Understanding diagrams of the late stages of stellar evolution

[rnielsen25]

TL;DR Summary

I'm having trouble understanding the two diagrams describing the composition of stars at the late stages of stellar evolution. I hope some of you can help me understand and read the diagrams.

Hi, I have an upcoming exam in astrophysics. We have been told to make sure we study all figures and tables, s.t. we are able to explain them. However, I ran into these figures below, in my lecturer's PowerPoint and I do not know where they originate and aren't presented in my book. I'm having some trouble understanding the two figures below, and I was hoping you could help me understand what information they contain.

The first figure is this one:

As the title indicates, this figure represents a 25-solar-mass star. I believe it describes the shell structure of the star at the end of its evolution (because of the iron core). However, I'm a little unsure how to read this graph exactly. Especially, I'm having trouble understanding what the y-axis describes? So the x-axis indicates the inner mass m(r) from zero to about 8 solar masses. I believe this makes me able to tell that the inner core from 0-2 solar masses contains iron, the next part gets a little messy. Does it describe that the next layer is a mixture of Ca, Ar, S, and Si and then the y-axis represents the fraction of these elements at the given layer?The next figure is this table:

So this figure represents the two stars, one of 15 solar masses and the other 25 solar masses.
I believe tau(years) is the time it takes the fusion processes to exhaust C, Ne, O and Si, respectively. Then $L_{\nu}/L$ must represent the amount of luminosity (energy) released from neutrinos compared to the normal luminosity.
So it tells us that a substantial amount of the energy released from the star is from neutrinos in the later stages of the stellar evolution?
Why is this important?

 

[Bandersnatch]

Does it describe that the next layer is a mixture of Ca, Ar, S, and Si and then the y-axis represents the fraction of these elements at the given layer?

Yes, the y-axis shows fractional composition on a logarithmic scale. Not of the whole star, but of the core (you can see it's a star of 25 solar masses, but the x-axis goes to maybe 8.5 - the region above composed of unburned hydrogen and helium is excluded).
The same graph is adapted in D.Prialnik's textbook (An introduction to the theory of stellar structure and evolution; fig. 9.18), with attribution to: The physics of supernova explosions S. E. Woosley, T. A. Weaver 1986, ann. rev. astron. astrophys., 24. The paper is not openly accessible, but your university library likely provides access, should you want to take a closer look.

So, e.g. going from the right side leftwards, you can see that there's a thin (by mass, not necessarily in terms of the radius) layer of mostly helium and hydrogen, which then turns into pure helium layer: H fraction goes to zero on the graph, and He jumps to the very top; probably represents the lower edge of the hydrogen burning shell.
Next, He fraction goes down maybe 10%, to make room for C, Ne, and O. As you enter the helium burning layer the fuel is being depleted and the burning products accumulate in the layer below. And so on until the 100% iron core.I can't really help you with the second one. Your initial analysis seems correct, though.