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High Mass Stars: Crash Course Astronomy #31

High Mass Stars: Crash Course Astronomy #31


Stars are in a constant struggle between gravity
trying to collapse them and their internal heat trying to inflate them. For most of a star’s
life, these two forces are at an uneasy truce. For a star like the Sun, the balance tips
in its twilight years. For a brief glorious moment it expands… but then blows away its
outer layers, leaving behind the gravitationally compressed core. It goes out with a whimper.
Well, a whimper from a two octillion ton barely constrained nuclear powered fireball. But more massive stars aren’t quite as resigned
to their fate. When they go out, they go out with a bang — a very, very big bang. In the core of a star, pressure and temperature
are high enough that atomic nuclei can get squeezed together and fuse. This releases
energy, and creates heavier elements. Hydrogen fusion makes helium, helium fusion makes carbon,
and each heavier element, in general, takes higher temperatures and pressures to fuse. Lower-mass stars like the Sun stop at carbon.
Once that builds up in the core, the star’s fate is sealed. But if the star has more than about 8 times
the Sun’s mass, it can create temperatures in its core in excess of 500 million degrees Celsius,
and then carbon will fuse. There are actually a lot of steps in this process, but in the end you get carbon
fusing into neon, magnesium, and some sodium. What happens next hearkens back to what we
found goes on in the Sun’s core as it ages: fuse an element, create a heavier one, then
that heavier one builds up until the core contracts and heats up enough to start
fusing it. So carbon fusion makes neon, magnesium, and sodium, and those accumulate.
The core heats up, and when it reaches about a billion degrees, neon will fuse. Neon fusion
creates more magnesium, as well as some oxygen. These build up in the core, it shrinks, heats
up to about 1.5 billion degrees, and then oxygen fuses, creating silicon. Then THAT
builds up until the temperature hits about 2-3 billion degrees, whereupon silicon can
fuse. Among a pile of other elements, silicon fusion
creates iron. And that’s trouble. Big, big trouble. Once silicon fusion stars, the star
is a ticking time bomb. But before we light that fuse, let’s take
a step back. What’s happening to the outer layers of the star? What do we see if we’re
outside, looking back at it? Because the star was born massive, it spent
its hydrogen fusing days as a blue main sequence star. Stars like this are extremely luminous,
and can be seen for tremendous distances. Like the Sun, though, a massive star changes
when hydrogen fusion stops, its core contracts, and then helium fusion begins. It swells up
just as the Sun will, but instead of becoming a red giant, it generates so much energy it
becomes a red supergiant. These are incredibly huge stars, some over
a billion kilometers across! And they are luminous. For example, Betelgeuse in Orion
is a red supergiant, and one of the brightest stars in the sky despite being over 600 light
years away. From that distance, you’d need a decent telescope to see the Sun at all.
And that’s nothing compared to VY Canis Majoris, the largest known star, which is
a staggering two billion kilometers across. We even have a special term for it: a hypergiant. As the core switches back and forth from one
fusion reaction to the next the outer layers respond by contracting and expanding, so a red
supergiant can shrink and become a BLUE supergiant. Rigel, another star in Orion, is a blue supergiant, putting
out over 100,000 times as much energy as the Sun! OK, let’s go back to the core. It now looks
like an onion, with multiple layers: iron is building up in the center, surrounded by
fusing silicon. Outside that is a layer of fusing oxygen, then neon, then carbon, then
helium, and finally hydrogen. You might think massive stars would last longer
because they have more fuel than lower mass stars. But the cores of these monsters are
far hotter, and fuse elements are far higher rates, running out of fuel more quickly. A star like the Sun can happily fuse hydrogen
into helium for over 10 billion years. But a star twice as massive as the Sun runs out
of hydrogen in just 2 billion years. A star with 8 times the Sun’s mass runs out in
only 100 million years or so. And each step in the fusion process happens
faster than the one before it. In an extreme case, like for a star 20 times the mass of
the Sun, it’ll fuse helium for about a million years, carbon for about a thousand, and neon
fusion will use up all its fuel in a single year! Oxygen lasts for only a few months. Silicon fuses at a ridiculously high rate;
the star will go through its entire supply in — get this — a day. Yes, one day. The
vast majority of a star’s life is spent fusing hydrogen; the rest happens in a metaphorical
blink of the eye. Silicon fuses into a bunch of different elements,
including iron. That inert iron builds up in the core, just like all those elements
did before, and just like before the iron core shrinks and heats up. But there’s a huge difference here. In all the previous fusion stages, energy
is created. That energy transforms into heat, and that helps support the soul-crushing amount
of stellar mass above the core. But iron is different. When it fuses it actually
sucks up energy instead of creating it. Instead of providing energy for the star, it removes
it. This accelerates the shrinking, compressing the core, heating it up even more. Even worse, at these temperatures and pressures
the iron nuclei suck up electrons that are whizzing around, which are also helping support
the core. It’s a double whammy; both major means of support for the star are removed
in an instant — silicon fusing into iron is happening so fast this literally takes a fraction
of a second once it gets started. The core gets its legs kicked out from under
it. It doesn’t shrink, it collapses. The gravity of the core is so mind-bogglingly
strong that the outer parts crash down on the inner parts at a significant fraction
of the speed of light. This slams down on the central core, collapsing from several
hundred kilometers across down to a couple of dozen kilometers across in just a few thousandths
of a second! The star is doomed. Because all hell is about
to break loose. Now, at this point, one of two things can
happen. If the star has less than about 20 times the Sun’s mass, the core collapse
stops when it’s still 20 or so kilometers wide. It forms what’s called a neutron star,
which I’ll cover in the next episode. If the star is more massive than this, then
the collapse cannot be stopped by any force in the Universe. The core collapses all the
way down. Down to a point. The gravity becomes so intense that not even light can escape. A black hole is born. We’ll cover black holes in a future episode
as well. But for now, what happens when the core collapses and suddenly stops? The core of the star, whether it’s a neutron
star or a black hole, is now extremely small with terrifyingly strong gravity. It pulls on
the star’s matter above it, HARD. This stuff comes crashing down at a fantastic speed and
gets hugely compressed, ferociously heating up. At the same time, two things happen in the
core. While this stuff is falling in, a monster shock wave created by the collapse of the
core moves outward, and slams into the incoming material. The explosive energy is so insane
it slows that material substantially. The second event is that the complicated quantum
physics brewing in the core generates vast numbers of subatomic particles called neutrinos.
The total energy carried by these little neutrinos is almost beyond reason: In a fraction of
a second, they carry away 100 times as much energy as the Sun will produce over its entire
lifetime That’s an incredible amount of energy. Now,
these little beasties are seriously elusive and hate to interact with normal matter; one
single neutrino could pass through trillions of kilometers of lead without even noticing. But
so many are created in the core collapse, and the material barreling down on the core
so dense, that a huge number of them are absorbed. This vast wave of neutrinos slams into the
oncoming material like a bullet train hitting a slice of warm butter. The material stops
its infall, reverses course, and blasts outward. The star explodes. It explodes. This is called a supernova, and it is one
of the most violent and terrifying events the Universe can offer. An entire star tears
itself to shreds, and the expanding gas blasts outward at 10% the speed of light. The energy
released is so huge they can be seen literally halfway across the Universe; they outshine
all the stars in the rest of the galaxy combined. The expanding material, called the supernova
remnant, forms fantastic shapes. The most famous is the Crab nebula, from a star we
saw blow up in the year 1054. The tendrils form as the material expands into the gas
and dust that surrounded the progenitor star. As remnants expand and age they become more
tenuous. Some have bright rims as they push into material between the stars; others form
complex webs of filaments. I’m often asked if there are any stars near
enough to hurt us when they explode. The quick answer is no. Even though supernovae are incredibly
violent, space is big. A supernova would have to be at least as close as 100 light years
from us before we start feeling any real effects. The nearest star that might explode in this
way is Spica, in Virgo, and it’s well over 100 light years away. I say “might” explode,
because it’s at the lower mass limit for going supernova. It might not explode at all. Betelgeuse will certainly explode some day,
but it’s too far away to hurt us. We’re pretty safe from this particular threat. I’ll note that after all this, there IS
another kind of supernova involving white dwarfs, which we’ll cover in a future episode
about binary stars. Happily, we’re probably safe from them too. Breathe easy. As terrifying and dangerous as supernovae
are, there’s a very important aspect of them you need to know. Supernovae
are capable of great destruction, but they’re also critical for our own existence. When the star explodes, the gas gets so hot
and is compressed so violently by the blast that it undergoes fusion, what astronomers
call explosive nucleosynthesis: Literally, creating heavy elements explosively. New elements are produced in quantities that
dwarf the Earth’s mass. Calcium, phosphorus, nickel, more iron… all made in the hellish forge of the
supernova heat, and flung outward into the Universe. It takes millennia or longer, but this material
mixes with the other gas and dust clouds floating in space. Sometimes, these clouds will be
actively forming stars — sometimes the collapse of the cloud to form stars may even be triggered
by the supernova slamming into it! Either way, the heavy elements created in the supernova will become
part of the next generation of stars and planets. Supernovae are how the majority of heavy elements
in the Universe are created and scattered. The calcium in your bones? The iron in your
blood? The phosphorus in your DNA? All created in the heart of the titanic death of a star.
That star blew up more than 5 billion years ago, but parts of it go on in you. Today you learned that massive stars fuse
heavier elements in their cores than lower mass stars. This leads to the creation of
heavier elements up to iron. Iron robs critical energy from the core, causing it to collapse.
The shock wave, together with a huge swarm of neutrinos, blast through the star’s outer
layers, causing it to explode. The resulting supernova creates even more heavy elements,
scattering them through space. Also, happily, we’re in no danger from a nearby supernova. Crash Course Astronomy is produced in association
with PBS Digital Studios. They have a YouTube channel with great videos — go, just go over
there, check their videos out. They’re fantastic. This episode was written by me, Phil Plait.
The script was edited by Blake de Pastino, and our consultant is Dr. Michelle Thaller.
It was directed by Nicholas Jenkins, edited by Nicole Sweeney, the sound designer is Michael Aranda,
and the graphics team, as always, is Thought Café.

100 comments on “High Mass Stars: Crash Course Astronomy #31

  1. So, Betelgeuse is a 0.5 magnitude star in Earth's sky, for comparison, if I were at Betelgeuse what would Sol's apparent magnitude be?

  2. I already know most of the stuff they talk about in these videos, but hearing it again in a very easy to understand and engaging format makes it enjoyable anyways.

  3. Now I got just one Question…
    How much Heat is Required to Fuse Iron into the next Elements ?

    I mean. Once these Guys become Black Holes. We cant really Check them anymore.
    But what if the Gravity is Strong enough to Fuse Iron into the next Element ?

    Could a Black Hole then Become an even Bigger Supernova ????

  4. I wish the video explained a little more about the creation of the heavier elements beyond iron.
    Are the heaviest elements created by the rapid collapse or at the moment just before the outward explosion or by the explosion shockwave. Also a comparison of the speed of the collapse compared to the speed of the resulting outward explosion. It seems like the collapse must be faster. But by what amount?

  5. Just the idea that neutrinos, the absolute epitome of smallness, are the main reason why the largest stars explode is about as poetic as reality can get.

  6. "in a fraction of a second, these neutrinos carry away 100 times as much energy as the sun produces IN ITS ENTIRE LIFE!"… OMFFGGG

  7. This was my second time crying in crash course astronomy.
    First time was the saturn episode.
    Not that I was sad, I just cry when I get super excited 😅

  8. I like this series of videos except that subjective and exaggerating adjectives are overused. I understand that these videos are meant to be approachable, but those adjectives are just unnecessary and not professional at all.

  9. What if we are all just a collective consciousness of our star which is why many ancient religions worship it

  10. I never understood how the star goes from imploding to exploding. Someone care to explain?
    Btw, awesome videos!

  11. Thanks you for another informative video! Can you please help me understand how stars can form with such wildly different masses?

  12. I'm under the impression that most of the heavy elements actually didn't come from the supernovas of massive stars but from colliding neutron stars and their junk they fling into space at that time.

  13. Thank you for explaining why iron fusion ends the life cycle of a star. Many documentaries glaze over those details.

  14. How are are elements bound iron made? You got me wondering this during your video. Good video btw thanks an please keep them coming they are quite interesting!

  15. Let me just say "WOW!!!!!!"
    I love PBS I seriously thought I've just about seen all there is to see about space and stars and black hole videos on YouTube I feel like a zombie just clicking away at boring same ol videos on YouTube but this video literally made me jump out of my zombified sleep state into a full attentive little kid learning about science for the first time again. I've never felt more intrigued learning the same thing twice. Very well done can't express how happy I am listening tl this channel rn. This is a summed up short version of how awesome j think this video is btw.

  16. Iron fusion does happen but since it is endothermic that's when the collapse occurs. It should be clearly stated that it isn't the existence of iron but the attempt to fuse iron that causes a the implosion.

  17. Strange really, iron is such a brittle and weak metal but when it turns up it can bring down the most powerful life giving force in the galaxy. Tars and Case will not approve.

  18. Why is it often said that a black hole is so powerful that even light cannot escape, yet there is always/often a jet stream of light and energy being ejected when the black hole is diagrammed?

  19. Incredible video! I wonder what is happening regarding the curvature of space-time when a star collapses and then goes super nova and possibly forms a black hole.

    1.) Does warping/curving space-time require energy?
    2.) When a star collapses to a black hole, is that the result of "breaking" the fabric of space-time or is space-time infinitely malleable?

    Thank you kindly.

  20. Phil Plait, you ARE a star my man, just don't go eating more than two point eight times the solar mass! Cheers for edutainment! X

  21. Sirius A and Sirius B could go super nova if that particular star system gets to close to a red dwarf star or a black hole and it disturbs and distorts Sirius B's orbit around Sirius A and it makes it orbit Sirius A much closer and Sirius B starts stealing mass from Sirius A.

  22. Look Doitis cost that mass for Earthe might Hit Domind AIRES Thomas McMahon MARS HOBSPHON might hit slfe in me Domind HOBSPHON Explodes

  23. I knew a little bit of that stuff already I would say about 25% you took me to another level and something very easy to understand. It is very appreciated and I enjoyed the video thank you very much to the Creator's I am going to subscribe.I'm picky as hell so that's about the best compliment I can give. Thanks again

  24. The largest known star is not VY canis majoris, in fact there are seven stars bigger than that. The largest star in the galaxy is UY scuti it is a hypergiant star of 8 solar masses.

  25. I used to enjoy these. Til I went to this Plait Twitter, and bashing the president in disrespectful way. So done with series.

  26. no danger from a Supernovae to us? …. what about the Gamma Ray Burst where they say it might caused mass extinction once on our Planet? 🙂

  27. I had no idea the timescales of heavier element fusion were so short :O that's mind boggling.. even more mind boggling that ghostly neutrinos are created in such volumes that they are responsible for the explosion.. like wtf.. they HATE interacting with matter :O

  28. hey why can't calcium or phosphorus be generated by regular fusion? they're pretty light elements? surely it's energetically favourable for some carbon or neon or nitrogen to fuse into phosphorus?

  29. Its going to be something like virus or ourself that kills us.. If we are lucky its gonna be a massive asteroid.. otherwise slow and painful..

  30. This means that if an piece of iron fell into a star, that iron would obviously melt but never change into other elements throughout the existence of the star?

  31. I think I found a problem in your logic. Let me explain: When a star grows older it gets hotter causing the outer layers of gas to expand which is exactly what we would expect to happen since gas tends to expand and diffuse evenly into its surrounding environment (especially when heated). Okay, I get that…so far so good. But, here’s the problem: If all that is true then stars could never form. The theory of stellar evolution would have us believe just the opposite – that large clouds of gas will somehow, someway contract as they heat up (presumably because of gavity). Sorry, but you can’t have it both ways. It seems evolutionists have a habit of picking and choosing whatever scenario they want to go with in any given situation only as it pleases them. So, which one is it?

  32. Every time I hear about iron being the death of a star, it always makes me look at my cast iron pots and think "What worlds have you destroyed?!"

  33. So if all Life rose and evolved to intelligence with one 1/2 Solarlife-Time then nothing precludes the existence of an intelligent species that once orbited the ProSolar Star that eventually exploded into the Nebula that birthed our Sun. .

  34. I would like to know whats going on when he says quantum physics brewing in the core generates neutrinos. Does anyone know what they think this entails?

  35. Sir kya aap isko hindi me convert krke bna skte h ……mko smajh ni aata eng me and m international space olumpiad ki tyari krri hu to apki videos jaruri h dekhna

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