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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. What is the universe actually made off? Let's look at three

  • different times in the universe and consider what it was made of. First

  • after the Big Bang. So if we draw a little pie chart here, the universe was

  • made from just hydrogen and helium and tiny little bits of lithium. We can just

  • cut it up like this, and and so this is hydrogen 75% and helium 25%, and it

  • pretty much adds up to 100% already but we'll just write it up here -- so lithium

  • is of the order of 10^(-10) which is really just a tiny tiny tiny

  • amount and we don't really need to worry much more about it. Now at a time

  • later, namely 4.6 billion years after the Big Bang, an important event happened, at

  • least for us humans, namely the Sun was born. From studying the chemical

  • composition of the Sun, as I will explain later, we can deduce what the universe

  • was made of at that time, and as it turns out the universe looked a little bit

  • different in its composition. It looked something like this, here we have

  • hydrogen, 70 1.6%,

  • 27% helium, and then this one, here, that's 1.4% of heavy elements. So

  • what we can see, and again we're going to go into more detail later, some of the

  • hydrogen got converted into helium. So it's less hydrogen about five billion

  • years after the Big Bang, but a little bit more helium, and helium, through various

  • steps, has been converted into heavier elements: a whole 1.4%. Now if we

  • then look at what things look like today, that's 13.8 billion years after the Big

  • Bang, you see that the heavy elements make up a whole 2%.

  • There's a little bit more helium and a little bit less hydrogen, so we have a

  • whole 2% of all the matter that are elements heavier than hydrogen and helium.

  • As you can see the two most important elements in the universe clearly are

  • hydrogen and helium, and really who cares about all of these other heavy elements?

  • They make today 2%, and in the early universe is was zero, and then a little

  • bit, and accordingly astronomers, already quite a while ago,

  • came up with the astronomers periodic table. That is pretty simple, actually. So

  • simple that I can draw for you here. It contains three things:

  • X, Y, and Z, and you can guess what X is: it's hydrogen. Hydrogen is pretty important in

  • the universe and it sits up there in the periodic table. Helium is also pretty

  • important, second most common element, it's in the top right corner there, and

  • then all the heavy elements combined, they are called "metals". And they

  • together make up Z -- so we can simply the universe pretty well: to just

  • hydrogen, helium and metals.

  • But of course we know that the devil is in the detail, and we are really

  • interested in these metals here because that is the subject of this lecture,

  • right, the cosmic origin of these heavy elements, and I should say here of course

  • that metals is not in terms of the chemistry is not the correct description

  • of all these elements that are found in the periodic table but, you know, we are

  • astronomers, so we get away with calling all the elements "metals", even if in a

  • chemical sense they are nothing but a metal. So that's

  • a little historical piece and people still use the term "metal", and we're going

  • to use it for our stars as well. Before we move on to the next topic I just want

  • to say that you can see the universe changes its overall composition with

  • time which means that stars formed at different times will naturally have a

  • slightly different composition as well. Because stars form from gas that is

  • available at a given place at a given time in the universe, and the star's

  • surface layer they do reflect the composition of the birth gas cloud. So we

  • are in a very lucky position that if we find stars born at different times and

  • we study their chemical composition, we can reconstruct how the composition -- the

  • makeup of the universe -- changed. And that's exactly what we're going to do.

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

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B1 中級

第2集:宇宙是由什麼組成的? (Ep. 2: What the Universe Is Made of)

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    林宜悉 發佈於 2021 年 01 月 14 日
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