Supernova (supernovae) Supernova (Supernovae)
- Where we left off in the last video, we had a mature,
- massive star, a star that had started forming a
- core of iron and it has enormous inward pressure on this core
- because as we form heavier and heavier elements in the core,
- the core gets denser and denser, and so we keep fusing more and more elements
- into it, and so this core becomes more and more massive, more and more dense,
- squeezing in on itself, and so. . . It's not fusing! That is not exothermic anymore!
- If iron were to fuse, it would not even be an exothermic process!
- It would require energy! So it wouldn't be even something that could help
- to fend off this squeezing, this increasing density of the core,
- so we have this iron here, and it just gets more and more massive,
- more and more dense, and so at some mass, already a reasonably high mass,
- the only thing that's keeping this from just completely collapsing
- is what we call Electron Degeneracy Pressure.
- Let me write this here. . .
- and all this means is we have all of these iron atoms
- really close to each other, and the only thing
- that keeps it from collapsing altogether at this earlier stage is that
- you have these electrons and these are being squeezed together
- we're talking about unbelievably dense states of matter.
- And electron degeneracy pressure is essentially saying that
- all these electrons don't want to be into the same place
- at the same time. I won't go into the quantum mechanics of it,
- but they cannot be squeezed into each other any more!
- So that, at least temporarily, holds this thing from collapsing even further.
- And in the case of a less massive star, in the case of a white dwarf,
- that's actually how a white dwarf maintains its shape!
- Because of the electron degeneracy pressure!
- But as this iron core gets even more massive, even more dense,
- and we get more gravitational pressure, so this is our core now,
- eventually even the electron degeneracy. . . I guess we could call it
- force, or pressure, this outward pressure, this thing that keeps it from collapsing,
- even that gives in! And then we have something called electron capture!
- Which is essentially the electrons get captured by protons in the nucleus!
- They start collapsing into the nucleus!
- It's kind of the opposite of beta negative decay, where you have
- the electrons getting captured, and protons getting turned into neutrons,
- you have neutrinos being released, but you can imagine an enormous
- amount of energy is also being released! So this is kind of a temporary (state?)
- and then all of a sudden, this collapses even more!
- Until all you have, and all the protons are turning into neutrons, because they're capturing electrons!
- So then you. . . What you eventually have is this entire core is collapsing
- into a dense ball of neutrons. You can kind of view them as one
- really really really massive atom, because it's just
- a dense ball of neutrons. And at the same time, when this collapse happens,
- you have an enormous amount of energy being released in the form of neutrinos!
- Did I say that neutrons are being released?
- No, no no. The electrons are being captured by the protons,
- protons turning into neutrons, this dense ball of neutrons right here,
- and in the process neutrinos get released!
- These fundamental particles, we won't go into the details here,
- but this enormous amount of energy. . . And this is actually is not really really well understood,
- of all of the dynamics here, because at the same time that this iron core is undergoing
- through this first it pauses due to the electron degeneracy pressure, and then
- it finally gives in because it's so massive, and then it collapses into this dense ball of neutrons,
- but when it does all of this energy and it's not clear how - because it would have to be
- a lot of energy,- because remember, this is a massive star,
- so you have a lot of mass in this area over here,
- but it's so much energy that that it causes the rest of the star to explode outward!
- in an unbelievably bright, or energetic, explosion.
- And that's called a supernova.
- And the reason why it's called nova, it comes from
- (I believe, I'm not an expert here) Latin for new!
- and the first time people observed a nova, they thought it was a new star!
- because something they didn't see before, all of a sudden,it looked like a star had appeared!
- because maybe it wasn't bright enough for us to observe it before, but when the nova
- occurred, it did become bright enough, it comes from the idea of new!
- But a supernova is when you have a pretty massive star, and its core is collapsing,
- and the energy is being released to explode the rest of the star out at unbelievable velocities.
- and just to fathom the amount of energy that is being released in a supernova,
- it can temporarily outshine an entire galaxy! And in a galaxy we're talking about
- hundreds of billions of stars! Or another way to think about it, in that very short period of time,
- it can release as much energy as the sun will in its entire lifetime!
- So these are unbelievably energetic events. And so you actually have the material that's not in the core
- being shot out of the star at appreciable percentages of the actual speed of light!
- So we're talking about things being shot out at up to 10% of the speed of light!
- That's 30,000 kilometers per SECOND! That's almost circumnavigating the earth every SECOND!
- So that's unbelievably energetic events that we're talking about here.
- And so if the star, if the original star, (these are rough estimates, people don't have a hard limit here)
- is 9-20 times the mass of the Sun, then it will supernova, and the core will turn into what is called
- a neutron star. Which you can imagine, is just this dense ball of neutrons.
- And just to give you a sense of it, it'll be something about
- maybe 2 times the mass of the Sun, give or take, 1.5 - 3 times the mass of the Sun.
- In a volume that has a diameter on the order of tens of kilometers!
- So roughly the diameter of a city! So this is unbelievably dense.
- And we know how much larger the Sun is relative to the Earth,
- and we know how much larger the Earth is relative to a city, but this is something more massive
- than the sun being squeezed into the size of a city. So unbelievably dense.
- If the original star is even more massive, if it's more than 20 times the Sun,
- Then even the neutron degeneracy pressure will give up and it will turn into a black hole.
- And that's - I could go into many of the details on that.
- And that's actually an open area of research still, on exactly what's going on inside of a black hole.
- But then it turns into a black hole, where essentially all of the mass gets condensed into
- an infinitely small and dense point, so something unbelievably hard to imagine.
- And just to give you a sense of it, this will be more mass
- than even three times the mass of the sun. So we're talking about an incredibly high amount of mass.
- So just to visualize things, here's a remnant of a supernova,
- this is the Crab Nebula, and it's about 6500 light years away.
- So it's still, from a galactic sense if you think of our galaxy as being 100,000 light years
- in diameter, it's still not too far from us on those scales.
- but it's an enormous distance. the closest star to us is 4 light years away,
- and it would take Voyager traveling at 60,000 kilometers/hour
- 80,000 years to get there.
- So this is a very - that's only 4 light years, this is 6,500 light years,
- but this supernova is believed to have happened 1,000 years ago, right at the center,
- and so at the center here, we should have a neutron star,
- and this cloud, this shock wave that you see here, this is the material traveling outwards
- from that supernova, over 1,000 years. The diameter of this sphere of material is 6 light years.
- So this is an enormously big shock wave cloud. And we believe that our solar system
- started to condense because of a shock wave created by a supernova relatively near to us.
- And just to enter another question that was probably jumping up in the last video,
- And this is still not really well understood. We talk about how elements up to iron, or maybe nickel,
- can be formed inside of the cores of massive stars.
- So you can imagine, when the star explodes, a lot of that material is released into the universe.
- And so that's why we have a lot of these materials in our own bodies.
- In fact, we could not exist if these heavier elements were not formed inside of the cores
- of primitive stars, stars that have supernovaed a long time ago.
- Now, the question is how do these heavier elements form?
- How do we get all of this other stuff on the periodic table?
- How do we get all these other heavier elements?
- And they're formed during the supernova itself.
- It's so energetic, you have all sorts of particles streaming out, and streaming in,
- streaming out because of the force of the shock wave,
- streaming in because of the gravity.
- But you have a mish-mash of elements forming, and that's actually where you have
- your heavier elements forming.
- And because (and I'll talk more about this in future videos) all of the uranium on Earth right now
- must have been formed in some type of a supernova explosion
- (or at least, based on our current understanding). And it looks to be about 4.6 billion years old.
- So, given that it looks to be about 4.6 billion years old, based on how fast it's decayed,
- (and I'll do a whole video on that), that's why we think that our solar system was first formed from some type
- of supernova explosion. Because that uranium would have been formed right at about
- the birth of our solar system. Anyway, hopefully you found that interesting,
- there's a fascinating picture, and if you go to Wikipedia and look up the Crab Nebula,
- keep clicking on the image and eventually you'll get a zoomed-in picture, and that's just
- even more mind-blowing, because you can see all the intricacy in the actual photo.
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At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
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When naming a variable, it is okay to use most letters, but some are reserved, like 'e', which represents the value 2.7831...
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At 2:33, Sal said "single bonds" but meant "covalent bonds."
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