Betelgeuse imaged, again

[Surlethe]

Universe Today

An international team of astronomers has obtained an unprecedented image of the surface of the red supergiant Betelgeuse, in the constellation Orion. The image reveals the presence of two giant bright spots, which cover a large fraction of the surface. Their size is equivalent to the Earth-Sun distance. This observation provides the first strong and direct indication of the presence of the convection phenomenon, transport of heat by moving matter, in a star other than the Sun. This result provides a better understanding of the structure and evolution of supergiants.

Betelgeuse is a red supergiant located in the constellation of Orion, and is quite different from our Sun. First, it is a huge star. If it were the center of our Solar System it would extend to the orbit of Jupiter. At 600 times larger than our Sun, it radiates approximately 100,000 times more energy. Additionally, with an age of only a few million years, Betelgeuse is already nearing the end of its life and is soon doomed to explode as a supernova. When it does, the supernova should be seen easily from Earth, even in broad daylight.

But we now know Betelgeuse has some similarities to the Sun, as it also has sunspots. The surface has bright and dark spots, which are actually regions that are hot and cold spots on the star. The spots appear due to convection, i.e., the transport of heat by matter currents. This phenomenon is observed every day in boiling water. On the surface of the Sun, these spots are rather well-known and visible. However, it is not at all the case for other stars and in particular supergiants. The size, physical characteristics, and lifetime of these dynamical structures remain unknown.

Betelgeuse is a good target for interferometry because its size and brightness make it easier to observe. Using simultaneously the three telescopes of the Infrared Optical Telescope Array (IOTA) interferometer on Mount Hopkins in Arizona (since removed), and the Paris Observatory (LESIA) the astronomers were able to obtain a numerous high-precision measurements. These made it possible to reconstruct an image of the star surface thanks to two algorithms and computer programs.

Two different algorithms gave the same image. One was created by Eric Thiebaut from the Astronomical Research Center of Lyon (CRAL) and the other was developed by Laurent Mugnier and Serge Meimon from ONERA. The final image reveals the star surface with unprecedented, never-before-seen details. Two bright spots clearly show up next to the center of the star.

The analysis of the brightness of the spots shows a variation of 500 degrees compared to the average temperature of the star (3,600 Kelvin). The largest of the two structures has a dimension equivalent
to the quarter of the star diameter (or one and a half the Earth-Sun distance). This marks a clear difference with the Sun where the convection cells are much finer and reach hardly 1/20th of the solar radius (a few Earth radii). These characteristics are compatible with the idea of luminous spots produced by convection. These results constitute a first strong and direct indication of the presence of convection on the surface of a star other than the Sun.

Convection could play an important role in the explanation of the mass-loss phenomenon and in the gigantic plume of gas that is expelled from Betelgeuse. The latter has been discovered by a team led by Pierre Kervella from Paris Observatory (read our article about this discovery). Convection cells are potentially at the origin of the hot gas ejections.

The astronomers say this new discovery provides new insights into supergiant stars, opening up a new field of research.

Pictures: http://www.universetoday.com/wp-content ... tel-f1.jpg, http://blogs.nature.com/news/thegreatbeyond/betel.jpg (this one is from last July, taken in Chile).

This goes up there with mapping out the temperatures of exoplanets. Who would have ever thought we'd be able to observe the surface of another star, let alone map out convection cells? I hope Betelgeuse goes supernova in my lifetime, by the way -- apparently, it's shrinking, which means the core is expanding, which means (maybe?) that it's starting a new fuel. A Betelgeuse supernova would be something to see.

[starslayer]

This goes up there with mapping out the temperatures of exoplanets. Who would have ever thought we'd be able to observe the surface of another star, let alone map out convection cells?

I would have. Optical interferometry is pretty damned amazing in what kind of resolution you can get.

I hope Betelgeuse goes supernova in my lifetime, by the way -- apparently, it's shrinking, which means the core is expanding, which means (maybe?) that it's starting a new fuel.

Betelgeuse shrinking does not necessarily mean anything about what fuel the core is burning at this point in time. The changes would only begin to happen over the Kelvin-Helmholtz (thermal) time of the star, which is at least a million years for most of them. In fact, when a star moves onto a new source of nuclear fuel, it to expands as the core contracts and heats up; it doesn't shrink as the core cools down. This is because each new stage of fusion requires far more energy per event than the last. So, for example, it is impossible to tell whether a supergiant is burning helium, carbon, or anything higher than that at its core; carbon fusion lasts a mere 300 years or so for a 20 solar mass star, far too short a time for the radiative perturbations to work their to the surface. The next steps take even shorter amounts of time, until silicon goes to iron via NSE in about a week to ten days at the most, followed by a collapse to a neutron star in less than a second; what happens immediately after that is quite a sight if you happen to be sitting in the cheap seats like us. I somehow doubt you'd want to pay top dollar for that particular show...

Now, you might think, "well, what about the time it takes for the core to contract? Surely, we'd be able to see something during that time." The problem with this is that stellar cores are basically isothermal in massive stars; the density is such that it simply doesn't take very long for thermal information to reach from one side to the other, so core contractions are relatively rapid compared to the Kelvin-Helmholtz time, and by the time carbon fusion starts, it's again too short a time to see any effects from the actual core to bubble up to the surface. The real fun during that part of a massive star's life is due to the interactions between the hydrogen and helium-burning shells that sit above the core.

I should note that most of this does not apply to the transition between hydrogen and helium burning (the move to the red giant branch is very easy to see), but Betelgeuse is well past that.

Going back to the fact that Betelgeuse is shrinking, I find it far more likely that that is due to the huge amount of mass that it has ejected; the dynamical time of most stars is an hour or less.

[Simon_Jester]

This goes up there with mapping out the temperatures of exoplanets. Who would have ever thought we'd be able to observe the surface of another star, let alone map out convection cells? I hope Betelgeuse goes supernova in my lifetime, by the way -- apparently, it's shrinking, which means the core is expanding, which means (maybe?) that it's starting a new fuel. A Betelgeuse supernova would be something to see.

...Ah, aren't we still in the "danger close" region for a Betelgeuse supernova?

Also, starslayer, could you define "dynamical time" for the non-astrophysicists* in the audience? I'd guess the time it takes mechanical changes to propagate across the star's volume; am I right?

*Or, in my case, non-astro physicists...

[starslayer]

...Ah, aren't we still in the "danger close" region for a Betelgeuse supernova?

No, we are not; we'd have to be about 50 light years away to see any significant damaging effects on the ground, and about 100 light years away to see any damage in space (satellites, astronauts, etc.).

Also, starslayer, could you define "dynamical time" for the non-astrophysicists* in the audience? I'd guess the time it takes mechanical changes to propagate across the star's volume; am I right?

Yes, that is correct; for example, the dynamical time of the Sun is about 15 minutes, so if you were to whack it really hard with a cosmic golf club, or move a massive object such as a planet close to it, the opposite side would feel the (rather large) vibrations in 15 minutes. Stars also tend to do one of two things when mechanically disturbed on such a scale: either collapse or blow up. They mostly blow up.

The two other timescales astronomers commonly use to describe stellar dynamics are the Kelvin-Helmholtz and nuclear timescales. The Kelvin-Helmholtz time is the time it takes for thermal perturbations to propagate throughout a star (usually from the core to the surface), while the nuclear time is the amount of time nuclear fusion will occur at the core (you can subdivide this between the different elements as well). The Sun's thermal time is about one million years, while it's nuclear time is about 10 billion years.

[Surlethe]

I would have. Optical interferometry is pretty damned amazing in what kind of resolution you can get.

It's black magic is what it is.

Betelgeuse shrinking does not necessarily mean anything about what fuel the core is burning at this point in time. The changes would only begin to happen over the Kelvin-Helmholtz (thermal) time of the star, which is at least a million years for most of them. In fact, when a star moves onto a new source of nuclear fuel, it to expands as the core contracts and heats up; it doesn't shrink as the core cools down. This is because each new stage of fusion requires far more energy per event than the last.

What I was thinking was that after ignition of the most recent fuel, the core will heat up and expand, which will cause the envelope to shrink.

So, for example, it is impossible to tell whether a supergiant is burning helium, carbon, or anything higher than that at its core; carbon fusion lasts a mere 300 years or so for a 20 solar mass star, far too short a time for the radiative perturbations to work their to the surface. The next steps take even shorter amounts of time, until silicon goes to iron via NSE in about a week to ten days at the most, followed by a collapse to a neutron star in less than a second; what happens immediately after that is quite a sight if you happen to be sitting in the cheap seats like us. I somehow doubt you'd want to pay top dollar for that particular show...

So the moral of the story is that by the time the radiative perturbations for carbon ignition have worked their way to the surface, the star has already blown itself up? I.e., the supernova shockwave overtakes the radiation?

Now, you might think, "well, what about the time it takes for the core to contract? Surely, we'd be able to see something during that time." The problem with this is that stellar cores are basically isothermal in massive stars; the density is such that it simply doesn't take very long for thermal information to reach from one side to the other, so core contractions are relatively rapid compared to the Kelvin-Helmholtz time, and by the time carbon fusion starts, it's again too short a time to see any effects from the actual core to bubble up to the surface. The real fun during that part of a massive star's life is due to the interactions between the hydrogen and helium-burning shells that sit above the core.

I guess that makes sense. The fusion shells would complicate things, wouldn't they?

I should note that most of this does not apply to the transition between hydrogen and helium burning (the move to the red giant branch is very easy to see), but Betelgeuse is well past that.

Going back to the fact that Betelgeuse is shrinking, I find it far more likely that that is due to the huge amount of mass that it has ejected; the dynamical time of most stars is an hour or less.

That would probably make more sense.

[Patrick Degan]

Has anyone managed to calculate a timeframe for Betelgeuse's eventual supernova or is there as yet too little data for an estimate?

[Singular Intellect]

Additionally, with an age of only a few million years, Betelgeuse is already nearing the end of its life and is soon doomed to explode as a supernova. When it does, the supernova should be seen easily from Earth, even in broad daylight.

I read that and thought 'that is going to be very cool to see!'...and then I remembered what kind of life spans we're talking about here.

Has anyone managed to calculate a timeframe for Betelgeuse's eventual supernova or is there as yet too little data for an estimate?

Hopefully we have enough time to develope Red Matter, before said star explodes and threatens to consume the entire galaxy.

[starslayer]

What I was thinking was that after ignition of the most recent fuel, the core will heat up and expand, which will cause the envelope to shrink.

That's not quite what happens. Right after the rate of nuclear burning begins to slacken, the envelope shrinks at least a little, because there is indeed less radiation pressure holding it up. However, the core immediately begins to contract under gravity, and it heats up immensely. This forces the envelope to expand again. In stars heavier than about two solar masses, as soon as the core begins burning the next fuel, it just stops contracting, because it reaches equilibrium. There is no expansion. In low mass stars like the Sun, once they run out of hydrogen in the core, the cores contract until they become degenerate (I misremembered my stellar evolution earlier; most stellar cores are not degenerate, but they still do tend to be isothermal). Nuclear burning in degenerate matter is thermally unstable, and all the helium goes up at once in the helium flash (Helium burning has a dependence of ~T^40, by the way). This is completely quenched by the envelope (so it's impossible to see from the outside), and the core expands until it isn't degenerate, at which point normal helium burning begins.

So the moral of the story is that by the time the radiative perturbations for carbon ignition have worked their way to the surface, the star has already blown itself up? I.e., the supernova shockwave overtakes the radiation?

Correct. More than likely, the photons from the ignition of carbon aren't even out of the radiative zone of the star by the time it blows itself up.

I guess that makes sense. The fusion shells would complicate things, wouldn't they?

Yes, because nuclear burning in a shell (especially helium burning) tends to be unstable. Two nuclear burning shells sitting on top of each other is never a stable configuration; in AGB (asymptotic giant branch) stars this leads to shell flashes. For very massive stars like Betelgeuse, you might see one or two before the whole thing goes kablooie - shell flashes have periods ranging from hundreds to thousands of years.

Has anyone managed to calculate a timeframe for Betelgeuse's eventual supernova or is there as yet too little data for an estimate?

There is unfortunately no way to tell exactly how much time Betelguese has left; it could be half a million years (it might still be in the middle of burning helium, which generally takes about a million years in ~20 solar mass stars), it could a few hundred years; hell, it could be next week. The only warning we'd get would come a few seconds to minutes (I forget exactly how long it is) before we saw the main explosion, in the form of an enormous neutrino and X-ray flash.

[Simon_Jester]

There is unfortunately no way to tell exactly how much time Betelguese has left; it could be half a million years (it might still be in the middle of burning helium, which generally takes about a million years in ~20 solar mass stars), it could a few hundred years; hell, it could be next week. The only warning we'd get would come a few seconds to minutes (I forget exactly how long it is) before we saw the main explosion, in the form of an enormous neutrino and X-ray flash.

From a scientific standpoint, I wonder if it would be worth it to keep a few automated telescopes around the planet trained on Betelgeuse. Sure, there's only a one in ten thousand* chance of it going off in the next century- but wouldn't there be a big potential payoff in terms of knowing that you're going to capture the whole event, along with any precursors you can identify after the fact?

*Something like that, anyway...

[starslayer]

From a scientific standpoint, I wonder if it would be worth it to keep a few automated telescopes around the planet trained on Betelgeuse. Sure, there's only a one in ten thousand* chance of it going off in the next century- but wouldn't there be a big potential payoff in terms of knowing that you're going to capture the whole event, along with any precursors you can identify after the fact?

Oh, there's no need for something like that. Neutrino detectors around the world would instantly see it, and the X-ray flash of an impending supernova was detected in one star a couple years ago, IIRC. Besides, a Betelgeuse supernova would be motherfucking BRIGHT, easily bright enough in the optical band to be seen at noon for months, and even brighter in UV and X-rays. Minutes after it happened, telescopes all over the world would be trained on it, so there'd be no real data lost.

[Simon_Jester]

Yes. On the other hand, I can imagine wanting to know what Betelgeuse looked like an hour or two before it exploded (so to speak). Would this be of no interest to the qualified astrophysicists for reasons I don't fully understand?

[Themightytom]

I wonder what kind of environmental effects a cosntant bright light in the sky would ahve. Where is Betelgeuse relative to Earh's orbit around the sun, would it be brighter in the summer or the winter?

If its summer time we're talking extra plant growth because of the night time light and it might confuse the hell out of nocturnal animals.

[starslayer]

Yes. On the other hand, I can imagine wanting to know what Betelgeuse looked like an hour or two before it exploded (so to speak). Would this be of no interest to the qualified astrophysicists for reasons I don't fully understand?

It might, but I wouldn't think so. Certainly not for any reason that I know of.

I wonder what kind of environmental effects a cosntant bright light in the sky would ahve. Where is Betelgeuse relative to Earh's orbit around the sun, would it be brighter in the summer or the winter?

If its summer time we're talking extra plant growth because of the night time light and it might confuse the hell out of nocturnal animals.

Go out tonight and look up, if it's not cloudy. Betelgeuse is the bright red star in the shoulder of Orion. Don't be fooled by Mars, the other really bright red thing in the sky. It should be somewhere near Regulus (bright star in Leo), about 20-30 degrees (2-3 fists held at arm's length, with thumb on outside against index finger) NW of Betelgeuse and Orion.

IOW, it's a winter star. In summer, it would only appear during the day. And the environmental effects would be no more than the full Moon's; a Betelgeuse supernova would be about that bright.

[Simon_Jester]

Yes. On the other hand, I can imagine wanting to know what Betelgeuse looked like an hour or two before it exploded (so to speak). Would this be of no interest to the qualified astrophysicists for reasons I don't fully understand?

It might, but I wouldn't think so. Certainly not for any reason that I know of.

[we need an "eyes cross in confusion" smiley]

Do you mean that seeing the immediate run-up to the supernova would be of interest, or would not be of interest?

I'm guessing "would not be," since anything interesting in the immediate run-up is happening in the core and will only be visible on highly non-visual parts of the spectrum, such as the aforesaid X-ray and neutrino emission.

[starslayer]

Do you mean that seeing the immediate run-up to the supernova would be of interest, or would not be of interest?

Likely would not be. I don't know of any reason why observing the star a month or two before it goes supernova - or even a day, or an hour or two - would show anything different from several years beforehand. However, I'm not up on the latest developments in the evolution of massive stars.

And holy shit, did I fuck up that url tag. Could a passing mod fix it?

[Surlethe]

That's not quite what happens. Right after the rate of nuclear burning begins to slacken, the envelope shrinks at least a little, because there is indeed less radiation pressure holding it up. However, the core immediately begins to contract under gravity, and it heats up immensely. This forces the envelope to expand again. In stars heavier than about two solar masses, as soon as the core begins burning the next fuel, it just stops contracting, because it reaches equilibrium. There is no expansion.

Okay, that's where I got messed up. I was thinking ignition of the next fuel would happen sort of like a helium flash, where (as you outline below) the core becomes degenerate, fuel ignites, entire core stores heat until it expands, which contracts envelope.

Oh, there's no need for something like that. Neutrino detectors around the world would instantly see it, and the X-ray flash of an impending supernova was detected in one star a couple years ago, IIRC. Besides, a Betelgeuse supernova would be motherfucking BRIGHT, easily bright enough in the optical band to be seen at noon for months, and even brighter in UV and X-rays. Minutes after it happened, telescopes all over the world would be trained on it, so there'd be no real data lost.

If Betelgeuse is a typical Type II, it will have an absolute magnitude between -16 and -20; that amounts to an apparent magnitude (at 200 pc) of -19 to -23. It will be the second brightest thing in the sky, after the Sun -- it will be between 17 and 30 times brighter than a full moon.

[starslayer]

Your math is backwards there, Surlethe; absolute magnitude is defined as the apparent magnitude at 10 pc. Assuming an absolute magnitude of -20, the apparent magnitude from 200 pc would be -13.5*, which just so happens to be about the apparent magnitude of the full Moon (-12.6).

*I used this relation: m = M + 5*(log(Dl) - 1), where m is the apparent magnitude, M the absolute, and Dl is what's called the distance modulus; within our galaxy, it's basically the distance to the objects in parsecs.

[Surlethe]

Hmm, that's exactly the relation I used. I guess I just mistyped it into the calculator.

[starslayer]

A quick BS check with magnitudes, for future reference: when using absolute magnitude, if the object is farther away than 10 pc, yet you get an answer that is brighter, something's wrong. With magnitude, smaller and more negative numbers are brighter.