PAUL: In this worked example
we're going to look at how you interpret
archaeological data
about galaxies. In particular
let's say you look at the a whole bunch of old stars in a galaxy like our own: what can you learn about the history of
the galaxy from it. This is what we call "Galactic Archeology".
So the first thing we can do is for each star we measure how old it is. Don't ask how, it's hard.
So what we can plot is a number of stars
against age
so these are very young stars and these star, say, are ten billion years old.
and let's say we see a curve that looks something like this.
Now what can we deduce from that?
Well it's fairly clear that the galaxy started forming about 10 billion years ago
and sometime around here maybe nine billion years ago
the bulk of the stars are being formed
how can we tell that? Well, what we're looking at here is low mass stars.
Low mass stars like red dwarfs have lifespans of hundreds of billions of years, so none of them have ever died.
Every red dwarf that's ever been born in the galaxy is still there.
so the fact that so many of them are about nine billion years old is telling us that about nine billion years ago
lots of them were formed
more recently the star formation rate has tailed off
OK, that's one clue, but now let's look at a slightly different graph.
Now let's look at the typical iron in a star
against age.
So this is the iron abundance.
And what's this going to look like? Well let's say it looks something like this.
Now what can we deduce from this?
what we can see is the very oldest of have no one
which is what you'd expect: these are the stars that were born out of the primordial gas from the Big Bang that was just hydrogen and helium
But very soon the stars have acquired quite a bit of iron. Where can that iron have come from?
Well it can't have been made in the middle of these low mass stars because
low mass stars don't really mix the marterial up, you're not going to see the iron made in their core on the surface.
Almost certainly what happened is that at the same time as the low mass stars formed, high mass stars were also formed.
They all seem to go together. But the high mass stars don't last very long.
and some fraction of them would have exploded to make Type II supernovae almost at once and dumped their iron out.
So the stars forming say here, just a hundred million years or so after the first stars formed
have been enriched by theiron squirted out by theType II supernovae
But very soon this curve starts flattening out, and that's becaus we're past the peak of the star formation.
Only a few stars are forming now and so there's not much going supernovae.
and then it stars climbing again over here about four billion years ago
four giga-years
What's happening here is we're starting to get the type Ia supernova forming, so the very first stars that were born over here
some of them were very massive and exploded right away, and gave us this amount of iron
But some that were a bit lower mass turned into white dwarf stars and eventually started exploding over here so six billion years later
and they're producing iron here, so what we're seeing here is an echo of the first burst of star formation
so what we can tell is here is the iron from Type II supernovae
and here is the iron from Type Ia supernovae
and we can tell there seems to have been about a roughly five billion year delay between the peak of star formation and when
the stars from there started turning into Type Ia supernova; white dwarfs that explode.