PROFESSOR: Have you ever wondered
how all the chemical elements are made?
Then join me as we are lifting all this data
secrets to understand the cosmic origin of the chemical
elements.
Let's summarize what we've learned so far about the older
stuff and how they can be used in our concept
of stellar archeology to understand what happened soon
after the big bang in terms of the chemical enrichment
and chemical evolution.
Still we have old stars, and we call them metal-poor.
And they are our tool.
They are our tool to study the early universe.
These stars are long-lived, so they have a low mass, something
like 0.6 to 0.8 solar masses.
And that means they have lifetimes
of 15 to 20 billion years.
And that means that they are still observable.
And that is very lucky for us.
And they are not just observable.
They are actually fairly easily observable,
because they're now located in the Milky Way.
Let's look at this again.
So this is 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-- actually two disks.
So this is the disk.
And we're about here, 2/3 on the way out.
And 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 make a lot of elements and you form more stars.
So the bulge is very metal-rich.
And 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 when you look into the spiral
arms that make up the disk.
But we look in a different place for the older 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.
And so that's called the halo of the disk.
It's a spherical envelope of the disk here.
And all the old stuff is parked there.
It's a bit of a junkyard.
Because when a galaxy forms, you have small systems
that actually come together and form a bigger system.
And then here you have a bigger system too.
And then they come together and make an even bigger one.
So that's called the hierarchical structure
formation paradigm.
And so this is the Milky Way.
And 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 are being spilled into the Milky Way.
And so this is how old stars actually
get into the outer halo of the Milky Way.
I should mention here that little dwarf galaxies are also
actually in the halo of the galaxy.
So they are also pretty old.
So these are entire little systems here
that are not completely shredded yet.
So they are just in the gravitational field
here of the Milky Way.
And they're orbiting around the disk.
And we also have globular clusters.
These are clusters of stars with up to a million stars.
They are also located here and down here.
And they are also really old.
We don't really know where they come from.
So the halo contains mostly three things--
globular clusters, dwarf galaxy, and lots of old stars.
And so with our telescope we can peek from here kind of here up
into the halo and down here and observe all the old stars
that are in this range.
So all in all, our metal-poor stars
are the local equivalent to what we call the high redshift
universe.
In a very complementary way, both metal-poor stars
and the furthest most distant galaxies
are used to study the early universe.
These faraway galaxies, when their 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 traveled 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 in the case
of these distant galaxies.
But the fact that we see them all 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.
And as we will see in the following,
these stars are really undisturbed.
And today, they look like just 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.