 Greetings and welcome to the Introduction to Astronomy. In today's lecture, we are going to talk about compact stars. These are things like white dwarfs and neutron stars, and what happens when they occur in a binary system? We've looked at white dwarfs and neutron stars separately, but some very interesting things happen when they are part of a companion system. So, let's go ahead and get started. So, we've looked at the individual stars, but what happens when one of these is in a binary system? Now, these are things that can happen. They don't mean they always will happen, but we will look at things like the novi and the supernovae, and then x-ray and gamma-ray bursts, as different things that can happen when compact stars have a companion star. So, let's start off with the novi. What happens with a white dwarf? We have a white dwarf star in a binary system. If the stars are close enough together, now, if they're far apart, nothing will happen, and it will just sit there as normal. However, sometimes when the star expands, the ordinary star expands and becomes a red giant, it will end up being close enough that material is pulled from one star to another and begins to form an accretion disk around the white dwarf. Now, the material in the outer layers of the star is hydrogen. So, hydrogen builds up on the surface of the white dwarf star here, and that process continues until the temperature gets high enough that nuclear fusion starts on the surface of the white dwarf. So, instead of nuclear fusion at the center of a star, here we have it happening on the surface, and the star will suddenly become hundreds or even thousands of times brighter. Now, this does not hurt the white dwarf star. So, it will give off a burst of energy, become a lot brighter, and then it will begin accreting material again. So, this can happen multiple times in the same system as long as material is being transferred from one star to another. So, a nova can be recurring. Now, we can also have a supernova. This process is exactly the same as a nova. The only difference is the question, what happens if this mass transfer pushes the mass of the white dwarf over that 1.4 solar mass limit? Remember, once you get over 1.4 solar masses, that star can no longer support itself against gravity and will collapse, and it will start to compress down, igniting all of the carbon within the star, and it won't just begin at the center, it'll begin throughout the entire star altogether and will then tear itself apart. So, that entire star will rip itself apart. So, here's an example showing what happens. We have two stars here, one star. Here, they're both on the main sequence. Here, the first star has evolved to become a red giant and not much happens. You might get mass transfer between the two, but that's not a necessity. Then, this star will become a white dwarf. When the second star evolves and becomes a red dwarf, it'll start the matter transfer onto the white dwarf star. And if this star is very close to that 1.4 solar mass limit, it will push it over, ignite the carbon, we call it carbon detonation, supernova, and it will just rip itself apart. And there will be nothing left behind of this star. It'll tear itself apart completely. And it's all because it pushed itself over that 1.4 solar mass limit, one too many objects that it could just not hold up against. So, we talked now about two types of supernovae. We talked previously about the type II supernova, which is the explosion of a massive star at the end of its life. That leaves behind either a neutron star or a black hole depending on the mass of the remnant. When we look at these, we see strong hydrogen lines in the spectrum. That's a way to be able to identify them. When we see a supernova, just seeing a star brighten doesn't tell us which of these two types it is. But if we take a spectrum, if we see strong hydrogen lines, that means it is a type II and a massive star at the end of its life. A type I supernova is a white dwarf star that exceeds that 1.4 solar mass limit and explodes, leaving nothing behind. This has no hydrogen lines. Remember what exploded? It was a big ball of carbon detonating and exploding outward and will not have significant quantities of hydrogen. These types of supernova are extremely important for distances. Remember our distance ladder, and we will come back to that with galaxies. These supernovae are going to be important because they are examples of standard candles. They have a standard brightness. Why? Every single one of these is the exact same type of object that explodes. They are all white dwarf stars, and they all have a mass of 1.4 solar masses. Exactly the same. Whereas a type II supernova, the mass might have been 20 solar masses or 30 solar masses or 40 solar masses, they'll all have different brightnesses. These are all identical, and we'll see them again when we look at determining distances. So those are two things that can happen with a white dwarf star. How about a neutron star? What can a neutron star do? What if instead of a white dwarf, we had a neutron star in a binary system? Well, the exact same thing as we looked at for a nova could happen here. You could transfer mass from a companion, just like we did to the white dwarf. Material would build up on its surface, just like it did on the white dwarf. The difference is that gravity is much stronger and temperatures are much higher. So instead of getting bursts of visible light, we get bursts of x-rays from the surface. In fact, many x-ray bursts have been detected. So we've detected many of these, and it is the same process as a nova. It is just a nova instead of being a white dwarf. It is a neutron star at the center here, and that is what is collecting the material, and its increased gravity and increased temperatures will cause it to undergo a burst of energy from its surface. Now, depending on how these happen, depending on the position, you also can use this to speed up neutron star rotation. So if the material is spiraling in one direction and the neutron star is rotating that same direction, you're giving it a little kick each time material hits it. It's hitting it in the same direction. Like pushing a child on a swing, you push them when they're going in the right direction and you can accelerate them upward. Well, this can accelerate pulsars to millisecond pulsars where they can be spinning 100 times a second. Up to almost the limit, even a neutron star has a limit to how fast it can spin before it would rip itself apart. Now, the last thing we wanted to look at here are the gamma ray bursts. Gamma rays, the most intense electromagnetic energy, and the gamma ray bursts were actually detected by the military in the 1960s. Why would the military detect them? Well, gamma ray bursts of gamma rays are associated with nuclear explosions. So tests from nuclear detonations on Earth were being looked for as to who was giving off testing nuclear weapons, and they were actually looking for those, and they found gamma rays from the sky, and now we know of thousands of those. And it's difficult to pinpoint their location. So it's very hard to see exactly where they are, which makes it hard to identify an optical or a radio counterpart. So it's hard to find the object. It took a long time for this because gamma ray telescopes have very poor resolution. Now, you might think if you remember resolution, short wavelengths gave you a really high resolution, and gamma rays are among the shortest wavelengths. However, gamma rays are impossible to focus. So even though they would have a high resolution theoretically because of that, or because you cannot focus them, they are unable to take advantage of that resolution. We have now been able to identify optical sources for some of these, and many of them are billions of light years away. So these are nothing anywhere nearby us that we are detecting. Now, when we look at the gamma ray bursts, we split them into two groups. There are two types of gamma ray bursts. There are long duration and short duration bursts. Now, the cut-off there is set at two seconds. So a long duration burst just means it lasts longer than two seconds. We believe this is caused by a stellar collapse of a star that lost its outer layers of hydrogen. Now, when it does that, when it collapses down, material then expels outward through this. All the gamma rays come out because it has less material to shield it from that. So instead of this happening well below in the core, now you have an exposed core where material is accelerating outward, and that material can then be emitted as gamma rays, and we can detect these as the long duration bursts, which may only be a few seconds, but at least two seconds in length. Now, the short duration bursts, on the other hand, are first of all less than two seconds in length. These are believed to be caused by neutron stars. We sometimes call this a kilonova, and that is from merging neutron stars as they spiral in together, and will eventually release gravitational waves as they combine and coalesce. And if you remember from the explosion that we get here, when we looked at what elements form, a lot of the elements, heavier elements, are often formed in collisions of neutron stars, such as we talk about here. So those short duration gamma ray bursts can also produce a lot of the elements that we see in the universe. So let's go ahead and finish up with our summary, and what we've looked at here is that compact stars in a binary system give rise to many different things. A white dwarf can become a nova or a supernova depending on the exact conditions, while neutron stars become x-ray or gamma-ray bursters. So that concludes this lecture on compact stars in binary systems. 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.