Ep. 3: Early Chemical Evolution
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 element. Let's look at the early chemical evolution. Some 14 billion years
ago, everything started with a Big Bang.
And it left behind the universe level made from just hydrogen and helium
gas. I mentioned before there's a tiny tiny little trace of lithium but we're not
going to worry about it. So we had a universe but they were no stars yet.
It was actually dark, and some people call that the cosmic Dark Ages. But
structure began to form and gas began to clump, and so eventually, in a gas cloud
the very first stars formed. Here are two of the first stars. And they were made
just from hydrogen and helium because the gas was just hydrogen and helium but for
energy generation purposes, these stars needed to have nuclear fusion go on in
their cores to produce energy. The first elements heavier than hydrogen
helium were created inside of these stars, and later when they exploded as
supernovae, they were expelled into this gas cloud. So we had the first elements
here, and of course they also gave the first light -- they lit up the universe for
the first time and changed it in a number of ways not just by bringing in
light but also by producing the first heavier elements. That set in motion
the chemical evolution of the elements. That's an ongoing process until today.
Now these stars, here, because they formed from just hydrogen and helium gas were
very massive: rather large, maybe something like a hundred solar
masses, so we use the unit of solar mass (this is a symbol for the Sun here) as a
unit. So 100 times as heavy as a Sun. The reason for that
is that hydrogen and helium gas has trouble getting really cold
making small stars. The important thing with massive stars is that they have a
really short lifetime. They have a short lifetime of maybe a few
million years only. A few million years is not much for on a cosmic time scale
which means they exploded pretty quickly as gigantic supernovae, and as already
indicated what happened at a later time these stars were gone but they left behind
all the heavy elements that they had created, and I should be specific here,
these heavier elements were all the elements made in fusion processes up to
(and including) iron. So we have fusion elements in the gas. With the onset of these
little other elements being in the gas, now the gas could clump much
better and actually make small stars. So the next generation of stars, this was
the first generation, and, here, now we have the formation of
second-generation stars, here is a second-generation star, here's a
second-generation star -- so small stars similar to the Sun and perhaps a little
bit less massive than the Sun actually -- but of course you're not going to make
just small stars you're also going to make big stars. So we have a few big stars
here, again, a few intermediate ones and the big ones, the massive ones, will
again explode on a pretty fast timescale and make more elements. But these little
guys, and that's the interesting part for us here now,
so the low-mass second generation stars,
they have very long lifetimes, of something like 15 to
20 billion years because they had masses (at least some of them)
between 0.6 and 0.8 solar
masses, so less massive than our Sun. As a good rule of thumb, I can write this
here, the Sun has by definition one solar mass, and that has a lifetime, or
the Sun has a lifetime, of 10 billion years. So everything that has less mass
will have a longer lifetime and everything that is more massive than one
solar mass will have a much shorter lifetime. Now, what does this
imply? Having a long lifetime? It means that these stars are still around. They
are around today, we can observe them today, we can see them today, and that
means that we can use them to study the composition of this early gas here
because that's when they're formed. They have incorporated in all their layers,
throughout, the composition of this gas cloud that was enriched by
these very first stars. So we have means to study the first stars and what came out
of their supernova explosions, and what happened at this very early phase here of
star formation, galaxy formation, and the formation of all the chemical elements.
If you then wind the clock forward, many things happened after, well, of course
more stars formed, many many more stars, more supernovae,
and with every generation of supernovae, more of all the elements was provided
(more of all the elements) because as we will see later, in some of these very
early processes (nuclear physics processes) all the elements from the
periodic table were created already at the earliest times.
Then with time, more and more of all of them were created. It's not like that the
universe (because it was just hydrogen and helium in the beginning) then took
time to build up all the elements and marched through the periodic table with
time -- that is not the case. Everything was produced from early on, and then just
more and more off it. And so, well, we have more stars and more chemical
enrichment but also the formation of the first bigger structures
including the Milky Way.
Because this all happened in small gravitationally bound systems and they
were then gobbled up later by slightly bigger neighbors and then those were maybe
gobbled up by the proto-Milky Way, and eventually the Milky Way formed, and
formed from the remnants of all these little systems. All
of this here happened as time went on until today. Today we have a universe
that's full of structure and and full of all the elements, well, a whole 2%
but I think we can consider this "full" at least, you know, if you're an
astronomy. And chemical evolution is an ongoing process. So in
the following, we're going to look in more detail at chemical enrichment and
chemical evolution and also stellar archaeology which is the way how you
all stars to trace these different early stages of chemical evolution.
