# Chemical and stellar evolution in stars

I'm having an exam soon where I have to discuss/describe the follow figure:

It's the first figure I have to explain, and it was on the same slide as the one below, so I'm thinking they are somewhat similar. But I can't seem to figure out what's going on in that figure ? "25 solmasser" is "25 sunmasses" if you were wondering. I'm pretty sure it tells me something about the time and evolution of each burning phase in a large mass star, i.e. hydrogen-burning, helium-burning, carbon-burning, neon-burning, oxygen-burning and silicon-burning. But it makes no sense (The first figure), and it's just on a slide with no text or explanation, so I'm not quite sure how to interpret it. So I was hoping maybe someone could give me some pointers?

I think the colloquial term for that type of plot is "spaghetti diagram" because you have a bunch of lines running across it. It's really the mass fraction as a function of interior mass. From our stellar structure equations, we have that $$\frac{dm}{dr}=4\pi r^2\rho,$$ which is derived from the mass-continuity equation, so you can relate the radius, $r$ (in solar radius units) to the mass (given in solar masses). This type of Lagrange coordinate is the standard for these plots though.
Basically, the plot shows the dominant element(s) as you go deeper into the star. The closer to 1.0 it is, the more you're going to find that element. The sharp peaks/drops arise from nuclear burning--fortunately for you, those are labeled at the bottom, this is not often the case! Note that towards the right side you see ${}^1{\rm H}$ starting to peak while ${}^4{\rm He}$ starting to fall, this is where hydrogen burning takes place; this also indicates that your plot extends much further than what is shown there (which makes sense, given that your plot extends to a little over 8 solar masses for a 25 solar mass star!).
• Thanks for the descriptive answer. I'm not quite sure about the bottom labels (lg($T$) and lg($\rho$)). What to those numbers represent ? And I'm not sure I fully understand the y-axis, even though you say it's the mass-fraction. Mass-fraction of what ? It doesn't seem to be the same as in figure 2, or am I mistaken ? – Denver Dang Dec 20 '14 at 19:48