 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about the neutron stars and pulsars, what is left over after a supernova explosion. So neutron stars and pulsars are really the same thing, and it just depends in one way how we're seeing them. So let's look at what happens first after a supernova explosion. A supernova will leave behind a remnant from the outer layers of the star, so that is expanding out into space. The core is compressed and can form one of two things. It can form either a neutron star or a black hole. We'll come back to black holes later. We're going to be looking at neutron stars right now. And there is a neutron star at the center of the crab nebula, that is the image we see of the crab nebula here today. How did we first discover these? Well, a neutron star very tiny, very hard to discover. However, Jocelyn Bell pictured here discovered an unusual radio emission that she detected from space in 1967 as a research student. They were extremely regular pulses. Here's the period, 1.33728 seconds. So those are significant figures. That is how accurate and how consistent these pulses were. That's really unusual how what can possibly pulsate once a second out in space. Objects are so large that they can't pulsate that quickly coherently. So like a star like the sun could not spin once a second. It would rip itself apart. So what kind of object could do this? Could they even kind of whimsically name the first one LGM for little green men? Could this be a signal from space? And that was one possibility, but then it was found that no, there was also another option that was quickly found, and that was that this was a neutron star, the compact core of the dead star that is about the size of a city. So as I said, no ordinary star could spin that fast and not rip itself apart. And then as we began to find similar objects, of course that kind of ruled out the little green men hypothesis because we're finding more and more of them. And then we found things spinning even faster. The crab pulsar was spinning 30 times every second. So it's hard to imagine what could possibly spin this fast. And the only thing that would have enough gravitation holding it together to be able to withstand that kind of rotational speed without ripping itself apart is a neutron star. Now how do we detect radio emission from it? Well, this is the pulsar behaves like a lighthouse. It is beaming out material. So as it collapses, the magnetic field intensifies. The magnetic field gets stronger and stronger so material can only escape along the axes. It can only escape along these two axes. The beams of particles cannot go through all of this magnetic field. This would be a donut shape going all the way around and only through these two parts where the magnetic field goes into the neutron star itself. So those beams of particles we can see only when the beam points toward Earth. So only when that happens to point toward Earth we get a pulse. So as it spins around we could get one pulse and then it could get another pulse as soon as it spins every single time. And that means that a lot of pulsars will remain invisible to us because their beams never point to us. If their beams never point in our direction by this model we will not be able to see them and that makes it very hard to detect them. So we're fortunate with things like the crab pulsar where we can detect the pulses from it. And there are many others showing us that there are many, many neutron stars out there and likely many more that we simply cannot detect. So how do we go about testing? Here's our model. How do we test our model? What is the evidence that pulsars are rapidly spinning neutron stars? Well we have a couple things. The masses measured for pulsars are in the correct range. So they have to be less than about three or four times the mass of our sun because that is the maximum that the neutron degenerate degeneracy can hold the star together. The pulsar beams will energize the nebula and keep it from keep it to keep it glowing. So where does it get the energy to glow? Well the rotation is gradually slowing. So we said it was that accurate but it is slowly slowing down over long periods of time. Now because it's slowing we can measure this and we can calculate the energy loss of the pulsar. If we compare that to how much energy the nebula is emitting they should be the same and that's what observations show. The energy loss from the pulsar is equal to the energy being emitted from the nebula. So again it helps us to confirm our models that everything seems to work out here and be consistent with a neutron star being the source of a pulsar. Now what happens? How long does a pulsar live? Maybe ten million years or so. They live a relatively long time not as long as many stars but a relatively long time. They finally slow down enough that the pulses cannot be seen. Remember that's where the energy is going. They are slowing down and eventually the pulses are no longer going to be visible. As the energy decreases the pulses are no longer going to be seen at short wavelengths. Right now the crab nebula is still pulsing in the visible light and in higher energy light. The nebula will only be radio pulses so the pulsars are slowly changing over time. Neutron stars themselves are really hard to detect. So the pulses are not pointed to earth. If they've slowed down too much to be able to produce pulsars that doesn't mean they cannot be detected. There is actually one that was detected by Hubble Space Telescope in 1992 and it is 400 light years away. We can measure it as 14 km in size and it has a temperature of half a million Kelvin, extremely high temperature. Remember the hot stars were like 30,000 and 40,000 Kelvin. This is much hotter. This is getting to be extremely hot not as hot as nuclear fusion at the center of a star but extremely hot for the surface temperature of any object. Now one other type of object that we get that is also related to these is what we call the magnetars. In a magnetar, first of all in a pulsar the magnetic field is extremely strong, much stronger than magnetic fields that we've dealt with so far. In a magnetar it's a thousand times stronger. We see from this one object known as Sagittarius 1806-20 something 40,000 light years away that it released a burst of energy, more energy in a fraction of a second than our sun will release in a hundred thousand years. So tremendous amounts of energy. It's thought that these might have a neutron core so it would be a neutron star at the core but has a mile thick iron surface above it and stresses on this. Think about the intense gravity. You could not stand on this surface. If you could somehow land on this you would be crushed flat by the intense gravity. That's how strong gravity is there. So while it's a solid surface it's not only is it extremely hot but even if it were solid and it cooled off enough if you tried to stand on it the intense gravity would just pull you down flat. So stresses within that iron surface produces cracks and star quakes on a scale unimaginable with earthquakes here on earth and that can release tremendous amounts of energy out into space. So these are some really really intense, pulsarly really intense objects but not really, I mean we have a rough idea maybe this iron surface does something that magnifies what we see for a typical neutron star or pulsar. So let's go ahead and finish up with our summary and what we've looked at is the neutron stars were first discovered as pulsars in bursts of radio emission. We looked at the lighthouse model to explain the pulsars. It also tells us why we do not see all neutron stars as pulsars and these neutron stars slowly lose energy as they age. Eventually the pulses stop and just the neutron star remains which would then be very hard to detect because it would be cooled off it wouldn't be emitting a lot of energy and would just be and be very tiny so it would not be very bright at all. So that concludes this 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.