Now, at this point, you may well be thinking, well that's a lot of theory.
I mean, yeah, here's a prediction. Our model's predict that the main
sequence staller will evolve into a red giant, and, woo, we find red giants, but.
That's not yet enough we want to see can I really show from observations that red
giants evolve at after main sequence stars in other words that one turns into
the other and we have a lab that will allow us to do this and I suggested that
this is going to be clusters because remember, stars in a cluster whatever
there size are all going to be formed at about the same time massive stars evolve
faster. The later stages of evolution are rapid
so if I let a cluster evolve for sometime and then stop.
I should expect to find, say, lots of O and B stars but no G stars.
That would be a reasonable thing if I find a bunch of G stars and O and B
protostars I've done something wrong. I need to imagine that the way that the
cluster evolves fits what model tells me. Flipping this around once I trust my
model I can look at a cluster and figure out its age by figuring out which stars
have evolved to what point. And moreover, it is in the context of
clusters that spectroscopic Peralax comes in to it's own.
When we look at a cluster, all the stars are about the same distance from us.
Whatever inter stellar extinction and reddening is going on because of.
Gust or clouds in between us and then, it's pretty much the same for all stars.
So, what we can do, is for these stars, brightness, relative to each other,
corresponds to luminosity relative to each other.
And color corresponds to color, relative to each other.
So, rather than taking detailed spectrum of each star, what, we have efficient
ways of, basically, taking three filtered.
photometric results through three filters doing what's called a color magnitude
diagram. Which basically plots the difference
between two colors, two filters, and the sum of the filters, or one of them.
that's basically brightness versus temperature.
And There are errors in the temperature, and
there is inaccuracies because of intergalactic rendering, but they tend to
move, inter stellar rendering, they tend to move the whole thing left to right.
There are. Unknown extinction that moves or distance
that moves everything up and down cause we can't determine an absolute velocity.
But by matching onto the main sequence we can get, and large numbers of stars, a
very accurate determination both of distance to the cluster and then of the
absolute luminosities of all the stars there.
So let's see a model simulation of how a cluster should evolve and then we'll
compare that to what we actually see. The star clock 2.0 is a very fun
Dos based simulation and what it's going to do is it's going to create for us a
collection of stars of various masses and.
I'm going to run the simulation, and I can stop it.
And we see that, let's see, about 800,000 years have passed.
Stars with masses nine solar masses and above have formed.
The ones with the higher masses are, in fact, beginning to move off the,
Main sequence. You see them moving off to upper right.
And later stars have not yet formed, so in 840,000 years, you get down to nine
solar masses. Let the animation run some more, your now
down to 1.6 million years and stars with a mass of, four solar masses have formed.
And the heavier ones are beginning to move off.
At this point, it pays to increase the time step because some things happen too
slowly, but you know, let's, let's take a few more slow steps.
Just to see what's going on, the heaviest stars will start leaving the main
sequence before the lightest stars show up.
Here we go, we're starting to see horizontal branch behavior.
And the heavy stars we said, or massive stars and except their main, the main
sequence earlier and we're seeing that before the sun varying in the sample
comes on the Into the picture.
Stars with masses of, say. Eight solar masses or more are already
done with their main sequence lifetime and have disappeared and notice the
repeditity with which horizontal helium branch.
behavior is gone. this star is, the 25 solar mass star, is
already done with helium burning and has probably generated a solar nebula.
And we're down to two one-half solar masses, and we again see very rapidly
zipping through the final stages of evolution.
Now that we get to the lighter stars, things slow down.
So that we indeed up my time step. And, now things will run ten times as
fast and we finally are almost ready to have our sun come on the picture and so
you see this is what the model predicts. These are our trajectories of,
Stellar evolution that we should expect to see and now stop this, any time I hit
a key and stop the simulation, this should be a possible configuration that
we can find in actual clusters so we go out and look around at clusters and make
these color magnitude diagrams and I will refer you to a beautiful database that
will make them for you and let's see how they jibe with reality.
So that was a nice simulation. We now know what theory predicts.
Now we go out to the lab, in other words the universe, and.
Do real clusters exhibit the properties that we saw any order of increasing
gauges they're very, very young cluster its called these kinds are called ob
associations they are clusters so young that.
Hot O and B stars are still on the main sequence so you don't expect to find many
solar mass stars on the main sequence. Here is the color magnitude diagram.
this is to be read very much like HR diagram.
In other words luminosity increases this way, temperature increases this way.
And we see here. It's very hard to figure out where the
main sequence lies but what is clear is that my guess probably somewhere around
here. Would be the main sequence, and only the
most massive of the stars have landed on it, and the less massive stars are a
collection of proto-stars. The structure becomes more definite when
you go to a young, but relatively developed cluster.
This is the Cone Nebula. That image that I prematurely erased is a
beautiful Hubble image which shows you, by the way, these dust cocoons that I
spoke about in which new stars are forming, but if we look,
At the eight, color magnitude diagram for this, we now see a well-defined,
[SOUND] Beginning of a, main sequence and, in fact if you drew this carefully
you would that the main sequence, let me correct myself.
Is probably something like this, and, what we are seeing is that the, more
massive stars have joined the main sequence and less massive stars have not
joined it yet, and the main sequence, the sort of the lightest ma, stars Already on
the main sequence would tell you how old the cluster is, a slightly older cluster
again this is familiar this is a pretty image of the orion nebula and here by now
by twelve million years of age we see a very well defined main sequence and what
we see this is sort of a classic. A jar diagram, that would be the main
sequence. We see that lighter stars are still on
their way down. Remember these are the evolutionary
trajectories down to the main sequence. And the most massive stars are already on
their trajectories away from main sequence.
So we have two sort of reference points here.
We have what's called the turn off point. That is the mass of those stars that are
just beginning to leave the main sequence.
And then we have, if you want, the turn-on point somewhere down here, which
is the mass of those stars that are just becoming main sequence stars.
And both of those, when you compare, have to give you.
Equal ages. More to the point when draws were called
isochrones, lines along which stars would lie at the same age and matches to the
cluster end. That's how we perfect our models, or the
people that do that perfect their models. We can look at an older cluster, the
Pleiades, well established cluster, 130,000,000 years old.
And again what we can see is that the. Main sequence turning point has moved to
the right. Note the main sequence turning point was
well to the left of zero over here and it is a little bit to the right of zero over
here. Older clusters have a redder, if you
want, a less massive turnoff point from the main sequence and there are very few
proto stars left. In fact, we're beginning to see here the
beginning of a population of A white dwarfs were one the more massive
stars that have already loss their shells here's an even older cluster 300,000,000
years here I've let the software helpfully designate for us
A few red giants and we very nicely see the main sequence turn off points here's
the precipe. Beehive.
You can see that it's an old cluster just by looking at it.
Look at all those reddish stars and very few that are blue.
And indeed the main sequence turning point is now, oh, at about +.3 In this,
color scale. So moving, consistently to the right.
Though there are a few stars, those marked over here in blue.
That don't belong. We also see, a well defined population
of. White dwarfs.
But what these blue guys are, we do not understand.
They are stars that are on the main sequence.
Even though they are, past the turnoff point.
And should already have been gone. We'll return to those momentarily.
Very old, open clusters. Here's M 67 and NGC 188 it's pretty easy
to tell that the latter is older than the former.
And finally, become the globblier clusters, the oldest clusters we find,
but also the richest. So they produce some very brilliantly
rich Color magnitude diagrams.
Here's the color magnitude diagram for M13.
And you see that with enough statistics you can find both the main sequence and
the turn off point and the entire red giant branch and the horizontal branch.
But no AGB stars that goes too fast. Even an M13 we don't find in the AGB
stars. And again you might ask, what's that guy
doing over there? That's another one of those blue
stragglers. they're particularly evident in this
image. So globular clusters are the oldest star
clusters. They're uniformly very old and indeed
this is a picture of a very beautiful, a very beautiful Hubble image of a pretty
globular cluster NGC6397. And you look at it and you know there's
something wrong. I understand where the yellow stars came
from, what is all this blue stuff? Globular clusters are old
It turns out that this happens a lot, particularly in globular clusters.
We have this phenomenon of blue stragglers.
The mechanisms are still a matter of some debate but probably a combination of two
things. These stars did not start out as massive
stars else over the billion year or three or five or 8,000,000,000 year history of.
These stars did not start out as high mass, the high mass stars that we find
them to be now. Because over the twelve or thirteen or
whatever billion year history of this globular cluster, they would long since
have evolved off the main sequence. They're still blue and on the main
sequence means they started out as less massive stars.
They became so massive either by mass transfer of a close binary phenomenon
we'll talk about later in the week. Or by the collision and merger, maybe
they started off as two or three. Less massive stars that merged up.
Its an, we don't see many stellar collisions out where we live in the
spiral arms where stellar densities are very low.
Our nearest neighbor is four light years away.
But globular clusters are much denser, collisions are conceivable.
And in fact it's lice, likely that both mechanisms take place.
And I spoke about the distinction between open clusters and globular clusters, and
I said, globular clusters are much, much older.
Astronomers had traditionally made this distinction
By calling distinguishing population one stars from population two stars and
initially it was a kinematic distinction, population one stars were the ones that
were moving around roughly the way the sun is around the milky way, population
two stars were moving in all kinds of other directions, their proper motions
and their doppler shifts told us they had high peculiar motion.
It turned out later that. But they what was underlying this
extinction what is now the definition of population one stars and population two
stars population one stars are middle rich.
That does not mean that there are stars made of iron.
The sun, with its trace contamination of carbon and oxygen and iron and calcium
and so on, and silicon is a population one star, that's called metal-rich.
The stars that we find in globular clusters are far more poor in metals,
and. so then we call the metal poor stars
population two. Now since we know by now that the carbon
and the calcium and the iron as we'll see.
That the sun incorporated into it's nebula.
That were there when the solar nebula formed were actually formed by
nucleosynthesis inside. A previous generation of stars.
What we conclude is that population two stars formed in regions where they were
essentially the first, or maybe the second generation of stars to form.
Whereas population one stars are forming in regions where there was much previous
star formation. Notice that its not a problem if the sun
say 5 billion years old. Because a g-type star lives 10 billion
years, but o and b-type stars that live say five billion years.
Can turn over many thousands of times in the time between, say, the formation of
the Milky Way 13 billion years ago, and the formation of the solar system 5
billion years ago. There was lots of time for lots of
generations of stars to have lived and, and scattered.
Their planetary nebulae around. And in fact the conjecture, since there
are some metals in the population two stars, the existence of a yet older
generation called population three. We've never seen them, I think.
But we believe they were there. These would have been the first stars in
the universe or in any region in the universe and they would have essentially
been pure hydrogen and helium stars. And we'll talk later about how that
changes their dynamics and why we think they must have existed.