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  • Have you ever wondered how all the chemical elements are made? Then join me

  • as we are lifting all the star dust secrets to understand the cosmic origin of the

  • chemical elements. We just talked about fusion processes and how elements are

  • made in stars, mostly for energy generation purposes.

  • Now let's look at how that actually manifests in a star as a whole because

  • as astronomers, we can observe stars but we can't really look inside of stars.

  • We can only see the surface.

  • There are a number of ways we can get clues from the surface of the star as to

  • what's going on in the core. Of all that, a really nice example of how

  • nuclear physics and astrophysics -- nuclear and astrophysics -- come together because

  • the nuclear physics governs what's happening inside the core, and then the

  • astrophysics provides what we can actually observe and both kind of need

  • to come together and work out. So over the last several decades, a lot of

  • progress has been made to put these two together and to understand what is now

  • called stellar evolution. That is actually governed by the nuclear physics

  • processes, specifically fusion, in the core. I wanted to share this with you because

  • it's very insightful. I'm going to draw what we call a Hertzsprung-Russell-

  • Diagram. It basically shows how a star evolves during its lifetime. We can

  • use the Sun as an example, and what we're going to have is hot stars here

  • and cool stars here, and then we have more luminous stuff up here and

  • less luminous stuff down here. And there is a certain track that looks like this

  • -- half of a Christmas tree -- and the Sun actually sits right now about here. One

  • can put any star in this diagram. You will see in a moment how we can

  • then learn about the evolutionary phase of the star and hence what's going on

  • inside of the core. The Sun is sitting here and we know on this

  • branch here, which we call the main sequence, that south burn hydrogen to

  • helium. How does this look? If we draw a star here, in the core

  • hydrogen is burned into helium, just like what we had in the previous section. The

  • star, when it moves on, and I should say that every star will kind

  • of start somewhere along the main sequence here, will stay there for

  • about 90% of its lifetime which means, coming back to the old stars for a

  • second, -- 90% of 15 billion years is about the age of the universe -- which means the

  • stars that started here when they were born in the early universe, they are just at the end of

  • this hydrogen to helium process which really means they haven't done anything

  • else but burning hydrogen to helium which really is the key to why these

  • stars don't show their age. They are just like what they've always been and we can

  • observe them today and infer things about the early universe from them today

  • because they haven't changed. That's the key here.

  • But if we look at the star that has a shorter lifetime and wants to

  • evolve, it will move up here, and it will move up here when the core -- let's see, this

  • was hydrogen here, and it has been converted to helium -- when we indeed have

  • just helium in the core, and there's no hydrogen left in the core. Then the star

  • will get a little bit rumbly and so it's going to start moving along here

  • and all sorts of things going on in the call because the thermostat is out,

  • there is no energy being produced right now, and so it what happens is the

  • star actually inflates to counter act that and it will move up here to become

  • very luminous and up here, we have the red giants.

  • They're called red giants because they're much bigger and more luminous

  • but they also cooler because they are bigger and so they are turn red and so

  • they have just a helium core, and what's happening is in an outer shell here they

  • is still hydrogen to helium burning going on in the shell.

  • And that provides a little bit of substitute energy, a little interim inner energy, to

  • the star as a moves up here. Then up here, we have something called the helium flash

  • which means the helium here in the inner core is now being converted to

  • carbon. How can I draw this? I'll make this go away...

  • So we eventually, helium gets converted to carbon, so eventually

  • we're going to get to a carbon core. And then we have helium burning further out

  • and hydrogen burning yet further out. When the helium burning

  • starts here, by the time it reaches here, it has this carbon core, so here it

  • reaches a helium core, this region. Then helium starts to burn and then

  • by the time you come here, we have the carbon core and then it moves up here

  • and this last part here it really depends on the mass of the star. The Sun

  • is actually not going to do much, it's going to just stick it out with a carbon-

  • oxygen core, here, and then turn into white dwarf and just cool down. so the

  • Sun actually a pretty boring star that has a pretty boring fate. If we

  • make the Sun much more massive, let's say 10 x more massive, it would move up

  • here in this carbon burning phase and a variety of later burning stages that

  • lead to iron. And then it would have an iron core up here

  • and you already know what's going to happen if a star has an iron core -- it

  • has lost its energy source and it will explode as a supernova. So before it

  • explodes, what's it going to look like? We have a whole bunch of these so called

  • shells, sometimes they refer to as onion shells. In the center, we have iron and

  • then they're silicon and all the other elements following out here, oxygen, carbon,

  • helium, let's drawn another one, hydrogen. There are a few more other elements being

  • produced in in minority processes so some of these shells are not pure in

  • these elements but that's kind of the basic idea (that is oxygen,

  • silicon, sulfur and others...) that you have a star that looks like that. So what

  • you see here is that whatever is happening in the core has a direct

  • impact of where the object sits on this diagram here. By measuring the

  • luminosity of the star as well as its temperature, we can place it on this

  • diagram and then learn in which evolutionary state the stars is currently in

  • which tells us what is going on in its core.

Have you ever wondered how all the chemical elements are made? Then join me

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B1 helium core star hydrogen carbon burning

Ep. 7: Element Production (Fusion) -- Part 2

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    林宜悉 posted on 2020/03/29
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