Ep. 8: Spectroscopy
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 talk about spectroscopy. This is the technique we
use to observe stars in order to figure out their chemical composition.
Now, you've probably all seen a rainbow. I really hope you have and what happens in
rainbow. Well, white light comes through a little water droplet and it gets split
up into the rainbow colors and we do the same thing with a spectrograph mounted
at a telescope. We take the star light and we split it up into its rainbow colors.
Now, what we see when we do this is not just the rainbow. Actually, we
see less than the rainbow because there are certain colors of the rainbow
missing. So if I draw this schematically here I have a rainbow, and let's say I
have blue here, and then green and yellow, what I will see also is that there is a
big line, something like this, missing here and it's black, and then there will be a
few things here and a couple there, and you know, many really really thin ones in
between that are hard to see. That missing part, or those missing parts here,
they contain all the information that we want. It's actually not the colors as
such, it's what's missing from there. Now how can we understand that? If we come
back to our stars and look at the stellar surface, let's draw a surface layer here,
and the core is here, you know that nuclear fusion is going on
in the core. It is really hot here and energy comes out of the core in the form
of hot photons. So we have these photons escaping from the core and they come and
pass through this outer layer here. Of course we are sitting here with
our telescope observing the stellar surface, right, as I mentioned in a
previous section that we can't look into the core, we can only observe the
surface here. And specifically, what we're observing is we're observing all the
photons that come off the surface. In this outer layer, we have hydrogen and
helium atoms because that's what the star is mostly made of. Hydrogen, helium... but
of course there are -- unless we're talking about the very first stars but that's a
separate story -- there will be other atoms in here: iron, magnesium, carbon, oxygen, and
so what happens is that all elements, hydrogen and helium as well, plus
magnesium and so forth, they absorb (let's draw them here), they absorb photons with a
very specific energy or wavelengths. That's equivalent. So what comes out
of here -- here's one that gets absorbed, all these
get absorbed, and then there are some that pass through -- so what we see here is
all the ones that came through and of course not the ones that were absorbed
by these atoms. So that's exactly what we see here, the colors is
everything that came through, and then the black lines here are the ones that are
missing. So we can see what's missing. All these iron atoms
here, they have absorbed all the photons at a specific color at a specific
wavelength and so that's missing. However, it is actually not entirely black-black as
only a certain amount is being absorbed, perhaps not completely. So what we
can measure is when we take a cross cut through this,
we are going to get something that looks like this, and so there is strong
absorption here, less absorption here, let's say this is our calcium -- that's a
calcium line here -- and these are three magnesium lines, these are two sodium
lines, then we can see from these line strengths here what the abundance of the
magnesium atoms here, here's another one, is, so line strength here corresponds to
abundance of magnesium atoms in the outer atmosphere. The nice thing of
course is that we, when we want to find the most metal-poor stars, so the oldest stars,
then we want to look for stars whose spectra have very weak lines. Let's say
like this because that means that only little calcium, magnesium, and sodium were
actually present in the star which means that the star must have
formed at a really early time when the cycle of chemical enrichment had only
gone around a few times. So this is the secret of spectroscopy, absorption
line spectroscopy. We take these kinds of data here and we measure the line strengths,
we measure how much is present here, and with the help of computer programs
we can and a whole bunch of physics, we can turn these line strengths here into in
abundance in the stellar surface, and that tells us about the formation time
of these stars.
