orbits to effectively weigh the star. But that's not the case with Betelgeuse, so instead we estimate its mass by measuring its luminosity and temperature and plotting that on the Hertzsprung-Russell diagram. We can then compare its properties to similar stars whose masses we already know. Now there are loads of uncertainties in these luminosity and temperature measurements. I mean, we are dealing with a pulsating boiling star after all. But most estimates put Betelgeuse in the 15 to 20 solar mass range, so while that's not super-accurate it's enough to tell us that Betelgeuse probably started out life as a B type star and when it finally does go supernova, it will likely leave behind a neutron star. But what does all this weird dimming have to do with Betelgeuse going supernova? Well nothing really. The outer layers of the star are very unstable but that really doesn't have anything to do with what's going on in the core, and it's the core that dictates when the star is going to die. But how long it takes to go through each stage of fusion depends on the mass of the star; the more massive it, is the shorter the stages. Well we don't have an exact mass of Betelgeuse, but even if we did we'd love to have some way of knowing what stage it's actually in because from there we could estimate how much time it really does have left. For example if it's still burning helium in its core, we are probably talking about 100,000 to maybe even a million years before it eventually explodes. But if it's advanced to the next stage and it's burning carbon, then it may have only a hundred years left. Unfortunately the only way to tell what's really going on in the core would be from its neutrino emissions but Betelgeuse is 640 light years away and neutrinos are extremely difficult to detect; there are just too few of the marriving at Earth to be distinguished from the billions upon trillions of neutrinos coming from the Sun every second. However it's not like we wouldn't have any warning at all before Betelgeuse does the ultimate firework. During the final collapse of the star's core anywhere, from 10 to the 57 to 10 to the 58 neutrinos are released in a powerful burst. These neutrinos escape a star a few hours before the shockwave rips it apart in a supernova. That would be more than enough neutrinos to set off every neutrino detector on Earth, so if you were to get a neutrino alert on your phone you might want to head outside in an hour and watch Betelgeuse explode. It's estimated that Betelgeuse will probably be at least as bright as a full moon and possibly brighter. Even if it went off in the summertime when the Sun is nearby in the sky, we would still be able to see it during the day. And you'd have plenty of time to catch the supernova at night in the autumn and winter because it should remain fairly bright for up to a year. We have a supernova progenitor star that is not too close as to be dangerous but not so far away that we can't study it in great detail when it finally does go off. It will be the first star to have been comprehensively studied before, during, and after its supernova. In the mean time Betelgeuse, is still the faintest that's been in recent history and quite possibly much longer. Although nobody has a crystal ball, it's probably going to return to brightness at some point. The only thing is we just don't know when that will be which makes the star so insanely cool and mysterious. That's all, thank you.
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