Ep. 5: Stellar Archaeology
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. Let's summarize what we've learned so far about the old stars
and how they can be used in our concept of stellar archaeology to understand
what happened soon after the Big Bang, in terms of chemical enrichment and
chemical evolution. We have old stars and we call them "metal-poor", and there are our
tool, our tool to study the early universe.
These stars are long lived. They have a low mass, something like 0.6 to 0.8
solar masses, and that means that they have lifetime of 15 to 20 billion years.
That means that they are still observable. That is very lucky for us,
and they are not just observable, there are actually easily observable
because they are now located in the Milky Way. Let's look at this again: so
this is a very quick drawing of our Milky Way. This is the bulge, the inner
part of our galaxy with a supermassive black hole in the center. And this here
are actually two disks. That is the disk, and we're about two thirds on the way
out. The bulge contains a lot of young stars, there's a lot of gas which means
you have formed a lot of stars which means you made a lot of elements and formed
more stars. So the bulge is very metal rich. The disk here is not quite
as metal-rich but still pretty enriched. Now, this is not the only
part of our galaxy. This is just the most visible part, namely the Milky Way band
on the night sky. That's from when you look into the spiral arms that make up the disk.
But we look in a different place for the oldest stars because they are kind of
located up here and below the disk: In
something that's called the halo. It's actually much larger than what
I'm drawing right now. That's called a halo of the disk. It's a
spherical envelope of this this disk here. All the old stuff is parked there.
It's bit of a junkyard, yeah, because when a galaxy forms, you have
small systems that actually come together and form bigger system, and then,
here, you have a bigger system, too, and then they come together and make an
even bigger one. That's called the hierarchical structure formation
paradigm. This is the Milky Way which means that these little guys
here kind of end up in the outskirts or at least a good amount of these little
guys end up in the outskirts but they are completely shredded apart. And what
is left are all the stars that have been spilled into the Milky Way. This is
how old stars actually get into the outer halo of the Milky Way.
I should mention here that little dwarf galaxies -- also actually in
in the halo of the galaxy -- they also pretty old, so these are entire little
systems here that are not completely shredded yet. They are just in the
gravitational field, here, of the Milky Way, and they are orbiting around the disk
and we also have globular clusters. These are clusters stars, actually clusters of stars with
up to a million stars, and they are also located here and down here, and they are
also really old. We don't really know where they come from but the halo
contains mostly three things: globular clusters, dwarf galaxies and lots of old
stars and so with our telescope we can peek from here and here up into the
halo, and here and observe all the old stars that are
in this range. All in all, our metal-poor stars are the local equivalent to what
we call the high-redshift universe. In a very complimentary way, both metal
poor stars and the furthest, most distant galaxies are used to study the early
universe. These faraway galaxies, when the light comes to us, we receive it from
this early time and this way we can figure out what this galaxy can tell us
about the early universe because it formed at that early time. Our metal-poor stars,
their light hasn't travelled for a long time. It has traveled maybe just
from here to us. That's a negligible amount of time
because these stars are today located in our Milky Way. But they are really old. We
see them as when they are old, not as when they were young as it's in the case
of these distant galaxies. But the fact that we see them all doesn't doesn't
matter to us. Because these stars don't get wrinkly or anything they just sit
there and they are just waiting for us to observe them. As we will see in
the following, these stars are really undisturbed and they just look like --
today they look just like what they did 13 billion years ago.
So that's a huge advantage for us, and of course we're going to
make use of it.
