Welcome back everyone. Today we're going to start the second of our four series of lectures on the story of modern astronomy. And our topic for this lecture is going to be stars. We're going to be looking at the, the life stories of stars. And we'll put this into a single question which is do stars last forever? And for moderns, now that we know so much about stars, you may think that's a silly question, but you have to understand for how long, for how much of human history, the stars were forbidden fruit so to speak. No one knew what they were, they were just these points in the the night sky, and as much as people could sort of imagine, the stars were eternal. They were the realms of the gods. So, that's our question, do stars last forever? And let's just talk a little bit about, what questions explicitly we're going to be asking. So, one place we can ask of course is our, most, nearest neighbor, in terms of, the stars. And that is the sun. So, what do we know about the sun, and how the sun works, and the sun's structure? How do we learn about, distant stars, right? So the sun is relatively close. But what about the other stars? They're at enormous distances. What makes stars different? Even as you go out at night and you look up and you see that there are some stars that appear to be bright, and there are some stars that appear to be dim. There are some stars that are red, and there are some stars that are blue. What goes into those differences? Some of it is just going to be accidents such as distance, some of it's going to be the internal structure of stars. How do stars form? Like one of the questions we want to ask about is their life story. So if the question is do stars last forever and the answer is going to turn out to be no then how do stars form? How are they born, what's their life story, and how do they die? So that's our last question, is how do stars evolve and how do they die? Alright, so let us begin with the sun. And let's just talk about the route that the sun has taken in the human imagination. So, in the beginning, as one can imagine. The sun, which is the bringer of all light and warmth, and life in some sense. Started off as a god. So the ancient Greeks saw the sun as a god, Helios, riding a flaming chariot across the sky and of course this occurs in many different kinds of of mythologies. The sun with its daily route through the sky was seen as a god. But Anaxagoras who was a, a Greek, part of the beginning of the Greek philosophers, the Greek natural philosophers, around 500 BCE, was the first to propose that the sun and the stars were really the same thing. There was no difference between the sun and the stars. Of course, he had to explain why the sun was so hot, and he thought it was a giant, burning stone. And you know, this got him into trouble as often astronomy can get you into trouble depending on your culture and he was nearly executed for this belief. Fast forward a couple thousand years and we get to the Industrial Revolution and you know, the Sun which was clearly hot and bright was thought perhaps it was made of coal, because coal at the time was seen as something, as a substance which produced a lot of heat and a lot of light. The problem however, that scientists worked out pretty quickly, is that you can figure out how long a lump of coal is going to last and, what you'd find is that even if the entire sun were made out of coal, that it wouldn't last long enough to account for what people already understood about the age of the Earth. So, what we see here is by the Industrial Revolution in the 1800s, people already are beginning to understand that the Earth is very long lived. Billions of years. And so the sun therefore also has to be billions of years as well. Now at the same time, another great revolution was happening in science. Which was very important, which was the introduction of Spectroscopy. People were able to take light, and pass it through a prism of fine enough resolution. That they began to see that when you spread light out in a prism, starlight or sunlight. You actually have individual lines. You have individual places where the rainbow spectrum is, cuts out. So we call these absorption lines, we're going to talk about them a little bit more as we go forward. But what was clear was that there was a, a lot of structure in the spectra of starlight and sunlight. And in fact that spectra was often, the kind of structure was very similar, which also gave people a clue. To the fact that how the similarity between the sun and the other stars. So with spectroscopy. And the ability to sort of take light, and, you know, beat it up, and get it to, give up its secrets. Is what really progressed, allowed astronomy to go from being astronomy to being astrophysics, because it was at this time also that people were first beginning to understand the relationship between how light is produced and the structure of matter. So I will talk about that a little bit more as we go along. So that was a very important advance, people being able to analyze starlight in great detail and then by the end of the 1870s people were also er, end of the 1800s people were also thinking deeply about the question of energy generation in the Sun. In particular Lord Kelvin a British physicist, had already worked out sort of the details of how long the sun could produce energy if it was just a big ball of gas, that was contracting under its own weight, and giving off a radiant heat, giving off heat as it contracted. And what he found there that the time scale for that was much shorter than what people knew already that the Earth had to the age of the Earth. So therefore the sun, they needed a power source for the sun. So that became a, very important question that existed for about 50 years. Until the 1920s, 1930s, when people began to learn enough about the structure of matter. And in particular, the fact that there was a nucleus at the center of every atom. That they began to understand that it was nuclear reactions that were actually creating, the sun, the energy, or releasing the energy that powered the sun. Okay, so that's the story in general of the route of the sun from god to a star. And now let us go on and what we're going to do next is talk a little bit more about light so we can understand how light was useful, so useful to us in understanding the nature of stars.
