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.