Hey, so, 
we spent a week looking at main sequence 
stars and we can apply really fun and 
exciting thinking and observation to 
learn a lot about them. 
That's 90% of the stars. 
We want to know about the others and one 
of the things that main sequence stars 
taughts us is that they're not eternal, 
they burn out. 
So if we see any they must be forming, if 
we can ask what happened where they came 
from, how it is that they formed, and 
what happens when they are no longer able 
to be main sequence stars. 
What happens to a star when its done, 
that'll be the topic of this week stellar 
evolution. 
The challenge that faces us when trying 
to understand star evolution is that 
stars evolve, but very slowly. 
The time scale for the sun's evolution is 
tens of billions of years. 
We do not get to watch a star evolve. 
we get a snapshot of the universe as it 
is now, and so we will need, again, to 
apply both painstaking observation and 
modeling and understanding of the science 
to pick piece together a picture of how 
stars evolve. 
I spoke about sort of population studies 
as the realm of sociologists, I think 
last week. 
probably a better analogy is the one I 
quoted on the title slide due to the 
famous British astronomer Sir William 
Herschel, 
who liken an astronomer to someone 
wandering through a beautiful garden and 
trying to understand by looking at the 
plants, how it is that they developed and 
evolved and developed and grow and 
you can relate if you wish the fully 
grown redwood to a sapling, even though, 
no living biologist has seen a 
the redwood sapling grow into a 4,000 
year old mature tree. 
We know enough about the way plants work 
to relate the two. 
And we have to learn enough about the way 
stars work to make the same 
observations about stars. 
one of the laboratories that we have to 
do this is star clusters. 
Clusters are, remember, are formed when a 
cloud starts collapsing. 
That means all of the stars in a cluster 
formed at the same time, 
comparing the snapshot that different 
clusters give us at different times since 
they were created allows us to compare 
what the status is of different stars. 
All the different stars in a cluster, 
when they are the same age and this gives 
us different snapshots of the population 
and this will be an important tool in 
understanding what we know about star 
evolution. 
So the plan for the week is start by 
talking about how stars form. 
We'll talk about the pre-main sequence 
aspect of solar evolution. 
We'll skip over the rather boring sedate 
main sequence and talk about all the 
exciting things that happen after the 
main sequence. 
We'll mention giants, 
and super giants, and dwarves, and 
remnants, and all kinds of exciting 
explosions. 
And, in the process we will have filled 
out the HR Diagram, explained all of the 
things that are not main sequence stars 
that we have found, and piece them 
together in an interesting pattern. 
We will discover many new phenomena and 
some new physics that will be required to 
understand them. 
More this week than in any previous week, 
we will be addressing questions to which 
the answers is not known not just to me 
but to mankind, 
and so, there will be a lot of well some 
people think this and some people think 
that. 
I think that's very exciting. 
we will be acquiring more steps on our 
construction of the Cosmic Distance 
Ladder:, ways to measure larger and 
larger distances to add more and more of 
the universe to our 3D picture of things 
to which we can determine the distance. 
We will not be discussing this week the 
end to, despite the fact that clusters 
play an important role in what we talk 
about, 
the dynamics of the formation of 
clusters, the molecular clouds the come 
from 
and the dynamics of how clusters evolve 
are going to be relegated to our study of 
galaxies, since those are structures 
above stars we will relegate them to 
galactic studies. Although as some of the 
aspects of the physics we study will be 
require relativistic physics and we will 
skip that because relativity will be the 
topic for next week and I hope you 
enjoyed this one.