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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. Why do stars shine? Why do we have sunlight every day? Well it's

  • because of nuclear fusion.

  • Nuclear fusion is going on in the core of the Sun.

  • Hydrogen to helium gets converted there and that gives the Sun enough energy to

  • sustain its luminosity for billions of years. How does this work?

  • Let's have a look. We want to reproduce what is going on in the Sun

  • and what's basically happening, is that 4 protons -- which also are just

  • 4 hydrogen atoms -- come together in a series of steps that we are going to

  • leave out for now, and they form a helium atom. That's made from 2 protons

  • and 2 neutrons. So we have some conversion of 4 protons here into

  • 2 neutrons into 1 helium. This actually works only because there is quantum mechanical

  • tunneling going on. That's a really cool thing. Ordinarily these positively

  • charged protons would actually repel themselves. But in the Sun it's

  • really quite hot. Not quite hot enough for them to all fuse straight up but

  • because of this tunneling effect it's hard enough -- just hot enough -- so that

  • these protons can combine to eventually form a helium nucleus. This kind of

  • tunneling effect is important for all subsequent fusion processes,

  • namely if we have another helium here

  • and another one, so if we put all of those together, we're going to get a carbon

  • nucleus. This is the carbon nucleus, and if we're going to add another helium to

  • that we're going to get oxygen. If we add more so-called alpha-particles

  • -- helium nuclei often called alpha- particles -- then eventually we're going to

  • get to iron. Now how does this help us understanding why the sun shines?

  • As it turns out, these lightest nuclei here are much less

  • tightly bound than the big ones like iron. That means we're going to get a

  • little bit more energy out of this than that. Now let's look at some details and

  • then come back to this. So if we're going to look at the constituent

  • here, 4 protons which make up one helium nucleus, and then we know 1 helium

  • nucleus consists of 2 neutrons and 2 protons. If we make

  • a little experiment and we weigh 1 helium nucleus and then we weight 2

  • neutrons and 2 protons separately, we're going to find out that the helium

  • nucleus actually weighs a little bit less than my initial constituents here.

  • Actually it's 0.73 percent that our final helium nucleus here weighs less

  • than these constituents. And that's really fantastic! This is called a

  • mass defect, and you've all seen the equation e = m * c^2

  • usually with a picture of Einstein attached. This here, this is a little

  • mass, a little mass difference. And if you stick that in here, you multiply

  • it with the speed of light squared which is just the constant, so just a

  • number, you're going to get out energy. And that is the energy that the Sun is

  • using to shine every day. This is the nuclear energy that stars produce. Now

  • this amount of energy that gets out becomes successively less if you go to

  • heavier and heavier nuclei and if you were to try to fuse two iron atoms

  • together, you're not going to get out anything. So iron atoms will not give you

  • any fusion energy with this here because this here is zero. Actually

  • you would need to put energy in if you

  • wanted to fuse two iron atoms. Obviously the star is going to have a

  • big problem because it doesn't want to, you know, put energy in and wants to get

  • to energy out. That's why, in the end, the star ends up with an iron core.

  • This is an iron core, here, these fusion processes have been going on in the

  • center, and growing larger and larger as more and more elements are being made.

  • And eventually, if there is a big fat iron core sitting there, and since it

  • can't get any energy out anymore, the star has a big problem

  • because it needs to have an energy source. Without that, it explodes

  • as a supernova.

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


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B2 中高級

第6集:元素生產(融合) -- -- 第一部分 (Ep. 6: Element Production (Fusion) -- Part 1)

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    林宜悉 發佈於 2021 年 01 月 14 日