The tension must be mounting, we've been, I've been leaving our star poised with
the core collapsing and we haven't asked. what happens then and so core collapse is
going to be the next topic obviously. And so what happens when the center of a
star collapses. You have this
Solar mass, few solar masses of, solid iron ru, rich core, collapsing and, it
stats with a sort of, typicle stellar core size, size of the earth and it
collapses down it turns out dramatically, in about a tenth of a second it collapses
to a size of a few kilometers and we'll talk about the remnant, in a later clip.
But, this gravitational collapse heats the, core to billions of Kelvin and the,
ambient thermal radiation that's going on there is now gamma rays and this is
terrible because as we discussed this needs to photodiscintigration of heavy
nuclei this breaks up all of the nucleotides that the star has been
diligently collecting. Working its way up from hydrogen to
helium to carbon to neon, ever so slowly. Are now completely eliminated, almost
completely eliminated by the hot radiation soup in the stellar core.
So whatever has not been ejected so far, is pretty much back to protons,
At the same time as the core collapses the out layers of the star find
themselves unsupported and at some point there's not what's called a homologous
collapse so the star collapsing As a system.
In some sense, the outer layers of the star are essentially in gravitational
free fall. They lose contact, they can't respond
faster than the speed of sound. They don't know in time that the core has
collapsed. And kind of like our slinky, the outer
layers find themselves suspended in space with nothing holding them up.
And they fall in gravitational free fall, achieving speeds as high as fifteen or
twenty% of the speed of light. So we're being dicey with ignoring
relativity in this situation. In the core as it collapses, as we said
electron degeneracy has completely been left behind.
Electron degeneracy will not support this core.
In fact the Pressures are so intense that electrons
essentially disappear. They are forced into inverse beta
processes, so that protons and electrons combine, leaving the star, the core, that
is essentially all neutrons. This is not to say there are not protons
and electrons left, but their concentrations are severely reduced.
You have a very, very neutron rich soup, with the extreme density.
Essentially the core has collapsed to the density of an atomic nucleus and beyond.
Does this stop the core what is left there that will be for the next video in
the mean time what happens in the atmosphere well within about a quarter of
a section the core is converted into neutrons and it achieved a super nuclear
density notice ten to the seventeen kilo's per meter squared
The core's at this point so dense that even light cannot escape.
On the other hand, many of the neutrinos formed in the inverse beta decay carry
off most of the energy of the gravitational decay of the core and
there's a lot of energy. This is only a few solar masses but they
are at very small R squares, the collapse is very intense and the power emitted in
neutrini. for the ten seconds following the
collapse, exceeds the combined luminosities of all the stars in the
universe. So, it's presumably, a good thing that
neutrini mostly did not interact strongly, this is an extreme luminosity.
the density as we will discuss, is so high, that even the neutrino departure
takes time. There's a neutrino sphere for awhile,
where even neutrinos are trapped. But they escape first, carrying off the
majority of the energy of the collapse. Now, when the core achieves nuclear
density or super nuclear density, the core collapse is I'm giving away the
punch line, does stop. And when the core collapse stops, you now
have the atmosphere, or the envelope, falling down at relativistic speeds on
top of this, now rigid, core. And this generates the mother of all
shockwaves. Now.
If everything I said were correct supernovi would look very different than
they do. And in fact the details of how the
explosion works out are still a very much a topic of simulation and debate and, and
research. An important role is played in the fact
that the shock wave is highly turbulent. If you really just compress, imagine
modelling an atmosphere that's compressing and then suddenly stops.
the atmosphere absorbs the shock wave instead of, as we observed being blown
away. What in practice happens is, that the
shock wave manages to propagate all way up to the surface of the star blowing
away the atmosphere, blowing away 96% of the mass of the 25 solar mass star and.
The, the, in the, in the atmosphere of this,
Shock wave propagating, matter is compressed and heated so much that it is
here that massive stars reproduce fusion, producing nucleotides all the way up to
iron and beyond. So all of the uranium and the gold and
the silver, the heavy elements are produced primarily, as we talked about, S
process nucleus synthesis in heavy stars and then injection into the atmosphere
before photo disintegration, but the, another important source of heavy
elements is. The shock wave in a supernova.
And. The atmosphere is blown away, it takes a
while for light to be able to escape but as the atmosphere expands and cools a
little bit, the, opasidy is reduced, and then light can
finally escape. It takes a few hours and the luminosity
in light, which is, as we'll see, factor of almost a hundred down from the
luminosity of nutrini is still significant.
It can be 30 million solar luminosity. So this again is the luminosity of the
entire galaxy on the scale of the luminosity of the galaxy coming out of
the star. The total energy released is some
humongous number ten to the 47 joules, in In light and more in neutrini, and the
phenomenon is what is called a type two supernova.
So, we've now seen type 1A supernovae and type two supernovae, they're qualitavely
different. Remember, a type 1A supernova was,
essentially, a, a nuclear explosion, the explosive fusion of a white dwarf made of
carbon. type two supernova is a core collapse
supernova. The energy source is gravitational
energy. Essentially, all of these ten to the 47
jewels are Kevin Helmhold's energy from the collapse of the core.
And so. Two very different mechanisms two
different fundamental forces driving these two types of extremely energetic
explosions and can we see these things well with the luminosity of 30,000,000
suns one would hope so the most famous one as mentioned in the quote at the
beginning of The, the, the title slide of this video
is the famous supernova of 1054 AD we have records for this from Japanese
sources, Arabic sources nate even Native American sources possibly, no mention in
European sources that I know of and the very famous Sung dynasty scholar
describing the. Guests star and the remnant of this super
nova is still visible its the famous crab nebula in Taurus m1 the first item on a
Messier's list of non comet objects this is a beautiful recent hubble image the
filimentary structure and all kind of properties of this nebula might come up
in a future clip. One should note that while supernova are
extremely explosive and energetic processes, it is a bit surprising that
the supernova remnant is so luminous a thousand years after the light from the
supernova reached Earth. when it was
When the, the 1054 supernova exploded it's reasonably nearby the star was
visible for about a week in daytime. So this was a very impressive effect,
artifact. There have been Milky Way supernovae
three times since then, or around that time.
In 1006, 1572 and 1604. The estimate is that there's a supernova
in the Milky Way every 300 years. Most of them, as most things are in the
Milky Way, are far away from us and are obscured by the dust.
The disk of the galaxy so we actually observe many more super novie in other
galaxies then we do in our own because we're we can drew them edge on and we
don't have to fight with Dust extinction and another fun example
is this supernova in 2011, M51 the beautiful whirlpool galaxy and the, you
can see a before and after shot here, before on the left, after on the right.
The arrow points to the bright new star that shows up in the galaxy.
That is a type two supernova. The fun thing about this one, as with
many supernovas of the modern era, is that it was discovered in fact by amateur
astronomers. There are a lot of us amateurs; I try to
count myself among the amateur astronomers but I don't perhaps qualify
and there's a lot of us looking at a lot of places that the professionals can't
all be looking. So the more citizen science involvement.
Supernovae are, we now have seen both core collapse and nuclear supernovae.
In general, there's a classification which, like many things in astronomy, was
made before the phenomena were understood and so is somewhat weird.
The spectral classification is that a supernova is.
Classified as a type 1a supernova, we now know that is the explosion of a white
dwarf, if it has strong silicone absorption
lines but no hydrogen or helium. In the spectrum, it's a 1b supernova if
it has weak hydrogen lines and strong helium.
Lines in the spectrum. It's a 1C if there are weak silicon lines
but no hydrogen or helium. And, finally, it's type two, the core
collapse we've been discussing right now, if it has strong hydrogen lines in the
spectrum and. It turns out we know what type 1a
supernovae are those are the nuclear explosion of a white dwarf it turns out
that type two and what are called one and 1c.
One because there are no strong hydrogen lines in fact our core collapse
supernovae resulting from stars that, to some extent or other, have lost their
atmosphere through the mass loss mechanisms we've discussed.
So the hydrogen and helium lines are absent or weakened.
Simply because by the time the core collapses most of the hydrogen and helium
envelope has been lost and is not present to absorb the radiation.