The Cosmic Classroom - Neutron Stars and Pulsars


Uploaded by vmargoniner on 16.11.2009

Transcript:
Hi. Welcome back to the Cosmic Classroom.
So, we'll now talk about neutron stars which are
the result of the evolution of very high mass stars.
So, if the star has enough mass, it will fuse hydrogen into helium,
then helium into carbon. It keep going on and fusing
heavier heavier elements and getting energy out of that.
But eventually it forms iron, and as we talked about before,
you cant get energy out of fusing iron. So, at that point the star
can't get energy to balance the gravity that's,
that's pushing on it. And the star then collapses.
Well, when the star collapses, so you have this, have this
iron core that then is too cold, it becomes a little bit too cold
to be able to stop the gravitational collapse.
In the end, gravity wins and it collapses.
Well, when this core of iron collapses,the electrons
combine with the protons to become neutrons.
So you'll lose the information about what kind of atom
there was there before and instead there was this
big bowl of neutrons. Cause this, so the density
is the same density as you would find in a nucleus of an atom.
Those are extremely high densities. So in this case, what
stops the collapse from going even further is what's
called neutron degeneracy pressure. It's that the neutrons are so
close together that you can think about it as they physically
touching each other. And that's what's stopping them
from collapsing even further. But it's extremely dense.
Its very very hot, because as soon as you see it,
because after all its the core of this very high mass star
and the temperatures that were needed to fuse something into
carbon were very very high. Now, if you thought that,
that white dwarfs are extremely dense and they are, neutron
stars are to blow your mind. How dense they are.
So, in order for you to have an idea of the density of a
neutron star, imagine getting that same table, the same teaspoon
that we brought before and we imagine in the white dwarf
that we would pack about a hundred people in there.
Now imagine instead getting the entire population of the earth
and packing it in that volume. That's how dense
the neutron star is. OK? It's unimaginable right?
Its very very dense and hot. So, a neutron star is sometimes
observed as a pulsar. So, a pulsar is a neutron star
that's rotating, rotating very fast and emitting light in just
one direction or in two directions. So, for example, this light has a,
you have a couple of nice drawings here.
So in, in the drawing here on the left, you have a neutron star.
The neutron star is rotating extremely fast, because of conservation
of angular momentum. We started with this
huge massive star that was rotating. Well as, as
the star collapses,the angular momentum needs to be conserved,
and the star spins really really fast. The star spins really really fast
with that there is, the magnetic fields are,
become even stronger than what they used to be in the
surface of the high mass star. And the star starts to eject
light in beams, in two beams. Very similar to what happens
in a, in a lighthouse, very similar to what happens in a lighthouse.
Pulsars can rotate, can spin so fast that they can spin
many times in one second and when they were first observed,
they were actually first referred to as LGM for Little Green Man,
because nobody could understand how something could be creating
a signal that was so fast and so uniform, so constant.
So, for a while people thought Oh, may be there are some
aliens out there trying to contact us in in,
in creating this signal. But, just pretty soon,
it was realized that that wasn't the case.
So,you can think about hearing a neutron star.
Instead of, you wouldn't be able to see the neutron star
blinking because it emits this jet of radiation
mostly in the radio and your eye cannot see radio.
But you could assign a beep every time, there is a peak
of intensity of light. And then you could hear
a pulsar. So here's an example of hearing a pulsar.
So every beep you hear is a, is correspondent to a
increase in the intensity that you observe here from mark.
So,for example, this one, it rotates 1.4 times each second.
About the Crab Pulsar, it's 30 times per second.
and it's really hard to, to discriminate there.
But actually there are pulsars that are even faster than that.
627 rotations per second. See if you can hear that. (Sound)
Alright, that, that at least woke you up, in case you got
a little bit bored, alright. Now you don't always observe
a neutron star as a pulsar. It depends on your location.
If you're in a good location you seeit as a pulsar.
If you're not, you won't. So let me show you this movie
in which you will be seeing the, the pulsar from different locations.
From some locations, you'll see it getting brighter and dimmer
and from others we wont. So, let's see it here.
Now your seeing it from the side. So if you are to measure
all the luminosity coming towards you, it wouldn't be changing.
Notice the blue lines are the magnetic field lines
that you can't really see, its just the drawing.
And there you go. Now it's going right at your face
and you would see being brighter brighter, brighter, brighter and brighter.
And each one of those would correspond to one beep
that I just showed you. OK? Another way to think
about it is I can pretend to be, I can pretend to be a neutron star.
So, let me say that I have two jets, one going that way,
one going this way. OK? And if I'm rotating and if
my arms are going this way, you'll see it and then you won't,
and then you will,and then you won't. Now if I were inclined like this,
maybe you wouldn't see me as a pulsar at all.
You would just see me as a neutron star.
The, the brightness would not be varying. So, its just,its just the question
of Geometry, its just the question of where you are located.
Now, I got a really good question from my student this week
about neutron star and I had to scratch my head a little bit.
"Professor, how come we have such strong magnetic fields
in a neutron star? After all,neutrons are neutral.
They don't have charge and I know that in order to have
a magnetic field, I have to have moving charge."
Excellent point, excellent questions. A neutron star is made mostly
of neutrons, but not completely. OK? So if you really look in detail
in this structure of a neutron star, you'll see that there are also
super conducting protons in there. There are also ions and
there are also electrons. OK? So, it's not just one big ball
of neutron, its one big ball of neutron with some
charge attached to it. So, just to finalize, we sent two
spacecrafts into space in the 70s. Those were called Voyager's One and Two.
They are about to leave, they, they actually left the solar,
the solar system. They already traveled
through all the planets, went through Pluto,
all the way getting out of the solar system and
in there, we put two gold plates one in each one of those spacecrafts.
So, the idea is that those plates if ever, if ever grabbed
by an alien civilization, will know about us.
We'll be able to tell them, Hey! We live
here on this planet. So, there's a lot of
information on the air. We tell them how we know
about the structure of the atom. We tell them that
we know about light. So there are many different things there.
And one of the pieces of information is a map to help the aliens find the sun.
So, the map is based on the location of pulsars all around us.
The idea is that if the aliens are smart enough to have
mapped all the local universe and know all the, the location
of all the pulsars, they should be able to get to us.
And thats it. I hope you'll know a little bit more about pulsars
and neutron stars.