 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 three 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 1.33728 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 sort pulses? And in a way, it was jokingly considered perhaps LGM, or LGM-1 meaning little green men. 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 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 it could 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. If we were to spin the Earth that fast, it would be ripped apart. It had to be something very compact, very small, and 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 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 would 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 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 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? How 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 pulse are here at the center, and the bluish lines here are the magnetic field lines looping around 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 will 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 pulsars 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 into the energy that is then being emitted by the nebula. So how do 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 will slow down enough that the pulses can no longer be seen. So the 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 pulsars that have slowed down, we are unable to see them in the visible, but we can still see them as radio 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're 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 half a million Kelvin, 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 we have 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 undetectable. So that finishes our lecture on neutron stars and pulsars. We'll be back again next time with another topic in astronomy. So until then, have a great day, everyone, and I will see you in class.