Supernova Explained: The Final Moments of a Massive Star

[00:00:00]Massive stars end their existence in a blaze of glory called a supernova.

[00:00:05]You've probably heard of them before.

[00:00:07]What you might not know is that a lot is happening in the core right before that explosion.

[00:00:12]The life of one of these stars is measured in millions of years, but there are several phases near the end that occur at normal human timescales.

[00:00:21]What are those moments like and why does this end in a supernova?

[00:00:25][Intro Music] Hey Crazies.

[00:00:30]Contrary to what you might have heard, not all stars end in a supernova.

[00:00:34]Anything less than about eight times the mass of our sun and the star will just kind of fizzle out gradually.

[00:00:40]There's no sudden catastrophic explosion.

[00:00:43]Today is about stars with masses greater than eight suns. Why, though?

[00:00:49]I was trying to give you the day off, but come on.

[00:00:52]Why does it take eight solar masses?

[00:00:54]Because of the way fusion works.

[00:00:57]See, the star’s own mass is trying to make it collapse.

[00:01:01]It's called self gravity.

[00:01:03]The only thing holding it back is the energy released from the fusion in the core.

[00:01:07]The star finds a balance pretty quickly that we call hydrostatic equilibrium.

[00:01:12]So after that, the star is stable?

[00:01:15]Well, for a while.

[00:01:17]Stars do have a lot of particles in them, but that fuel supply is finite.

[00:01:21]After millions of years of fusing hydrogen into helium, the core will run low on hydrogen.

[00:01:28]Not so much that hydrogen fusion stops, but the supply will get low enough that the fusion can't support the star's weight anymore.

[00:01:35]Can it find a new equilibrium? Yeah, but only so many times.

[00:01:39]How many times? That depends on the mass of the star.

[00:01:43]Red dwarfs can only turn hydrogen into helium, so they only have one equilibrium available.

[00:01:49]But stars like our sun will eventually be able to turn helium into carbon.

[00:01:53]So they have a second equilibrium they can use later in life.

[00:01:56]The largest stars have six different equilibriums and go supernova when the sixth one fails.

[00:02:05]Feel free to holler if you have any more questions.

[00:02:08][Transition Music] To be clear, it's not like all these processes are happening at the same time, at least not at first.

[00:02:16]For the main bulk of a star's life, hydrogen fusion is all that's happening.

[00:02:21]As a massive star runs low on hydrogen, it has to switch to helium.

[00:02:24]But hydrogen fusion doesn't just stop.

[00:02:27]The core now has two regions: Helium fusion in the inner core and hydrogen fusion in the outer core.

[00:02:33]Helium will eventually run low as well, and in about a 10th of the time.

[00:02:37]So the star will have to switch again to maintain equilibrium.

[00:02:41]Each successive transition increases the temperature of the inner core and occurs at a faster rate.

[00:02:47](Off Camera) Is it faster because it's hotter?

[00:02:49]Yes, mostly, but it's not the only reason.

[00:02:51]When you fuse nuclei, you often end up with less of the new thing.

[00:02:56]For example, it takes four hydrogens to make one helium, and it takes three heliums to make one carbon.

[00:03:02]That means you have less fuel for the next stage.

[00:03:05]But later there are stages where it's just about adding helium, so it's not always a factor.

[00:03:10]And temperature is definitely the bigger factor.

[00:03:12]If it were just the fuel, you'd expect helium fusion to be only four times faster, when it's actually ten times faster.

[00:03:20]It's complicated, is what I'm saying.

[00:03:22]The point is you can see these last few stages happen at normal human timescales.

[00:03:27]This is the blink of an eye, astronomically speaking.

[00:03:30]These are the last few moments of a massive star's life immediately before it supernovas.

[00:03:36]By that time, the core looks like an onion.

[00:03:38]There are so many layers of fusion.

[00:03:40]But the process happening in the center is the most important.

[00:03:44]It's the hottest and releases the most energy.

[00:03:47]If that inner core fails, it's all over.

[00:03:50][Transition Music] The whole reason these fusion reactions can support the star's weight is because they release energy.

[00:03:58]And the only reason they release energy is because successive nuclei are more tightly bound.

[00:04:04](Off Camera) How does binding them together release energy?

[00:04:06]Oh, great question!

[00:04:08]Binding is a type of negative energy.

[00:04:10]You can think of it like an energy deficit.

[00:04:13]A nucleus being lower on this graph means it's more tightly bound and has a bigger deficit.

[00:04:19]That deficit is paid for by a release of energy equal to the difference.

[00:04:23]Conservation of energy shall not be violated!

[00:04:26]Imagine two basketballs are being rolled toward either side of a hill with a ditch in the middle.

[00:04:32]If they're moving slowly, they'll just roll back down.

[00:04:35]In a sense, they repel each other.

[00:04:37]If they're moving fast enough though, they'll make it all the way into the ditch and get stuck.

[00:04:41]We could say those balls are bound to each other.

[00:04:44]This is basically what fusion is.

[00:04:46]What matters is the depth of these ditches in our analogy.

[00:04:50]You can see the middle of this one is deeper than the ground outside.

[00:04:54]There's a gravitational energy difference that got released as heat or something.

[00:04:58]The process is said to be exothermic.

[00:05:01]A similar thing happens with the atomic nuclei in a star.

[00:05:05]In the outer layers, they're not moving fast enough and they repel each other.

[00:05:08]Like the slower basketballs, it's as if they can't quite make it up a hill.

[00:05:13]But stellar cores are hot.

[00:05:15][Spring Noises] The nuclei are moving so fast that they essentially don't have time to repel each other.

[00:05:21]Fast fast!

[00:05:23]They make it up their hill, so to speak, and they get bound to each other.

[00:05:26]The energy difference is released, but in a more of subatomicy way.

[00:05:30]You know, like with a super energetic photon or a positron or neutrino or something.

[00:05:35]Every nucleus exists in a tiny ditch on a giant cliff face.

[00:05:40]Connecting all the ditches gives us the binding energy curve, which we saw for a brief moment earlier.

[00:05:45]And as larger nuclei are built, there's a downward trend.

[00:05:48]Energy continues to be released.

[00:05:50]The process continues to be exothermic... up to a point.

[00:05:55]After an atomic mass of 62, the energy trends back upward again.

[00:05:59]Any further change to the right doesn't release energy anymore.

[00:06:02]It absorbs it.

[00:06:04]The process is said to be endothermic.

[00:06:06]Endothermic fusion is catastrophically bad news for a star.

[00:06:10]The last and shortest stage of a massive star's life is silicon fusion, which is when it makes a whole list of larger nuclei until it hits that upper limit.

[00:06:19](Off Camera) That happens when the star makes iron, right?

[00:06:22]Actually, that's a common misconception.

[00:06:24]Surprise, surprise!

[00:06:26][Transition Music] Full disclosure, this was recently a huge surprise for me too.

[00:06:32]I had to unlearn some things.

[00:06:34]This silicon fusion stage isn't all that fancy.

[00:06:37]It's just adding helium over and over again.

[00:06:40]Helium happens to have two protons and two neutrons.

[00:06:43]So, any time that gets added to another nucleus: the total atomic number will go up by two, but the total atomic mass will go up by four.

[00:06:52]It's Periodic Table time!

[00:06:54]Starting at Silicon-28, these are the elements that get made in that final stage.

[00:06:59]But the star’s not making the typical isotopes of these elements, at least not in large quantities.

[00:07:04]While each step is two boxes at a time, the masses don't always match.

[00:07:09]Normal, everyday stable iron is actually iron-56.

[00:07:14]We're four neutrons short.

[00:07:15](Off Camera) Iron-52 is still iron though, isn’t it? Kind of, but it's not stable.

[00:07:20]Besides, the star doesn't stop at iron anyway.

[00:07:23]Silicon fusion can go all the way up to nickel-56.

[00:07:26]Nickel is the end of the line, not iron.

[00:07:29]Yes, Nickel-56 is unstable and will decay into iron-56 if left alone.

[00:07:34]Except it is not left alone.

[00:07:36]There are high energy photons zipping around, breaking nuclei apart in a process called photodisintegration.

[00:07:42]Which would be a cool band name.

[00:07:44]Anyway, remember how the tipping point for binding energy occurred at an atomic mass of 62?

[00:07:50]Well, stars never make it that far.

[00:07:52]Thanks to photodisintegration, the stellar fusion limit is a little lower at 56, and it certainly doesn't make isotopes this stable.

[00:07:59]The stellar fusion chain is actually up here at less stable binding energies.

[00:08:04]All those high energy photons that were supposed to be supporting the star’s weight, they're breaking nuclei apart instead.

[00:08:10]There isn't a mass large enough to overcome this problem.

[00:08:13]Catastrophic failure is imminent.

[00:08:16][Transition Music] These massive stars begin their life calm but end in a panic.

[00:08:23]First, there's hydrogen fusion, which lasts millions of years.

[00:08:27]Then there's helium fusion, which lasts hundreds of thousands of years.

[00:08:31]Next, there's carbon fusion, which only lasts thousands of years.

[00:08:35]After that, the panic sets in.

[00:08:37]Neon fusion only lasts for years, like regular Earth years.

[00:08:42]Oxygen fusion only lasts for weeks.

[00:08:44]And finally, silicon fusion only lasts for days.

[00:08:48]The star is frantically looking for heliums to smash into its bigger nuclei.

[00:08:52]But it's running out.

[00:08:54]To make matters worse, the high energy photons that were supposed to be helping are now working against the star.

[00:09:00]Without sufficient energy released from the core, the star collapses on itself.

[00:09:04]When that core reaches a critical density, which happens quickly, it becomes a neutron star.

[00:09:09]An outward shockwave is created, powered by a rush of neutrinos from the formation of the neutron star.

[00:09:16]So much energy is released in that instant that the star explodes in a supernova, which temporarily outshines its entire galaxy.

[00:09:24]It's the most spectacular event in the whole universe.

[00:09:27]And all it took was eight suns worth of mass and waiting millions of years.

[00:09:32]So until the next one, remember, it's okay to be a little crazy.

[00:09:36]Thanks to everyone who supports the channel.

[00:09:38]I'm overwhelmed with your generosity.

[00:09:40]A special thanks goes out to our new Asylum Counselor: Joe K. Kim and our new Einsteinium crazies: Dan Sullivan and William Hutchison.

[00:09:49]If you'd like to support the channel, there are links down in the doobly-do.

[00:09:54]Imaginary numbers are just as real as real numbers.

[00:09:57]While it is true that imaginary numbers can have real world consequences, like with alternating current in quantum wave functions, they do that through a phase angle.

[00:10:06]There's no way to put a phase angle on mass and have it make any sense.

[00:10:10]Anyway, thanks for watching!

[00:10:13]Surprise, surprise!!