 Greetings and welcome to the Introduction to Astronomy. In this video, we are going to look at the Habitable Zones Lab and simulator that will be used to look at what the region is around a star that could be considered habitable. And you will look at a couple of different things here. There's some background information for you to review. And then there are two simulators that we're going to look at, the Circumstellar Habitable Zone Simulator, and the Milky Way Habitability Explorer. So we are going to look at those two to really be able to try to understand how this works. So we'll go ahead and take a look first at the Circumstellar Habitable Zone Simulator. And when we open that up, what do we see? Well, as always, we see a number of different windows here. First of all, we can see the solar system here to scale, and we're starting off looking at our solar system with the Earth. And there are a number of different settings we can do from general settings, or in planet settings, where you can select specific systems, if you like, or leave it on none selected, and then you can choose the initial star mass and the initial planet distance. And that will show you where those are, and then it shows you where the star is on an HR diagram. And then finally, down here, we have the timeline controls, where we can see what time we are set to, how long we have been, the star has been through since its formation. This shows the habitable zone, and where it is, whether that star remains in the habitable zone. Because what we'll see is the habitable zone does not remain the same, and a planet can end up being too close to the star or too far away at different times during the star's life. And this is where we see at the bottom, where we are in that star's life, for the sun here, about halfway through its life. And here's the point where it would stop fusing hydrogen, become a white dwarf, and eventually destroy the planet. But just because the planet is destroyed there, doesn't mean that it hasn't lost the capability to support life before that. So let's take a look at what we can do here. We can adjust and find out the limits of the habitable zone by dragging the planet. And you'll see that as you drag the planet further out, eventually it will turn from an Earth-like planet to more of a Mars-like planet. And you can find where that level is here at a little more than one astronomical unit. If you go further away than that, then the planet is no longer in the habitable zone. You can also move it in and find the inner edge of that. And that's at about 1 1⁄8 of an astronomical unit. And you can see that if you get closer in than that, then the planet no longer becomes habitable. So for our solar system, we really have just one planet in the habitable zone, and that would be our Earth. Now if this also can change over time, so if you watch where the Earth is right now and how long will it take, well, we can run the simulation forward and note that the habitable zone has been moving, that it is moving further away from the sun over time. That is because the sun is slowly moving and slowly warming up and getting brighter, putting off more energy over time. And that means there will come a time if you actually run this forward enough that the Earth would no longer be habitable. So there will come a point here, and we can adjust this manually, if you like, to find out that point that means after about a little over 5 billion years that the Earth would no longer be within the habitable zone of the sun. Not because of anything that's happened to the Earth, its orbit is not changing. But the sun has become brighter and will therefore be giving off more energy, and it will make the Earth hotter. So that habitable zone is sliding outward, and eventually there will come a time where Mars would be within the habitable zone. However, because of the size of Mars and its lack of an atmosphere, that still doesn't mean it would be habitable like the Earth. Just being within the habitable zone is not sufficient to say that something is indeed habitable. There are other criteria that are important as well when we look at those. So we can change those. We can also look at specific systems, if we like. And you can select, for example, 51 Pegasi. Find a star that has a planet, but it's very, very close to the star. And what we notice when we look at that is that the planet has changed its structure. It shows that it has a dividing line between the two. And that is that this is now tidally locked. So it is locked to the star so that one side is always facing the star and very hot. One side is always pointed away and therefore very cold. So this would be a very bad thing for life. So this planet would not be likely to have life. First of all, it's way outside of the habitable zone. And secondly, it's tidally locked to its star so that it would not have a good chance of having liquid water on its surface. Now, the other thing that we want to look at is the second simulator. And that is the galactic Milky Way habitability explorer. So as we look at this, this is a little bit of fewer controls here. In fact, just a couple things to look at. And what we're looking at is the probability of a catastrophic event. And this would be a supernova occurring within 50 to 100 light years, which would be sufficient to wipe out life on a planet and sterilize it. Supernova occurring that close to us would wipe out life. And we're very fortunate on Earth that there aren't any stars that would become supernova within that kind of distance from us. However, there are other stars that are within this. And the closer you get to the center of the galaxy, as you get closer and closer to the center, the probability of that extinction risk does increase. So that makes you think, OK, let's get far away from the center and minimize that risk of extinction. But we also have to look at the abundance of heavy elements. If there are not sufficient heavy elements, things like iron and silicon that make up a lot of the Earth, then we would not also not be able to have life. And those depend on supernovae to have formed. So as you get further and further out towards the edge of the galaxy, the abundance of heavy elements decreases, and therefore, you're less likely to have Earth-like planets. So you can actually play with the slider a little bit here, move that in and out. And as you move closer and closer, you get more heavy elements, but you also get a higher probability of extinction risk. Eventually, you reach a point where that extinction risk becomes tremendous. And if you go too far out, while the extinction risk has gone down, we do have that there's less heavy elements. So again, you want to find something in between some kind of middle range where there's less likelihood of a catastrophic event and still sufficient region that has sufficient amount of heavy elements to be able to form Earth-like planets. Now you'd also have to look at, and this simulator does not look at this, but one thing you also would want to consider is how close you are to the spiral arms. So being close to a spiral arm would also have a lot more of these massive stars. So even a star like the Sun very close to one of these would still be much more likely to have a supernova go off nearby, even if it is in the outer portions of the solar system. So these are two things that we can look at here to really tell where within the Milky Way the best chance of finding a habitable star would be. So that concludes this video on looking at the habitable zones simulators. We'll be back again next time to look at another one of these. So until then, have a great day, everyone, and I will see you in class.