This video is sponsored by Magellan TV. welcome back to Launch Pad, I'm Christian
Ready, your friendly neighborhood astronomer. 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 video, but first I did want to take a moment and thank Magellan TV for
their sponsorship. Magellan is a new type of streaming service with documentaries
in space science nature and history. So far I've enjoyed their programs on
black hole starships, the majestic earth, and I'm really enjoying their episode on
super solar storms. New programs are added weekly and can be watched anytime
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Documentaries are streamed in Full HD and even in 4k without any interruptions
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Astronomy who sign up below will get an extra month of streaming absolutely free
so make sure you check them out. 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. Nw 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 24th 1987. But that supernova was in the Large
Magellanic Cloud which is a satellite galaxy to the Milky Way. Even though it's
a 168,000 light-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 supergiant". 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 of 420 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 supergiants, 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 supergiants
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. This image of Betelgeuse
really shows off its blobby shape with a huge hot spot in the upper left...at least
that hot spot was there at the time this image was made; it's possible that
Betelgeuse has developed some exceptionally dark regions since this
image was taken. 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 cool down 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 supergiants because there really aren't
that many of them that are nearby enough to study in great detail. Pinning down
Betelgeuse's mass would really help us to improve our models, but it lacks a
visible companion. 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. In our
last video on how high mass stars evolve and die, we saw that their cores compress
and grow smaller as they heat up to fuse heavier and heavier elements. 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 them arriving 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. Still, my preference would be to have it go off on
a clear winter's night when I just happened to be looking at it. I think that
would be very nice. But you know what? What it's doing now is also really cool.
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 meantime 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. So I invite you to go outside from time to
time and take a look at Betelgeuse while it's still visible in the winter
sky. See if you can watch the star change in brightness! A huge thanks to my
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to be kept up to date on Betelgeuse or learn more about just how stars live
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new video. Until next time, stay curious my friends.