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 super giant, a type B super
giant collapsed in a large magellanic clowd, 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 occured 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 you 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 1987, so it was 1987A. And
the remnants of the supernova are still under careful observation, we're still
seeing the development. the 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 can see an image of the
surroundings of the explosion, and then a close-up on the right-hand side, which
shows you the ring that 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 biploar emission of the star through mass loss prior to the
supernova explosion. And what we're seeing where these two
extreme rings on the left and right are at a distance of 20 light years from the
supernova. These are glowing where the height,
where the light from the supernova is hitting the ejecta from the previous mass
loss. similarly, the glowing ring 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, are crashing 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 a
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 were back to those Neutrinos.
So, what transpired is that neutrino detectors, although they were not
designed to be neutrino 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 interacting 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 light years away which is where the
neutrino exploded and you realize that this means the supernova admitted about
ten to the 58 neutrini which since we know their average energy would have
carried ten 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 modeled supernovi, we were talking about
red super giants, 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 are 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 arrived prior to the
light is that the density in the shock wave the supernova explosion, the
compression is so extreme that for the first three hours light cannot escape
but neutrinos escape first. So, the neutrino sphere breaks first,
neutrinos has escaped. The ejecta have to thin for three more
hours before light ceases to be trapped and so measuring this time difference,
gave us a very good handle on checking the, 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 learned 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 ice cube, so to conduct a modern neutrino experiment, you
take a tank 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 a 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, 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 amount 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 ice cube project is doing is lowering a bunch of photomultipliers.
Here's a, a depiction 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.