We talked about the end of a high mass star.
It ends its life in Type II supernova resulting
from the collapse of its iron core. There
is a star...we, at least those of you who
took Astronomy 1-PO1, will remember. Betelgeuse.
It is a red super giant. It is a part of constellation
Orion, the Hunter. It's in the shoulder of
the Hunter.
What I'm going to show next is, in fact, a
video composed of images taken over a long
period of time and then compressed, so that
it could be a minute or so video. That particular
star, Betelgeuse, can actually go supernova
perhaps in our lifetime. Nobody knows for
sure.
I just want to show this so that you can see
how unstable these super massive red giants
are near the end of their life. This is from
YouTube and excuse the cheesy music, I'll
try to decrease the sound.
[music]
[silence]
Dr. Mitrovic: So you see how volatile its
surface is. These are the actual shots taken
of Betelgeuse.
[silence]
Dr. Mitrovic: It may or it may not happen
that this window given, 10,000 years, is optimistic.
It can take maybe even a million years for
this to occur.
As I mentioned last time, most of the energy
released in Type II supernovae is carried
away by neutrinos. Only 0.1 percent of energy
is carried by visible light. Although the
energy carried away by the light is quite
substantial, because they can be seen from
very large distances, nevertheless it's just
a small fraction of the total energy that
is released when the gravitational energy
contained in the core was eventually released
in Type II supernovae.
There is one hallmark of these Type II supernovae
which distinguishes them from type IA, and
that is their spectra contain hydrogen spectral
lines, from hydrogen in the top layers of
the red giant. Remember, type IA supernovae,
their spectra contain no hydrogen spectral
lines, while one can find the hydrogen spectral
lines in the spectrum of Type II supernovae.
You see all of a sudden brightening in the
sky, when there is an explosion, and how do
you find which type of supernova it is? You
analyze the spectrum. If there is no hydrogen,
then it's type IA. If you see the hydrogen
spectral lines, that is a Type II supernova.
The question is, what is left after Type II
supernovae? First of all, there is a gas ejected
by the explosion, which slams into the interstellar
material, gas and dust surrounding the star.
It starts glowing, and that is what we call
supernova remnant.
Here is a photograph of this famous Crab Nebula.
This is so-called Crab Nebula. It's one example
of a supernova remnant. It is the remnant
of supernova that went off in 1054 AD, and
it was observed and documented by Chinese
astronomers. It turns out that supernovae
have been also observed by famous astronomers
that we talked about in Astronomy 1P01, during
their lifetime. Tycho Brahe saw one. So did
Kepler when he was very young.
The important thing about this particular
one is that Chinese astronomers didn't just
say, "Well, we saw a brightening." They, of
course, didn't know what was happening. It
was just a guest star that appeared and was
there for a long time, but they also marked
its location in the sky, so that actually
we now know that this particular glowing and
expanding gas is actually located around the
point where the supernova went off.
It turns out, as we will discuss later, that
there is a neutron star here at the center.
In this particular case, in addition to supernova
remnant, a neutron star was left behind at
the center.
The question is, what evidence do we have,
for instance, for the neutron stars? There
are objects that were named pulsars, for the
reasons I'll explain shortly, that are in
fact just fast-spinning neutron stars. Often,
we describe what is going on using so-called
lighthouse model for a pulsar.
Here is a fast-spinning neutron star, and
I'll explain why its spin is fast. Its spin
axis is not aligned with the magnetic axis
-- that is, the north-south magnetic pole.
They are not pointing in the same direction.
What is happening is that some of the electric
charges are pulled off the surface of a neutron
star, because neutron star is not just neutrons.
In fact, a neutron in the free space is not
stable. It decays after a few minutes into
a proton and electron. It's stable in the
nucleus when it's surrounded by other nucleons,
other neutrons and protons.
A neutron that is on its own is not stable.
It transforms into a proton and electron.
It decays into those two particles.
Say neutrons on the surface of the neutron
stars, they don't see nucleons everywhere.
There is empty space on one side, and as a
result, some of them will decay into charged
particles. These charged particles are forced
to spiral around magnetic field lines. It
turns out that when you have electric charge
moving along a circular or more complex spiral
path, it produces electromagnetic radiation
that is aligned with the axis about which
the charges spiral.
Electromagnetic radiation is produced, and
it's highly directional.
If I may, imagine I am a pulsar, and a vertical
axis is my spin axis, but my magnetic north-south
direction is along my extended hand. As I
spin, there's radiation in this direction.
If it happens to cross you, you will detect
the pulse. Then after one complete rotation,
again, you see the pulse.
If this highly-directed beam of radiation
happens to cross the path of the Earth around
the sun, we will observe a pulse. There's
whole range of wavelengths of electromagnetic
radiations. In radio part, sun is visible.
Then, there are also x-rays, gamma rays. If
the beam of this radiation points towards
the earth, we will detect the pulse. If the
beam points to us on Earth, we'll detect it
as a pulse.
They were first observed by a graduate student
at Cambridge in 1967, Jocelyn Bell. She detected
pulses of radio waves that were coming in
regular intervals 
of 1.33728 seconds. In the period after the
Second World War, Brits used to dominates
the field of radio astronomy.
Here, you see one of these antennas that could
receive radio waves because they actually
were first to develop radar during the Second
World War. They were very advanced in this
sort of technology, antennas, this and that.
They were really the leaders.
You should appreciate how difficult discover
this was because, at that time, the antenna
would pick up all kinds of signals from space.
The data were recorded on paper tape. She
was able to see among all these signals on
a long paper tape these blips that appeared
in regular intervals of 1.33 and somewhat
seconds.
One had to be very tenacious, very determined
actually, and observant to be able to notice
that. Her supervisor was quite a big name
in astrophysics. Initially, they didn't know
what it was. They thought that maybe there's
some other civilization out there trying to
communicate with us by sending regular radio
pulses. This source was initially called LGM
for little green men.
Then, a few more were discovered. It became
clear that it cannot be some other civilization
trying to send us signals because there are
other sources. Just statistically, it's not
very likely to happen. They abandoned the
name LGM, little green men, for a pulsar.
By now, over 1,500 pulsars have been discovered.
In fact it turns out that at the center of
that Crab Nebula, there is a pulsar that actually
can be detected at visual wavelength. This
is a pulsar in the center of the Crab Nebula.
That particular Type II supernova that created
Crab Nebula supernova remnant, the one that
went off in 1054 and was observed and documented
by Chinese astronomers. It left behind the
neutron star.
You can see how the period here is .033 seconds.
It brightens up every .033 seconds and then
goes dim and so on and so forth. When it is
bright, the beam of radiation points towards
us and so on. You may ask the question. Why
is the spin of a neutron star fast? The reason
is the conservation of angular momentum. I'll
try to explain using an analogy that we are
all familiar with.
Soon when the Winter Olympics start again
and you watch the figure skating, you'll see
it again. We have a spinning iron core. The
initial star was spinning. Those of you who
took Astronomy 1P01 will recall that the sun
is rotating about once a month. It spins around
its axis. Here, we have a spinning iron core.
It has some spin rate. Then, it collapses
into much smaller object.
Let me erase this. As I say, this could be
understood by analogy, a spinning skater.
At the end of performance, the skater has
arms spread out and starts a slow spin. Then,
he or she pulls in the arms. As a result,
the spin becomes much faster.
The angular momentum is a quantity that is
given by the product of the mass of the object,
the average distance from the rotational axis,
and the speed with which the object is spinning.
The mass of the skater, and also of the iron
core that eventually collapses into a neutron
star, is fixed.
If the object contracts, if the average distance
of all parts from the rotation axis decreases,
for this to remain the same, the speed with
which the object is spinning has to go up.
That's why, as the skater pulls in the arms,
the spin rate increases. Also, that's precisely
the reason why the spin rate of collapsed
core, as it shrinks in size, increases as
well. That's the reason why these neutron
stars have high spin.
The rate of the spin decreases over time as
the spinning neutron star loses its energy
because it's emitting radiation. That radiation
carries some energy, and over time, that will
reduce the rotational energy, because radiation
took away some energy. The spin rate of neutron
star decreases over time, the rotational energy,
via radiation.
There is no free lunch. The energy is conserved.
If that radiation took some energy, that had
to be on account of rotational energy of the
neutron star.
