 Welcome to the Introduction to Astronomy. In this lecture we are going to talk about the formation of stars, so how stars go about forming from the material in the interstellar medium. Now let's just take a moment to review and go over some of the basics of stars. And we have stars that are, most stars are main sequence stars, much like our Sun, and they produce energy through nuclear fusion. That means they're converting hydrogen into helium at the rate of 600 million tons every single second. Now how much of this would depend on the mass of the star, a more massive star would be converting more, a less massive star would be less than this. Mass ranges of stars come from a little less than a tenth of the mass of the Sun to about a hundred solar masses. These small mass stars are more common, the main but the massive main sequence stars are hotter and brighter. The lowest massed main sequence stars are the ones that are cool and faint. And a galaxy like our Milky Way has the gas and dust to make billions of stars. So let's look at an example of one of these star-forming regions and that would be the Orion Molecular Cloud. This is one of the nearest star-forming regions, only about 1500 light-years away. Only a little bit of it is visible, much of it is very dusty and invisible to us. And we see it here, the four stars that outline the body of Orion are present as well as the three stars in the belt, which make Orion relatively prominent and easy to see. Down below we have the stars in the belt are about 5 million years old. So they're still relatively young. The sword hanging down from the belt contains stars that are less than a million years old. And the trapezium stars in the Orion Nebula here are about 300,000 years old. And we can zoom in, let's take a look at those trapezium stars. And here they are in visible light on the left, visible light on the left here, and in infrared on the right. And there we see those stars. They're hard to see in visible light because of all of the dust around them. However, when we look at them in infrared, the infrared penetrates the dust and gives us a much clearer view, not only of those stars, but of other stars, some of the 2,000 stars that are forming in this region. So this is one of those nearby star-forming regions. We see far more, in fact, far larger ones elsewhere in the universe. Now how do we go about clearing out the Nebula when the star cluster forms? Well, the stars will have strong stellar winds and that will clear away the material. And that is much like the sun clears out the solar system. It has a relatively strong stellar wind and that was able to push away at material. And we can see in our image here that when a cluster of stars is forming, the winds from that will push material back until it starts to recede. So it'll start pushing this material back outward and will clear out this. Now that will compress and we'll have more stars forming. So that will push away the remaining material. Massive stars do not live very long and end their lives in supernova explosions, which will also help push material back. Both of these compress the nearby clouds and continue that star formation process. So what stars have already formed are beginning to do the next set of stages. Now when we look at stellar birth and we see some areas of this, first of all, we cannot see it directly in visible light. Visible light does not penetrate, so we use infrared or radio waves. Infrared for the later stages and radio for the very earliest stages that penetrate the dust. The initial collapse is a very short period of time. Now here we see sometimes what is known as the pillars of creation, young stars that are in the process of forming. Now as I mentioned, you cannot see them in the visible light, but if we look in the infrared with the same area, and this is an image from the James Webb Space Telescope, we see how different they look, how much material is not visible. So we see some dusty areas when we look in visible light, when we look in a little more in the infrared, we see far more detail there and we see details in the structures. The places that are still dense are those dense knots where stars are forming currently. So you form a dense core within a clumps of material and most of that star formation is going on at the very peaks here, where parts are sticking up. That's where they are trying to eat away, the stars are trying to eat away at this dust and gas, but enough material has collapsed here to make it dense enough to be resistant to that collapse. Now here we can see how it looks, what we feel stellar birth is like. We start off with a large cloud and we form a dense core within that material. So we start off by forming a dense core of material and then that begins to collapse down into a disc so it collapse along the axis and the gravitational force becomes stronger and becomes dominant and a rapid collapse then begins and material, it's much easier for the material to fall inward from the polar axis than it is along the rotation. So it's a lot easier to fall down and that's why things come into a disc of material. Now as that disc forms, the star becomes what we call a T-tory star, which occurs for stars that are similar to our sun. Again, it's hidden in the dust cloud so we need to look at these in the infrared. We also get very strong stellar winds that will come here and will clear out the remaining material. So here we see jets of material coming out along that polar axis so as the star is clearing out it actually sends out jets of material. It's hard for that material to come out along this axis. It's much easier to go out perpendicular to the disc that is formed and that will clear out the remaining material giving us a disc, the jets temporarily which form what we call a herbig aro object and we can see an example of sketch of one of those here. So in that we have the star that is in the process of formation. We have an accretion disc of material around it. We see the jet. The herbig aro object is actually where that jet of material strikes the interstellar medium and causes it to glow. So it's not directly the star, it's actually the interstellar, material in the interstellar medium being excited. So this is a sketch of how it happens. Let's look at an actual one here. So this is the herbig aro object. The star is hidden at the center here and the bright spots would be what we would see that would glow in brightness. That's where the jets of material strike the interstellar medium and begin to glow. Now these are the very earlier stages. Then the star starts to settle down. So the winds will diminish, the jets will diminish and eventually disappear. That disc will likely begin to form planets. So we'll start to form planets and we'll look at exoplanet formation shortly and the wind and radiation pressure clear out the nebula, emptying it and the star and planetary system will remain. Now we can look at this on the HR diagram which as you recall is a plot of temperature and luminosity. We look at the temperatures on one axis, increasing to the left, we have the luminosities increasing upward. And when we plot those, we find that most stars will live their lives on the main sequence here, which is the red line. But as a star evolves, it will change its position and what that means is that its temperature is changing and its luminosity is changing. Overall, the temperature will increase while the luminosity decreases. So that's what we see with any of these stars as they're forming, the temperature is increasing means they're moving to the left, the luminosity is decreasing meaning they're moving down. So the stars tend to move down and to the left. The core reaches a critical temperature when nuclear reactions begin and that is when the star settles in on what is called the zero age main sequence. That means that it's as it's when it first becomes a star. However, if we have an object less than 0.075 solar masses, the nuclear reactions will never begin. It will never reach a high enough core temperature for nuclear reactions. And that becomes what is known as a brown dwarf, which we will look at again elsewhere. So let's go ahead and finish up here with our summary. And what we've looked at this time is that star formation begins in a cool molecular cloud in space. The material collapses and we form things like disc and jets of material around the star and we can use the HR diagram to be able to study these different stages of stellar evolution. So that concludes this lecture on formation of stars. 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.