I can't resist discussing what is probably
the most famous supernova of the recent times.
Many of you are too young to remember this, but I was a graduate
student in 1987 and the story of the supernova is part of my education.
So I'll tell you about it.
The story starts about 168,000 years ago
when a blue supergiant, a type B supergiant
collapsed in a Large Magellanic Cloud.
The fact that the collapsing star managed to make
it back from red giant to blue giant was surprising
to theorists, but because this supernova occurred so close
to the solar system and the nearby Large Magellanic Cloud.
In fact, the progenitor star, being a giant
star, was very luminous and was known and
had been observed.
And this is I think, the only case where we
have observations of a star before and then, of the supernova.
Remember, stellar evolution is, by and large, a slow process.
This is one of the few cases where we really get to see
a before and after image of the same star and can study it.
And this forced some modifications to theory.
The light from the supernova reached us
in 1987 and it was the first supernova observed in 87 so it was 1987A.
And the remnants of the supernova are still under careful observation.
We're still seeing the development.
This sequence of Hubble images starting from 1994 and ending in 2006
shows you a ring like structure, we'll just talk about it, around the supernova.
The center is
the remnant and then this ring, which
is progressively becoming more and more glowing and
developing this blob like structure, is an
exciting thing to, to, to pay attention to.
Even more famous, you can see it in the
left, you see an image of the surroundings of
the explosion and then a closeup on the right
hand side which shows you the ring I mentioned before
as well as two larger rings.
I think this was referred to as the three ring circus when it was first observed.
And it took a lot of work by theorists to figure out what it is that we are seeing.
The model that describes it is contained here.
This was a large blue supergiant star. It had been experiencing mass loss.
There was, around it, this envelope that is what this blue haze
there is attempting to describe in this rendition is the emission.
Remember, we talked about this the past emissions of, the bipolar
emission of the star through mass loss prior to the supernova explosion.
And, what we're seeing where there these two extreme rings on the
left and right are at a distance of 20 lightyears from the supernova.
These are glowing where the high, where the light from the
supernova is heating the ejecta from the, previous mass loss.
similarly, the glowing green surrounding the sort of waist of the hourglass is an
area where ejecta from the supernova, matter
from the supernova traveling much more slowly,
is now crashing in. The remnants of the star's atmosphere is
now, or crushing in to the previously existing gas from previous mass loss.
And that's heating and compressing them and that is the glow that we are seeing.
And, as I said, all of this is under study.
But this was not even, but all of this, for all its
excitement, was not the most exciting aspect to me of supernova 1987A.
Yes, we're back to those neutrinos.
So what transpired is that neutrino detectors, they,
though they were not designed to be nuetrino detectors,
were active at the time they were looking
actually for signals of proton decay, and found none.
But the people who were looking for proton decay had large tanks of water that they
were observing for small flickers of light, which
we now know to be a very good way
to detect neutrinos.
And it transpired in the weeks after the detection of the light from supernova
1987A, is that a burst of neutrinos, a total of 20 or 21 in three detectors
worldwide, were detected, at about the time the supernova exploded.
And you can again remember that, remembering
that most neutrinos go through most detectors completely
without interrupting.
The fact that we saw 20 of them, you can figure out how many
actually have to go through the tank in order for 20 to be detected.
You multiply that by the 4 pi r squared for 160,000 lightyears
away, which is where the neutrino exploded and you realize that this
means the supernova emitted about 10 to the 58 neutrini which, since
we know their average energy, would have carried 10 to the 46 Joules.
This was a subluminal supernova. It's a little,
it was a little bit less than luminous than a typical type two supernova.
The reason for this as we now understand, having seen
the progenitor, is because this was a blue giant star.
It had gone horizontal branch.
It had shrunk.
The ejecta had to escape a deeper gravitational well.
They were more
tightly bound. This was not a star.
Typically, when we model supernovae, we we're talking about red
supergiants, very bloated, most of the atmosphere is at large distances.
In this case, all of the atmosphere would have
had to have been blown away from very close,
relatively, to the star's core and this means that
less energy was available to escape in the supernova.
And then the fun thing about these neutrinos is when
you track exactly when they were detected, they were
detected about three hours before the light was detected.
Now neutrinos, I said, are not quite massless but they're almost massless.
They travel essentially at the speed of light but they certainly travel
no faster than light incorrect news from a couple of years ago notwithstanding.
The reason neutrinos tra, arrived prior to the light is that the
density in the shockwave the supernova explosion, the compression, is
so extreme that for the first three hours, light cannot escape.
But neutrinos escape first so the neutrinos' fear breaks first.
Neutrinos escape.
The ejecta have to think for three more hours before light ceases to be tracked.
And so measuring this time difference gave us a very good handle on checking
them, our models of supernova and many things
were verified and other things have been corrected since.
This discovery, in sort of inadvertent
launched a new field called neutrino astronomy.
There are many new experiments planned to try to study the sky just as we
learn new things when we turned on radio
telescopes and X-ray telescopes and gamma ray telescopes.
It is hoped that observing the universe in
this, for essentially, the first time
in this non-light radiation, non-electromagnetic radiation,
will give us a handle on phenomena that have not previously been observed.
There are many neutrino experiments.
One of the most fun ones to think about is this experiment called IceCube.
So to conduct a modern neutrino experiment, you
take, in fact, a very, very pure water.
The water has a lot of protons and
electrons in it.
And if you happen to have a neutrino passing by, well mostly, nothing happens.
But if it happens to interact with an electron, that electron or
the products will be moving at a high speed through the tank and
they will emit a characteristic radiate kind of light called Cherenkov radiation
and then you put in light detectors called photomultipliers to detect the radiation.
So all you need is a great
big tank of very pure water with a lot of photomultipliers attached to it.
And you need to hide this deep underground because otherwise, cosmic rays and
various kinds of contamination on the
surface will, will overwhelm the neutrino signal.
So you hide it deep underground where only neutrini can get.
And one of the best place to do, the places
to do this is actually in Antarctica near the South Pole.
It turns out that polar ice, ice is not particularly transparent as we know it,
but if you compress it, all of the impurities, it turns out are squeezed out.
deep, a mile hundreds of meters below
the surface, polar ice is very, very transparent.
So you have, ready made, a deeply
submerged huge collection of very pure water.
And what the IceCube project is doing is lowering a bunch
of photomultipliers.
Here's a this picture of the grid into Antarctic ice,
and hooking them up to computers to try to detect neutrinos.
And this is one of the more unusual and one
of the more interesting telescopes that we have in the world.
And, since I was there for the actual
excitement, I had to share this story with you.