Greetings and welcome to the
Introduction to Astronomy.
In this video, we are going to
talk about two different types
of objects, which are
actually different versions
of the same thing.
And those are the neutron
stars and the pulsars
and these are two of the
things that can be left over
after a supernova explosion.
In reality, they are
different versions
of the same thing a pulsar is
just a neutron star, which is
seen in a certain perspective.
So all pulsars
are neutron stars.
But all neutron stars are not
necessarily seen as pulsars.
So let's look a little
more about these.
And what we see first is what
happens after a supernova
explosion.
Now we've seen images
like this before.
This is the Crab Nebula which
is the supernova explosion that
occurred in 1054.
And what happens
behind is that it
leaves the remnant the
outer layers of the star
are expelled out into space.
But the core left behind here
can be one of two things.
So the core left behind can
either be a neutron star
if it is less than
about three solar masses
or it can be a black hole.
If it is greater
than 3 solar masses.
And that is not a
very hard number.
We don't know
exactly what that is.
But it's roughly in that
vicinity of about three times
the mass of our Sun.
And it all depends on what
is left behind in that core
after the supernova explodes.
So the vast majority
of the material
gets expelled out into
space that we see here.
And that core left
behind is what we
want to start looking at today.
Now let's start looking at
how these were discovered.
How did we discover neutron
stars in the first place?
And this discovery was made
by Jocelyn Bell in 1967.
She was a research student.
And she found unusual
radio emissions,
which we see an image of here.
And what she found
is that it was
an extremely regular emission.
So the pulses of radio
emission that she was getting
were coming every 8 seconds.
This is extremely accurate.
So they were very
consistent the time
between from one
pulse to the other
was very consistent and regular.
So it was not just randomness.
There was something very
organized going on here.
So what kind of object could
create such a sort pulses
and in a way, it was jokingly
considered perhaps LGM or LGM-1
meaning little green men.
Is that was this a sign of life?
So was it actually
a sign of life out
in the universe because we
could send a signal like that
with such a period,
but a natural sources
would not create
something necessarily
that regular and
very hard to imagine
how they could do something
that short of a period.
What could do something
that would pulse
just once every second or so
most astronomical objects,
things like stars and
planets and galaxies
would not be able to pulsate
on such a short time frame.
So what could this object be
what had Jocelyn Bell found.
And one of the things
that we figured out
is that first of
all, there was no way
that an ordinary star, it
could not be an ordinary star.
So that was not possible because
it could not spin fast enough
without tearing itself apart.
So if we would try to spin
the Sun once a second.
The centrifugal forces
would tear it apart.
We would try to spin
the Earth that fast.
It would be ripped apart.
It had to be something
very compact, very
small an extremely strong
holding itself together.
And then we began to find
other similar objects that
were discovered.
In fact, in the Crab
Nebula the crab pulsar
was spinning 30 times a second.
So not just a once a second.
But 30 times a second, which
made the problem even harder.
How could you spin
something 30 times a second.
It's just not possible.
There is nothing
that wouldn't hold up
to those kind of structures
except for as we now
know a neutron star.
This is the compact
core of the dead star
maybe about the size of a city.
So it is something only
about 10 kilometers across.
And if it is spinning this fast.
It is approaching spinning
at the speed of light.
The outer layers it is getting
to a large decent fraction
of the speed of light.
If you try to spin something
this small, this fast.
So it's spinning very quickly.
But the neutron star
is a dense enough
that it can actually hold
up to the forces that
would try to rip it apart.
So a neutron star could
actually survive this.
So how are we detecting this?
Why do we detect pulses from
some of these neutron stars?
So what kind of model would
be able to explain that.
Well, what we use is what
we call the lighthouse model
in which the pulsar behaves
like a lighthouse beaming
material that we can then see as
it collapses the magnetic field
intensifies.
So the pulsar here at the
center and the bluish lines
here are the
magnetic field lines
looping around to the pulsar
and as it collapses down.
They become much more intense
and that forces the particles
to beam out along the two axes.
The charged particles trying to
leave the pulsar cannot cross
those magnetic field lines.
So they can only exit
along the magnetic axis.
So we get a very
tight beam of material
heading this direction.
Another tight beam of material
heading this direction,
we can see the pulsar
only when the beam
points towards the Earth.
So if we are looking from this
direction, we see no pulsar.
If we are looking
from this direction,
we do not see a pulsar.
The neutron star.
Is there.
But this is one of the
reasons many pulsars remain
invisible to us because
their beams never
point in our direction.
So we are not able to see them.
In that case as a pulsar
although we could technically
detect them as a
neutron star just
as a lone neutron star,
which we will see later.
So how can we test this model?
Well, let's take a look.
What we can look at.
What is the evidence?
So in any science,
we want to look
for what the evidence is
that pulsars are really these
rapidly spinning neutron stars.
We can measure the masses.
And they fit in
the correct range.
So that's good.
We have a way to energize the
pulsar beams will energize
the nebula and keep it glowing.
So it's another
source of energy.
Where does the energy come from.
Remember that energy has to be
conserved it cannot be created
or destroyed.
But what happens is that the
rotation of the neutron star
slows.
So what was spinning 30 times
a second for the Crab Nebula
pulsar.
Then we'll slowly over a million
years or so slow down and only
be spinning once a second
or once every two seconds.
So that would slow down.
So we know where the energy
can actually come from
and observations have shown
that this energy loss based
on calculations of the
pulsar slowing down
is equal to the
energy being emitted.
So this tells us
where the energy
comes from because the
energy is balanced.
The energy that the
pulsar is losing
is going in to the
energy that is then
being emitted by the
nebula so how did
these pulsars change over time.
Well, let's take a look here.
And pulsars can live for
about 10 million years.
So they have a relatively
decent lifespan.
And eventually,
they finally, we'll
slow down enough that the
pulses can no longer be seen.
So that rotation will
slow down long enough.
And as the energy
decreases the pulses
can no longer be seen
at short wavelengths.
So for something
like the crab pulsar
we can see it in visible light
it actually pulses on and off
and we can see that
in visible light
for older pulses of pulsars that
have slowed down slowed down.
We are unable to see
them in the visible,
but we can still see them
as radio pulses pulses.
They no longer give
off enough energy
to energize visible
light but they
do in the radio part
of the spectrum.
So eventually, the pulsars
will slow down and not
be able to be detected neutron
stars are difficult to detect.
First of all, if the pulses are
not pointing towards the Earth
they are almost impossible to
see or if they have slowed down
enough they've spun down to
slowing down enough that they
cannot produce pulsars.
So trying to find just a
neutron star is very hard.
We find the vast majority
of them, in fact, almost all
of them when they are pulsars.
However, in 1992, we were able
to detect a lone neutron star
as we see in the image here.
It doesn't look like a whole
lot just a very hot object.
Nearly nearly half
a million Kelvins.
So very high temperature and it
is only about 400 light years
away relatively
close to us based
on the size of our galaxy
of 100,000 light years away.
This is only 400
light years away.
And it is about 14
kilometers in size
comparable to the size
expected for a neutron star.
So they can be
detected and this could
be one where the pulses simply
aren't pointing at us anymore
or never have been
pointing at us.
It could be now has slowed down
and not be producing any pulses
but it is simply not
detectable as a pulsar
but it is one case where you've
been able to detect a neutron
star all by itself.
So let's finish up here
as we do with our summary
and what we've looked
at in this lesson
is first of all neutron
stars were first
discovered as pulsars giving off
rapid bursts of radio emission.
We can use the lighthouse
model to explain the pulses
and why we do not see all
neutron stars as pulsars.
Some of them are
simply not pointing
in the correct
direction neutron stars
will slowly lose
energy as they age.
And eventually the
pulses will stop.
But the neutron star will
remain behind pretty much
on detectable.
So that finishes our lecture
on neutron stars and pulsars.
We'll be back again next time
for another topic in astronomy.
So until then, have a
great day, everyone.
And I will see you in class.
