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Transcript
0:00	- What would happen if
0:01	a star exploded near the earth?
0:04	Well, the nearest star to Earth, of course, is the sun,
0:07	and it is not going to explode,
0:09	but if it had eight times the mass,
0:12	then it would go supernova at the end of its life.
0:15	So what would that look like?
0:17	Well, as noted by xkcd if you held up
0:20	a hydrogen bomb right to your eyeball and detonated it,
0:25	that explosion would still be a billion times less bright
0:30	than watching the sun go supernova from Earth.
0:36	That's how insanely powerful supernova explosions are.
0:39	They are the biggest explosions in the universe.
0:42	When we see supernovae in other galaxies,
0:45	they are brighter than the combined light
0:48	of hundreds of billions of stars, so bright, in fact,
0:52	that they appear to come out of nowhere.
0:55	On the 8th of October, 1604,
0:57	the astronomer Johannes Kepler looked up into the night sky
1:01	and noticed a bright star he had never seen before.
1:05	It was brighter than all the other stars in the sky
1:08	and about as bright as the planet Jupiter.
1:12	On moonless nights, it was bright enough to cast a shadow.
1:16	Kepler published his observations
1:18	of this star in a book called "De Stella Nova,"
1:20	which means "about a new star" in Latin.
1:24	Kepler thought he was witnessing the birth of a new star,
1:28	but it was actually a star's violent death.
1:32	Over the following year and a half,
1:33	the light faded until it was no longer visible,
1:37	but the name stuck.
1:39	Even once we learned what was really happening in the 1930s,
1:42	the violent final explosion for stars
1:45	between about 8 and 30 solar masses
1:47	has been called a supernova.
1:51	But how exactly a star explodes is not
1:54	what most people think.
1:57	For most of a star's life, it exists in a stable balance.
2:00	In its core, it fuses lighter elements together
2:03	to make heavier ones, and in the process
2:05	it converts a small amount of matter into energy.
2:08	This energy is really what keeps the star
2:11	from collapsing in on itself.
2:14	Gravity compresses the star,
2:16	but that force is counteracted by the pressure generated
2:19	by the movement of particles inside the star,
2:22	and by the pressure of photons released by fusion.
2:26	So in effect, stars are propped up by their own light.
2:32	If the rate of fusion drops at the center of the star,
2:34	the temperature and the pressure decrease.
2:37	Gravity starts winning, compressing the star,
2:41	but this increases the temperature and pressure in the core,
2:44	which increases the rate of fusion.
2:46	It's a stable self-regulating system, but there's a problem.
2:51	Stars have a finite amount of fuel,
2:54	which over time gets used up.
2:56	Our sun is about 5 billion years
2:59	into its 10-billion-year lifespan.
3:02	There are stars dozens of times more massive than the sun,
3:05	which you would think would live much longer,
3:07	but they actually use up their nuclear fuel faster
3:11	A star 20 times the mass of our sun has
3:13	a lifespan of just 10 million years,
3:16	and more massive stars burn hotter, and even brighter,
3:20	but for much shorter lives.
3:22	For 90% the life of a star,
3:24	the core is only hot enough to fuse hydrogen into helium,
3:29	and when the hydrogen runs out,
3:31	fusion slows, gravity compresses the core
3:34	and its temperature increases to 200 million degrees,
3:38	at which point helium fuses into carbon.
3:41	There's enough helium to power the star for around
3:44	a million years, but as the helium runs out,
3:47	the core is, again, compressed and heated.
3:49	Carbon starts fusing into neon,
3:51	which lasts about 1,000 years,
3:53	and then neon fuses into oxygen for a few more years,
3:57	then oxygen to silicon for a few months
4:00	and at 2.5 billion degrees, silicon fuses
4:03	into nickel which decays into iron.
4:07	Now, at the heart of this giant star,
4:08	there is an iron core building that's only
4:12	a few thousand kilometers across.
4:15	Iron is where this pattern stops.
4:17	Instead of liberating energy as it fuses
4:20	into heavier elements, it actually requires energy.
4:24	Iron is the most stable element.
4:27	So it actually takes energy both
4:28	to fuse it into heavier elements
4:30	and to break it down into lighter ones.
4:33	Both fusion and fission reactions ultimately end up at iron.
4:38	The iron core grows,
4:40	but the crush of gravity becomes greater
4:42	and greater as the rate of fusion drops.
4:45	When the iron core is about 1.4 times the mass of our sun,
4:49	which is known as the Chandrasekhar limit, the pull
4:52	of gravity is so strong that something totally wild happens.
4:56	Quantum mechanics takes over.
4:58	Electrons run out of room to move,
5:01	and they're forced into their lowest energy states,
5:04	and they then become absorbed by the protons in the nucleus.
5:09	In this process, the protons turn
5:11	into neutrons and release neutrinos.
5:15	With the electrons gone, the core collapses,
5:18	and fast, at about 25% the speed of light.
5:22	So what used to be a ball
5:23	of iron 3,000 kilometers in diameter becomes
5:27	a ball of neutrons just 30 kilometers across.
5:30	Essentially, it's a neutron star.
5:33	With no outward pressure to hold it up,
5:36	the rest of the star caves in.
5:38	Also, falling at a quarter of the speed of flight,
5:41	it hits the neutron star and bounces off,
5:44	creating a huge pressure wave.
5:46	But this kinetic energy isn't quite enough
5:49	to start a supernova explosion.
5:51	No, the thing that really kicks it off
5:53	is the humble neutrino.
5:55	Now, I normally think of neutrinos
5:57	as particles that do basically nothing.
6:00	I mean, they interact so rarely with matter
6:02	that right now there are 100 trillion neutrinos
6:05	passing through your body per second.
6:08	It would take a light year of lead just
6:10	to give you a 50-50 chance of stopping a neutrino,
6:14	and that's because they interact only
6:15	through gravity and the weak force
6:18	but in a supernova, when the electrons are captured
6:21	by the protons, an unbelievable number
6:23	of neutrinos is released, around 10^58.
6:28	You would think they would just fly off
6:30	at nearly the speed of light, but the core
6:33	of a supernova is incredibly dense,
6:35	about 10 trillion times more dense than lead
6:39	and as a result, it traps some of those neutrinos
6:43	and captures their energy,
6:45	and this is what makes a star go supernova.
6:48	A particle that is millions
6:50	of times less massive than an electron that barely interacts
6:54	with anything is responsible for some
6:57	of the largest explosions in the universe.
7:01	In that explosion, only 1/100 of 1%
7:05	of the energy is released as electromagnetic radiation,
7:08	the light that we can see.
7:10	Even then, supernova have enough energy
7:12	to outshine a whole galaxy.
7:14	About 1% of the energy is released as the kinetic energy
7:18	of the exploding matter, but the vast majority
7:21	of the energy is released in the form
7:24	of neutrinos, and neutrinos are actually
7:27	the first signal we detect from supernovae,
7:31	and that's because after they're generated
7:33	in the core, they can escape
7:36	before the shockwave reaches the surface,
7:38	where the light that we see is generated.
7:41	So neutrinos can arrive on Earth hours before the photons,
7:44	giving astronomers a chance to aim their telescopes
7:47	at the right part of the sky.
7:51	I actually used to work
7:52	at a neutrino observatory back in college,
7:54	and I would work the graveyard shift
7:56	between midnight and 8:00 AM.
7:58	So if I detected a really big increase
8:01	in the neutrino flux during my shift,
8:02	it was my job to call and wake up scientists,
8:06	so they could go look out for a supernova.
8:09	Now, that never actually happened,
8:10	but we did have some close calls.
8:12	Now, I need to clarify a couple things.
8:15	First, not all really massive stars explode.
8:18	As they collapse, some form black holes instead,
8:21	which means they do not go supernova
8:24	and second, there's another way to make a supernova.
8:27	Sometimes a white dwarf star, which is incredibly dense,
8:30	pulls matter off a nearby star, and when it's mass reaches
8:34	that Chandrasekhar limit of 1.4 solar masses,
8:37	the white dwarf collapses, creating a supernova.
8:41	This is actually the type of supernova that Kepler saw
8:43	in 1604, a supernova 20,000 light years from Earth.
8:49	Now, because the shocks are asymmetric,
8:51	supernova explain neutron stars that can move really fast.
8:56	There's a neutron star we've observed with a velocity
8:58	of 1,600 kilometers per second, and we think that was caused
9:04	by a very asymmetric supernova explosion,
9:07	sent it shooting off in the other direction.
9:10	Despite only recently learning about how supernovae work,
9:13	humans have been observing them for thousands of years.
9:17	Ancient Indian, Chinese, Arabic
9:19	and European astronomers all observed supernovae,
9:23	but they are quite rare.
9:25	In a galaxy like our Milky Way,
9:27	consisting of 100 billion stars,
9:30	there are only about one or two supernovae per century.
9:34	A particularly amazing example is the supernova of 1054,
9:39	when the light of a supernova 6,500 light years away
9:42	reached the earth and was recorded by Chinese astronomers.
9:47	If we look to where that supernova was recorded,
9:50	we see the Crab Nebula.
9:52	It is a giant remnant of radioactive matter,
9:56	left behind by the explosion.
9:58	In the 1,000 years since the explosion,
10:01	the remnant has grown to 11 light years in diameter.
10:05	Supernovas produce a lot of cosmic rays.
10:08	Cosmic rays are actually particles,
10:10	mainly protons and helium nuclei,
10:13	and they travel out at very,
10:15	very nearly the speed of light.
10:17	They have a tremendous amount of energy.
10:20	So at what distance could
10:22	a supernova cause problems for life on Earth?
10:25	The closest stars to us, besides the sun,
10:27	are the three stars in Alpha Centauri.
10:30	They are 4.4 light years away, but stars do move around
10:35	and on average, a star gets within one light year
10:38	of Earth every 500,000 years.
10:41	So what would happen if such a star went off?
10:46	- Yeah, so within a light year, you're easily
10:48	within a danger distance from just the kinetic energy.
10:52	So I think even at that distance,
10:54	you're looking at possibly blowing the atmosphere off.
10:58	- But we would also
10:59	have other problems to worry about.
11:01	Supernovae create conditions that are hot enough
11:03	to fuse elements heavier than iron.
11:06	In the months after the explosion, these elements
11:09	undergo radioactive decay, producing gamma rays
11:12	and cosmic rays.
11:14	Less than 0.1% of the energy produced
11:16	by a supernova is emitted as gamma rays
11:19	from these radioactive decays,
11:20	but even this tiny percentage can be dangerous.
11:24	At a few light years from a supernova,
11:26	the radiation could be deadly, though most
11:29	of it would be blocked by our atmosphere.
11:33	Now, the earth is protected from solar and cosmic radiation
11:36	by our atmosphere, and specifically by ozone molecules,
11:41	three oxygen atoms bonded together,
11:43	but high energy cosmic rays from supernova can come down
11:47	and break apart nitrogen molecules in the atmosphere,
11:52	and then these bond with oxygen atoms,
11:55	which can then break apart ozone,
11:58	and so we can lose a lot of our ozone
12:01	if there's too many cosmic rays coming
12:03	from supernova events, and that can expose us
12:05	to all kinds of dangerous radiation coming in from space.
12:09	We actually see an increase
12:10	in atmospheric NO3 concentrations,
12:13	coinciding with supernova explosions.
12:16	A supernova within 30 light years is rare,
12:19	only happening maybe once every 1 1/2 billion years or so,
12:23	but a recent article suggests supernovae could be lethal all
12:26	the way out to 150 light years away,
12:30	and so those would be much more common.
12:32	We actually have evidence
12:34	for a supernova that went off 150 light years
12:36	from Earth 2.6 million years ago.
12:39	It would've been seen by our early human ancestors,
12:42	like Australopithecus, and we know this
12:45	because there are elements present on Earth
12:47	that could only have been deposited by a recent supernova.
12:51	In sedimentary rocks at the bottom of the Pacific Ocean,
12:54	scientists have found traces of iron-60,
12:57	in a layer that was deposited 2.6 million years ago.
13:02	Iron-60 is an isotope of iron
13:04	with four more neutrons than the most common type of iron.
13:08	Iron-60 is really hard to make.
13:10	Our sun doesn't make it, nor is it produced, basically,
13:13	anywhere else in the solar system.
13:15	Iron-60 is made, basically, exclusively
13:18	in supernova explosions,
13:20	and iron-60 is radioactive.
13:22	It has a half life of 2.6 million years.
13:25	So every 2.6 million years,
13:27	half of the sample decays into cobalt-60.
13:31	So all of the iron-60 that was around during the formation
13:34	of the earth, 4.5 billion years ago,
13:36	has definitely decayed.
13:38	So the iron-60 that the scientists measure
13:41	is proof of a recent supernova.
13:43	Scientists also measured trace amounts of manganese-53
13:47	in the same sediments, giving further evidence
13:49	supporting the idea that recently there was
13:52	an explosion of a nearby supernova.
13:55	The supernova that happened 2.6 million years ago
13:58	wasn't catastrophic for our ancestors,
14:01	but some researchers hypothesized that it could be related
14:04	to the mass extinction, which is seen
14:05	at the Pliocene-Pleistocene boundary
14:08	in the fossil record around the same time.
14:11	This extinction wiped out around 1/3 of marine megafauna.
14:15	The idea is that the cosmic rays
14:17	from the supernova hit particles in our atmosphere,
14:20	creating muons, which are charged particles
14:23	like the electron, only more than 200 times heavier.
14:26	The muon flux for years after the supernova
14:29	would've been 150 times higher than normal,
14:33	and the bigger the animal, the larger
14:35	the radiation dose it would've received from these muons,
14:38	which is why megafauna were so disproportionately affected,
14:42	and what's more, the animals that lived
14:45	in shallower waters were more likely
14:47	to become extinct compared to the ones that lived at depth,
14:50	where the water would've protected them from muons.
14:54	Further evidence for these recent nearby supernovae comes
14:57	from our place in the galaxy.
15:00	You know, if you look in the space
15:01	between the stars in our galaxy, on average,
15:04	there are around a million hydrogen atoms per cubic meter.
15:08	That may sound like a lot,
15:09	but it's basically a perfect vacuum
15:11	but for hundreds of light years
15:14	in all directions around our solar system,
15:17	you find there are 1,000 times fewer hydrogen atoms.
15:21	It's like they've all been blown out somewhere,
15:24	and our solar system is existing in this cosmic void,
15:28	inside a low density bubble.
15:31	So that is evidence for maybe tens of supernovae
15:34	that would've blown all this material outwards,
15:38	but there are cosmic explosions that are even
15:40	more deadly than normal supernovae, gamma ray bursts.
15:44	Gamma ray bursts were discovered by the Vela satellites,
15:47	which were looking for Soviet nuclear tests
15:50	but on the 2nd of July, 1967,
15:52	the satellites detected a large burst of gamma rays,
15:56	which were coming from space.
15:59	There are two main sources of gamma ray bursts,
16:02	mergers of neutron stars and the core collapses
16:05	of gigantic stars called hypernovae.
16:08	Hypernovae are caused by stars that are
16:10	at least 30 solar masses and rapidly spinning.
16:14	Their collapse leads to an explosion 10 times more powerful
16:18	than a regular supernova, and it leaves behind a black hole.
16:23	The gamma ray bursts caused by hypernovae channel most
16:27	of their energy into beams
16:28	which are just a few degrees across.
16:32	If there was a gamma ray burst within 6,000 light years,
16:35	it would decrease the ozone level enough
16:38	that it could be catastrophic.
16:40	To put this distance in context, a sphere with a radius
16:43	of 6,000 light years contains hundreds of millions of stars.
16:49	On October 9th, 2022, astronomers detected one
16:52	of the most powerful gamma ray bursts ever measured.
16:55	It was powerful enough to measurably affect how
16:58	the ionosphere bounces radio waves.
17:00	The effect on the ionosphere was
17:02	around the same as a solar flare,
17:05	but this gamma ray burst was located
17:07	in a galaxy 2.5 billion light years away.
17:12	Astronomers speculate that a gamma ray burst
17:14	could have caused the Late Ordovician mass extinction,
17:17	which wiped out 85% of marine species 440 million years ago.
17:23	There is no direct evidence,
17:25	but gamma ray bursts are common enough
17:27	that it is estimated that there has been
17:29	a 50% chance that there was
17:31	an ozone-removing, extinction-causing GRB
17:34	in the vicinity of Earth in the last 500 million years.
17:38	So if a supernova or a gamma ray burst were
17:41	to go off near the earth now,
17:43	that would be pretty catastrophic but in an ironic twist,
17:47	we kind of owe our existence to these sorts
17:50	of explosions because 4.6 billion years ago,
17:54	it was probably the shockwave from a nearby supernova
17:58	which triggered the collapse of a cloud of gas
18:02	and dust that gradually coalesced to form our solar system.
18:06	So the sun, the earth and all of us wouldn't be here today
18:11	without the explosions of nearby stars.
18:22	Figuring out how supernova explode was incredibly difficult.
18:25	It took a combination of astrophysics,
18:27	particle physics, computer science and mathematics,
18:30	and if you wanna develop a better understanding
18:32	of our universe, then you should check out
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