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.