 Greetings and welcome to the Introduction to Astronomy. In this video we are going to talk about star formation, so how stars actually form and the processes that are involved there. So we're going to start off looking a little bit at just some of the basics of stars. So what we look at is how do stars work? Well, stars, main sequence stars specifically, which are stars like our Sun, produce energy through nuclear fusion. And what that means is that they are converting hydrogen into helium at the rate for our Sun of about 600 million tons of hydrogen into helium every single second. However, the Sun has so much hydrogen in it that it can do this for 10 billion years without running out of hydrogen. Stars overall come in a big range of masses from about eight one-hundredths the mass of the Sun, a little under a tenth the mass of the Sun for the lowest mass star possible. And the most massive stars, a little more in question, up to about a hundred solar masses. Sometimes we see things going a little bit higher than that and often when we start to find things that are too much higher than that, when we find stars that we think might be 200 times the mass of the Sun, it usually turns out to be several stars close together. Now of these, the small mass stars, the low mass ones, are much more common. So we see far more of these tenth of a solar mass stars than we do of the hundred solar mass stars. However, these hundred solar mass stars are the brightest. They are the hottest and the brightest so they are the easiest ones to see. We can see these over great distances whereas we can only see these very small stars, the cool and faint ones, if they are very, very close to us. And when we look like a galaxy like our own Milky Way, it has gas and dust to make many billions of stars. So let's take a look at one of the star-forming regions within our galaxy, which is the Orion Molecular Cloud. And this is one of the nearest star-forming regions, 1500 light-years away. Now if we remember what a light-year means, that means we see it as it was 1500 years ago. However, the star formation process is slow, so it's not that things have changed drastically over 1500 years. So it might not look a whole lot different than it does right now. Now you may recognize the outline of Orion. There's Orion's belt, the sword, which is the Orion Nebula, which includes the Orion Nebula calling coming down, and some of the bright stars, Betelgeuse and Rigel, that make up part of the body of the hunter. Now, only some parts of this are visible. We can see some of the areas, the great loop here coming around Orion, and some of the other nebulae, which are emission nebulae caused by the emission of hydrogen gas giving off a very bright glow. But it's hard to see into this area because it is so dusty. When we look at this, the material that's here has formed over a period of time. So the stars in the belt are about 5 million years old. These are relatively older stars, about 5 million years. The stars in the sword going down from the belt in here are about a million years old. And the trapezium stars in the Orion Nebula itself are only a few hundred thousand years old. Now we can look at those trapezium stars in a little more detail, and here we see them in an image. This is visible light here, and this is in the infrared. So this is the same image. And you can see those stars kind of buried within all the dust here. In the infrared, they're much more visible. The infrared light is much better able to penetrate the dust to be able to let us look in to this stellar nursery. So here we are looking at the trapezium stars within the Orion Nebula. And these are only a few hundred thousand years old. So these are some of the most massive and hottest stars that we can see. But overall, we do see over 2,000 stars in this Orion region. So it's not forming just these very bright stars that we see, but lots of others. And this is, again, one of the nearest star-forming regions that we have to us here in the Milky Way. And it's an ongoing source of star formation. So when you often see images of nebulae, a lot of them that we see come from the Orion region with the Orion Nebula pictured in part here being one of the main ones. So how do we go about, once we've, as we're forming stars, how are we going to go about clearing that nebula? How are we going to clear that out? Well, we need to have this. We use the stellar winds of material. So as these young stars form, the young stars give us a very strong stellar wind which pushes material away. And that will clear away some of the major material. Many of the massive stars do not live very long, only a few million years. And they will undergo what we call a supernova explosion where they will implode as they build up iron in their core and then massively expand back out. Both of these serve to compress nearby clouds and continue the star-forming process. So we can see in the image a star cluster that is forming and as its radiation pressure pushes out, it pushes back against the nebula and the material left behind. And that material then gets compressed and more star formation continues to go on. So here's the stars that have formed. Here's the stars that are still in the process of forming and things like shock waves from supernovae and the compression from the stellar winds will enhance that star formation and continue the process. So that will eventually clear it out leaving behind just a cluster of stars at the end and eventually the nebulae itself, the nebulosity will be gone. So what is the process of the birth of a star? Let's take a look at that. And again, direct observations are not possible because of dust. So we cannot see that visible light makes it invisible to see the star formation process. However, infrared or radio waves can penetrate the dust and therefore allow us to kind of get a look or a glimpse into what is happening inside some of these clouds. First, we have the initial collapse which is relatively short, taking only thousands of years. And again, while that sounds like a long time to us, astronomically speaking, that is an extremely short amount of time. You start to form a dense core within a clump of material. And within that, the gravitational force will grow and eventually become dominant. So once you get enough gravity forming at the center here, it begins to compress and more material will fall in. The rapid collapse then begins as material begins to collapse into a disc. Now it's a lot easier for the material to fall in along the poles than it is along the equator. Material that is rotating around in the disc will have a much harder time losing that rotational energy. Material that is just scattered around not having a lot of rotation will be able to compress a lot easier. So the material will tend to collapse into a disc as the clouds and bits of clouds collide with each other and lose energy dropping down to a lower level. Now, as this happens, we begin to form for solar mass stars what we call T-tory stars. T-tory stars are, again, hidden in the dust clouds. We see them in the infrared and they are materials that will then have outflows of jets. So jets of material, but the T-tory stars have jets of material that come out and can then impact into the rest of the material. So the strong stellar winds that form will clear out this material, there'll be jets of material coming out and eventually we'll have the protostar left behind. Now what those jets do as they come out is to impact into that remaining material. So we form what we call a herbig aro object named after the two astronomers who studied these very early on. And here we have the protostar at the center. So the protostar is in here at the center and we have the accretion disc of material around it and then perpendicular to that we have the jets coming out this way and this way that then impact material around it and this is what we call the herbig aro object. It's not the protostar, it's not the accretion disc, it is the point where the jets of material strike the interstellar medium and heat that up and cause it to glow. Now this is a sketch of it, what does this actually look like? Well here's an image of it and this is an image of what we would call one of these early, early forming stars. The star itself would be shrouded in the center and hidden by the dust but the jet of material coming out can still be seen and then impacts over here in brighter areas as it impacts would become the herbig aro objects as that material impacts into the interstellar medium and causes it to glow. So this is a very early stage of star formation and especially common for the T-Tori stars for stars like our own. So as it goes through this as it's cleared out the nebula and it's formed this herbig aro object or T-Tori star, now we begin to settle in to the main sequence. So the star begins to settle down, the protostar calms down, the stellar winds, the jets diminish, the disc of material around it will likely form planets. We see that planets are very, very common and the stellar winds and the radiation pressure will slowly clear out the nebula but that clears out the dust and the gas particles leaving behind the star and most likely a planetary system behind. Now we can also look at stellar formation using the HR diagram. So we can look at an image of that here. As a star changes, it will change its position on the HR diagram that has nothing to do with the star itself moving around, it has to do with the temperature and the luminosity changing. So when the temperature and luminosity change, we're plotting the temperature and the luminosity here when they change its position on this main sequence will change. So if we look here for a protostar that is collapsing, the tracks are slightly different depending on the mass. They all start up in the upper right hand side of the main sequence which also contains red giant stars. However, red giant stars are not buried within nebulae so these stars are very hard to actually see with regular telescopes. So what happens overall is that the surface temperature will increase much more for a very high mass star going up to tens of thousands of degrees maybe only a little bit for a low mass star maybe only increasing few hundreds of degrees but in general the temperature will increase and the luminosity will decrease. Why is the luminosity decreasing? Well in a sense the star is becoming smaller. The large stars are up here so a star up in this corner is very large as it works its way down this direction the star will become smaller. Eventually when you reach the critical temperature of a little over 10 million degrees nuclear reactions, nuclear fusion begins in the core and the star will settle on what we call the zero age main sequence which is the reddish line down below here. So that is where the star will settle and then it will slowly change over its main sequence lifetime. That main sequence lifetime may be millions of years for a very massive star. It can be billions of years for a star like our sun here and it could be a trillion years for a very very low mass star. If the mass is low enough less than about .075 solar masses it will never achieve high enough temperatures in its core for nuclear reactions to begin and that becomes what we call a brown dwarf star. So a brown dwarf is a failed star a star that was never became hot enough in its core for nuclear reactions to begin. So let's finish up here as we do with our summary and what we've looked at is star formation begins in a very cool molecular cloud in space and the Orion region was a part of one of these that we looked at. As the material collapses down you form a protostar and you get to see a disk and a jet that will often form and we can use the HR diagram to study the early stages of stellar evolution and also later stages as well. So that concludes our discussion of star formation. 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.