This is an introductory astronomy survey class that covers our understanding of the physical universe and its major constituents, including planetary systems, stars, galaxies, black holes, quasars, larger structures, and the universe as a whole.
Core-Collapse Supernovae
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來自 Caltech 的課程
演变中的宇宙
382 個評分
從本節課中
Week 5
與講師見面
S. George DjorgovskiProfessor
Astronomy
So let's talk about the other kind of supernovae, supernovae who come from
the collapse of very massive stars at the end of their thermonuclear life,
and the remnants always look like these nice shells that are expanding.
So to freshen your memory, thermonuclear evolution of really massive stars
precedes by going from lighter elements to heavier.
First hydrogen to helium, then helium to carbon and oxygen and
then going to silicon and iron.
And at that point, you can no longer do thermonuclear
fusion because it's a thermal reaction, and you have to put in extract.
So massive stars, if they're massive enough, can reach central densities and
temperatures to create chemical elements all the way up to iron, and that's it.
But then you have this steel structure.
You have silicon burning into iron into core
surrounded by a shell that burns silicon and so on.
All the way you get into lower density,
lower temperature layers burning hydrogen to helium in the end.
And there is still normal stellar interlope left above that.
This is where hydrogen in the supernova remnant comes from.
Okay, so what happens when you can no longer burn stuff in the core?
You just created enough iron.
There is no silicon left to burn.
And it's the equivalent of why stars leave main sequence and
climb through a giant branch.
Well, in this case, there is no way to create any more energy
that will support star against its gravity.
Essentially, thermonuclear fire stops.
And because of that the core collapses.
There's strong gravity.
There's strong pressure.
And two different things can happen.
Iron can photodissociate with gamma rays.
You can also have inverse beta decay if you have sufficiently energetic particles.
Notice that that process releases two things,
neutrons and neutrinos, electron neutrinos.
And they're both important because neutrons will end up as the building
material of what was left of a neutron star.
Neutrinos fly away and carry most of the ash.
So, these two processes take away the energy [INAUDIBLE] the core collapses.
You get the shockwave.
There's usually bounce to the shockwave.
And that bounce can actually explode the star.
The physics of this is fairly complicated and not entirely fully understood.
People who do numerical simulations of exploding stars have only recently
managed to really have a full convincing simulations of how stars explode.
One interesting tidbit about this is that neutrinos
can be an important agent of this explosion.
Now neutrinos hardly interact with matter at all, but they do interact.
Now if you put a lot of particles, protons and neutrons in their way,
such as you have in massive star, then there is a reasonable probability for
these neutrinos to actually Interacting with that material and
see where a neutrino pushing the star and all that.
At least that's the theoretical model.
So the collapse of the core releases binding energy that's created.
That creates a shock that goes outward.
That shock is what explodes the star.
And that's again indicated here.
So the energy of the supernova comes from the release of the binding
energy that suddenly you're packing a substantial fraction of star's mass.
At least two stellar masses, more if possible,
into a tiny remnant that's kilometers in size.
And that binding edge is then converted into kinetic energy and
luminosity in both electromagnetic and neutrinos.
And this is where supernovae come from.
Now, theory says that 99% of the energy will be actually emitted in neutrinos.
And until 1987, that was just a theory.
But then supernova exploded, bringing a giant cloud and
several of those deep underground neutrino detectors, which are usually built to
detect solar neutrinos captured neutrinos coincident with this optical explosion.
And the amount of energy in those neutrinos,
it's like a dozen of them, was exactly right.
It's exactly what theory predicted.
Should be the neutrino flux.
Here on earth from something in the Magellanic Cloud.
So that was a very powerful demonstration that we actually do
understand why massive stars, I mean how massive stars explode.
And finally, let's talk about supernova remnants.
So you have the envelope of the stars now being pushed out by the shockwave.
It is pushed out in speeds that
first actually exceed 10,000 kilometers per second, eventually slow down.
Within it is all the newly synthesized material products of nuclear synthesis.
Some of which is done in star before it exploded some of which is done during
the explosion and this is how interstellar medium is enriched by heavy elements.
So, every atom in your body of elements
heavier than say oxygen comes form a supernova explosion.
Think about that, that's pretty cool.
They're all built out of stellar ashes.
And these remnants can last 100s of 1000s of years that fade out slowly,
and they emit in all different frequencies because along with stellar envelope, their
magnetic fields that they're being trapped in the plasma and there carried out.
Then there are all kinds of interactions just like you have in solar atmosphere,
interactions magnetic field and plasma can produce nice little flares.
They accelerate electrons and
so these things are observable in a variety of ways.
And there is a large number of these in milky way,
only a few are really visible in visible light, but
many can be seen in radio or x-rays, and they look like this when they.
This is the famous crab nebula of course.
Remnant of a supernova that exploded, at least that we've seen the light from,
in year 1054.
And it's probably the most famous supernova or supernova remnant.
It was well-studied by Chinese astronomers, also Japanese, Koreans.
It was also seen by the North American Indians.
And they're famous pictographs in Chaco Canyon showing this new star and so on.
And it was brighter than Venus for many weeks, so couldn't miss it, right?
And until recently,
there were no records whatsoever of this supernova in European records.
Now, it could not have been cloud over the entire Europe for
a few months in a row, especially this was in the summer,
and the explanation was that, well, this kinda clashed with
churches teaching that the heavens are perfect and nothing new can happen.
It cannot be changed.
Therefore, this thing could not be happening.
And so nobody dared write anything down.
There were some that were recently found and
they thought this was a sign of Pope so and so dying and going to heaven.
Interesting.
Right?
So, even today, this nebula is expanding at high speed.
These nice filaments are the beauties in plasma that Is very turbulent.
There's a magnetic field mixed with lots of free electrons.
It emits in x-rays and everything and it's also home to the crab nebula pulsar.
The first pulsar that was discovered, and we'll talk about pulsars next time.
And finally here is Huanaco Casiapia A, and
it's got its name because it was a first and brightest radio source
seen in constellation of Casiapia, because it's pretty big, it was easy to identify.
This is the remnant of a supernova that exploded in 1658.
That's, I think, Tyco and, certainly, Kepler have seen.
The picture here is composite of the infrared, optical, and
x-ray images from different satellites.
