 Greetings and welcome to the Introduction to Astronomy. In this video we are going to talk about compact stars and what happens to them in binary systems. And compact stars that we have looked at so far are the white dwarf stars that is the in state of something like our sun and is compacted down to about the size of the earth and we had neutron stars which occur after a supernova explosion and are compacted down to closer to the size of a city. So let's go ahead and get started here. And what we see is that in terms of compact stars we have looked at the individual ones as I've said. We've looked at the white dwarfs and the neutron stars and but what happens when one of these stars is in a binary system? We know that half the stars in the universe are in binary systems. So that implies that there are a lot of these times when a compact star is going to be present in a binary system. And what we have is we can get a number of different things. We can get Novi, we can get Supernovae, we can get X-ray and Gamma-ray bursts and we want to kind of look at all of these. Now the supernovae that we get here are different than the type 2 that we look at. The type 2 that we looked at previously was a massive star at the end of its life. In this case we're going to see a different type of supernova. So let's look at each of these in turn. So what we have, first of all, let's look at the Novi. What is a Nova? Well, if we have a white dwarf star in a binary system, they may be close enough that material can transfer. So you have the regular star on this side, the white dwarf star at the center here, and material is being transferred from one to the other. So if they are close enough together, material can be pulled off of the ordinary star and into what we call an accretion disk around the white dwarf. And as that material spirals around and in towards the white dwarf, it then builds up on the surface. Now what is building up there? Well, we know that the outer layers of a star are mostly hydrogen. So hydrogen is now building up on the surface of this white dwarf, and eventually the temperature can become high enough that nuclear fusion will actually start on the surface of the white dwarf. And this star, we'll see, when we look at this, we will see it become hundreds or even thousands of times brighter. And this is the kind of thing that can happen over and over again. It does not damage the white dwarf star. So the material can build up to the point where it explodes as a nova, and then 50 or 100 or 200 years later, we can do the same thing. We can actually produce a nova recurring. It will recur again in the same system. So some of these can occur over and over again as long as this star is still transferring material. Now generally, this would happen when this star evolves and becomes a red giant star. Remember that when it becomes a red giant, it becomes many times larger, and that allows the outer layers to actually get closer to the white dwarf star and allow the mass transfer to begin to take place. So a nova is one thing that can happen. Another thing that can happen is a white dwarf supernova. It is exactly the same process as for a nova that we looked at on the previous slide. But the question here is what happens if the mass transfer pushes the white dwarf over the 1.4 solar mass limit? In that case, the star can no longer support itself against gravity. So it will begin to collapse and essentially the entire star will explode. So our image shows that a little bit of that here. What would eventually happen is you would have the main sequence star and you would start have two stars first. One would evolve and become a red giant and eventually end up as a white dwarf. When this main sequence star becomes a red giant, then we would have mass transfer occurring and material is being transferred onto the white dwarf. If this white dwarf is at 1.4 solar masses, then a little extra material can push it over that limit and it will collapse and essentially a carbon fusion begins throughout the star all at once throughout that white dwarf and that will rip the star apart causing a massive explosion, which we call a type 1 supernova. Now that differentiates it from the type 2 supernova, which is when a very massive star reaches the ends of its life and ends up forming iron in its core. This is actually a white dwarf star that is right at the limit of 1.4 solar masses. If it gets pushed over that limit, it cannot survive. So that means that we do have these two types of supernovae. So what we can look at and just to give some of the ideas here, we have the type 2. That is a massive star at the end of its life and leaves behind a neutron star or a black hole. The different one way to differentiate them is that their light curves are slightly different and that a type 2 will show hydrogen lines in the spectrum because the outer layers of that star were a lot of hydrogen. A type 1 supernova on the other hand occurs when a white dwarf star exceeds the 1.4 solar mass limit and explodes. In this case, nothing is left behind. We do not see hydrogen lines because the white dwarf was made primarily of carbon and the little tiny bit of hydrogen that was transferred does not give very strong hydrogen lines as it would in a much more massive star exploding. These are extremely important for determining distances to distant galaxies. They are what we call standard candles or standard bulbs and that is because each of them, every single time it occurs, it is the same type of object exploding because we know their mass. Every single one of these is a 1.4 solar mass white dwarf star. So there should be little difference between them when they explode. We should see the same brightness occur and the same patterns in their light curves and that means that we can use them to then determine distances because they should all reach the same maximum brightness. So what can happen for other objects? Those are some things that can happen with white dwarfs. What if we have a more massive object instead of a white dwarf? What if we have a neutron star present? Well, we can take a look at that as well. The same kind of process can occur. You can have a neutron star in a binary system and the mass transfer can occur just like it did to a white dwarf. You can have the ordinary star here and the neutron star here and you can have material transferred into an accretion disk that then spirals around down to the neutron star. Material would also build up on the surface but the difference is that you have much stronger gravity and much higher temperatures that occur than it would with a white dwarf star. That gives us instead a burst of x-rays from the surface. So instead of a burst of visible light as we get in a nova an x-ray burst is essentially a nova but instead a neutron star is the compact object instead of a white dwarf. These are, many of these have been detected and interestingly enough, depending on the positioning these can actually speed up the neutron star rotation. So if you are sending material from a star into a neutron star if you kick it up and push it the way it is spinning, if it is spinning this direction in the first place then if you give it a little kick this side kind of like pushing a child on a swing you can give it a little bit of boost of a boost and that will allow it to speed up its rotation and in fact we find what we call millisecond pulsars pulsing in thousandths of a second and at the absolute limit of what a neutron star can actually do without ripping itself apart and these are believed to be spun up by mass transfer from other objects. Essentially each time you transfer a little bit of mass to the neutron star you kick it up give it a little bit of a push again as pushing a child on a swing and that would cause it to spin a little bit faster until it reaches this limit. Now we see x-ray bursts but we can also find gamma ray bursts so let's take a look at those and gamma ray bursts were first detected back in the 1960s in looking for gamma rays from nuclear detonation so military satellites were detecting these first because a nuclear blast would give off a lot of gamma rays and they were looking for these detections. Now we have detected thousands of these from space. One of the problems is it is very difficult to pinpoint the location and this is because gamma ray telescopes have very poor resolution. It is very hard to focus gamma rays as compared to focusing say radio waves or visible light and that makes it hard to find the optical or radio counterpart. So where is the burst occurring we need to say that if we can only judge that it's in this region there could be multiple objects that could be the source of this burst and we don't know which of them it is. After some work we have been able to detect optical sources for some of them and many of them are found to be billions of light years away. So what are these gamma ray bursts then? Well there are two types there are the long duration bursts and the short duration bursts these are defined and given the limit of two seconds so a long duration burst is something that lasts longer than two seconds a short duration burst lasts less than two seconds. We believe that the long duration bursts are caused by the collapse of a star which has lost its outer layers of hydrogen so as the star collapses down and gives off all of this energy the energy is then beamed and gives us a gamma ray burst these are the longer ones. Now we believe that the short duration bursts are caused by colliding neutron stars two neutron stars orbiting each other and then spiraling in closer and closer together over time and eventually coalescing into one single object. Some of the evidence for this we have things like the kilonova and gravitational waves. Gravitational waves have now been detected from black holes and are a prediction that would occur for any massive objects that are moving and massive objects moving fast like two neutron stars collapsing would be believed to give off gravitational waves and that is getting to the point where we can now detect these. So the difference is again what happens if some of the outer layers have been pushed out of a star we may get in that supernova explosion instead of a supernova we may actually get the gamma ray burst because the outer layers are no longer present we only have the inner layers of the star in a short duration burst again we're looking at a different process where two neutron stars are actually colliding together so we can look at an image I kind of drew that on there but we can actually look at that as an image as well and when we see them here they are actually giving off those gravitational waves that will travel out into space the closer they get together the more rapidly they move and the greater the gravitational waves therefore making them easier to detect any object moving gives off gravitational waves any object with mass at least but they are so weak that they are difficult to detect unless you have high mass objects things like neutron stars or black holes that are moving very very quickly so let's finish up here as we do with our summary and what we find is that compact stars in binary systems give rise to different types of phenomena the white dwarf stars can give us novi or supernovae things that we can see in visible light neutron stars can give us x-ray or gamma ray again more compact object then can give us higher energy events so we associate the x-ray bursts or the gamma ray bursts with neutron stars and bursts of visible light for novi or super even supernovae are believed to be caused by white dwarfs in a binary system and remember that most stars in the universe half the stars are part of binary systems and that means that there are a lot of cases where this can occur so that concludes our lecture on compact stars in binary systems we'll be back again tomorrow for again next time for another topic in astronomy so until then have a great day everyone and i will see you in class