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Where we left off in the last video, we had a mature,

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massive star, a star that had started forming a

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core of iron and it has enormous inward pressure on this core

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because as we form heavier and heavier elements in the core,

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the core gets denser and denser, and so we keep fusing more and more elements

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into it, and so this core becomes more and more massive, more and more dense,

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squeezing in on itself, and so. . . It's not fusing! That is not exothermic anymore!

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If iron were to fuse, it would not even be an exothermic process!

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It would require energy! So it wouldn't be even something that could help

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to fend off this squeezing, this increasing density of the core,

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so we have this iron here, and it just gets more and more massive,

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more and more dense, and so at some mass, already a reasonably high mass,

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the only thing that's keeping this from just completely collapsing

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is what we call Electron Degeneracy Pressure.

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Let me write this here. . .

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and all this means is we have all of these iron atoms

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really close to each other, and the only thing

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that keeps it from collapsing altogether at this earlier stage is that

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you have these electrons and these are being squeezed together

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we're talking about unbelievably dense states of matter.

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And electron degeneracy pressure is essentially saying that

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all these electrons don't want to be into the same place

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at the same time. I won't go into the quantum mechanics of it,

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but they cannot be squeezed into each other any more!

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So that, at least temporarily, holds this thing from collapsing even further.

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And in the case of a less massive star, in the case of a white dwarf,

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that's actually how a white dwarf maintains its shape!

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Because of the electron degeneracy pressure!

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But as this iron core gets even more massive, even more dense,

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and we get more gravitational pressure, so this is our core now,

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eventually even the electron degeneracy. . . I guess we could call it

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force, or pressure, this outward pressure, this thing that keeps it from collapsing,

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even that gives in! And then we have something called electron capture!

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Which is essentially the electrons get captured by protons in the nucleus!

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They start collapsing into the nucleus!

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It's kind of the opposite of beta negative decay, where you have

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the electrons getting captured, and protons getting turned into neutrons,

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you have neutrinos being released, but you can imagine an enormous

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amount of energy is also being released! So this is kind of a temporary (state?)

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and then all of a sudden, this collapses even more!

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Until all you have, and all the protons are turning into neutrons, because they're capturing electrons!

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So then you. . . What you eventually have is this entire core is collapsing

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into a dense ball of neutrons. You can kind of view them as one

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really really really massive atom, because it's just

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a dense ball of neutrons. And at the same time, when this collapse happens,

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you have an enormous amount of energy being released in the form of neutrinos!

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Did I say that neutrons are being released?

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No, no no. The electrons are being captured by the protons,

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protons turning into neutrons, this dense ball of neutrons right here,

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and in the process neutrinos get released!

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These fundamental particles, we won't go into the details here,

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but this enormous amount of energy. . . And this is actually is not really really well understood,

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of all of the dynamics here, because at the same time that this iron core is undergoing

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through this first it pauses due to the electron degeneracy pressure, and then

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it finally gives in because it's so massive, and then it collapses into this dense ball of neutrons,

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but when it does all of this energy and it's not clear how - because it would have to be

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a lot of energy,- because remember, this is a massive star,

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so you have a lot of mass in this area over here,

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but it's so much energy that that it causes the rest of the star to explode outward!

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in an unbelievably bright, or energetic, explosion.

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And that's called a supernova.

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And the reason why it's called nova, it comes from

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(I believe, I'm not an expert here) Latin for new!

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and the first time people observed a nova, they thought it was a new star!

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because something they didn't see before, all of a sudden,it looked like a star had appeared!

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because maybe it wasn't bright enough for us to observe it before, but when the nova

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occurred, it did become bright enough, it comes from the idea of new!

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But a supernova is when you have a pretty massive star, and its core is collapsing,

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and the energy is being released to explode the rest of the star out at unbelievable velocities.

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and just to fathom the amount of energy that is being released in a supernova,

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it can temporarily outshine an entire galaxy! And in a galaxy we're talking about

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hundreds of billions of stars! Or another way to think about it, in that very short period of time,

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it can release as much energy as the sun will in its entire lifetime!

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So these are unbelievably energetic events. And so you actually have the material that's not in the core

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being shot out of the star at appreciable percentages of the actual speed of light!

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So we're talking about things being shot out at up to 10% of the speed of light!

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That's 30,000 kilometers per SECOND! That's almost circumnavigating the earth every SECOND!

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So that's unbelievably energetic events that we're talking about here.

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And so if the star, if the original star, (these are rough estimates, people don't have a hard limit here)

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is 9-20 times the mass of the Sun, then it will supernova, and the core will turn into what is called

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a neutron star. Which you can imagine, is just this dense ball of neutrons.

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And just to give you a sense of it, it'll be something about

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maybe 2 times the mass of the Sun, give or take, 1.5 - 3 times the mass of the Sun.

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In a volume that has a diameter on the order of tens of kilometers!

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So roughly the diameter of a city! So this is unbelievably dense.

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And we know how much larger the Sun is relative to the Earth,

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and we know how much larger the Earth is relative to a city, but this is something more massive

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than the sun being squeezed into the size of a city. So unbelievably dense.

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If the original star is even more massive, if it's more than 20 times the Sun,

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Then even the neutron degeneracy pressure will give up and it will turn into a black hole.

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And that's - I could go into many of the details on that.

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And that's actually an open area of research still, on exactly what's going on inside of a black hole.

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But then it turns into a black hole, where essentially all of the mass gets condensed into

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an infinitely small and dense point, so something unbelievably hard to imagine.

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And just to give you a sense of it, this will be more mass

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than even three times the mass of the sun. So we're talking about an incredibly high amount of mass.

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So just to visualize things, here's a remnant of a supernova,

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this is the Crab Nebula, and it's about 6500 light years away.

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So it's still, from a galactic sense if you think of our galaxy as being 100,000 light years

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in diameter, it's still not too far from us on those scales.

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but it's an enormous distance. the closest star to us is 4 light years away,

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and it would take Voyager traveling at 60,000 kilometers/hour

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80,000 years to get there.

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So this is a very - that's only 4 light years, this is 6,500 light years,

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but this supernova is believed to have happened 1,000 years ago, right at the center,

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and so at the center here, we should have a neutron star,

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and this cloud, this shock wave that you see here, this is the material traveling outwards

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from that supernova, over 1,000 years. The diameter of this sphere of material is 6 light years.

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So this is an enormously big shock wave cloud. And we believe that our solar system

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started to condense because of a shock wave created by a supernova relatively near to us.

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And just to enter another question that was probably jumping up in the last video,

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And this is still not really well understood. We talk about how elements up to iron, or maybe nickel,

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can be formed inside of the cores of massive stars.

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So you can imagine, when the star explodes, a lot of that material is released into the universe.

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And so that's why we have a lot of these materials in our own bodies.

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In fact, we could not exist if these heavier elements were not formed inside of the cores

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of primitive stars, stars that have supernovaed a long time ago.

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Now, the question is how do these heavier elements form?

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How do we get all of this other stuff on the periodic table?

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How do we get all these other heavier elements?

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And they're formed during the supernova itself.

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It's so energetic, you have all sorts of particles streaming out, and streaming in,

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streaming out because of the force of the shock wave,

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streaming in because of the gravity.

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But you have a mish-mash of elements forming, and that's actually where you have

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your heavier elements forming.

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And because (and I'll talk more about this in future videos) all of the uranium on Earth right now

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must have been formed in some type of a supernova explosion

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(or at least, based on our current understanding). And it looks to be about 4.6 billion years old.

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So, given that it looks to be about 4.6 billion years old, based on how fast it's decayed,

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(and I'll do a whole video on that), that's why we think that our solar system was first formed from some type

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of supernova explosion. Because that uranium would have been formed right at about

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the birth of our solar system. Anyway, hopefully you found that interesting,

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there's a fascinating picture, and if you go to Wikipedia and look up the Crab Nebula,

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keep clicking on the image and eventually you'll get a zoomed-in picture, and that's just

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even more mind-blowing, because you can see all the intricacy in the actual photo.