Fusion occurs via the proton-proton chain (see the ancient animations as hydrogen turns into helium in the core.

This can be schematically shown as:

When the core becomes pure helium, the temperature will be too low for helium fusion to occur. So, core energy generation stops and now G > P and the core begins to collapse and HEAT . This transition looks something like this:

Now something complicated happens: As the core collapses and heats, some of this heat is transferred to a an annular region around the core which begins to fuse. This is known as hydrogen shell burning . As the core continues to collapse the shell gets hotter and hotter and, in fact, exceeds the previous temperature that held for the hydrogen burning core.

Thus the hydrogen burning rate in the shell continues to increase and the star outputs more energy (its luminosity increases). The shell burning source also provides additional Pressure which causes the outer part of the star to expand and cool. The star is not on the Red Giant Branch its luminosity is increasing and its surface temperature is decreasing.

The structure of the star looks something like this:

Eventually the core will become hot enough for the fusion of helium into carbon via this process:

The star is once again stable. It has core energy generation and P=G; The star is now a Red Giant and its radius is 100 -- 1000 times larger than when it was a main sequence star.

As the helium core turns to carbon, the star once again finds itself in a state where the core temperature is too cool to fuse carbon. The core then collapses and heats.

This heat is again transferred to shells around the star but in this case there is a double shell source of both hydrogen and helium burning. The resulting pressure is so great that the outer layers of the star are simply driven away exposing the hot (carbon core). This stage of stellar evolution is very fast (a few hundred thousand years) and the star is said to be a Planetary Nebula, although this has nothing to do with Planets!. An image of a planetary nebula is shown here:

The blue star in the center of the nebula is the hot carbon core. The green ring is the now ionized outer layers of the star being blown away from the core.

For low mass stars, the core will never be able to fuse carbon and it will just continue to shrink until it is stablized by electron degeneracy.

Electron degeneracy is a state of matter that occurs at very high densities. Their is an exclusion principle that says you can't put electrons arbitrarily close together. They will start to move faster to avoid this. Moving faster is equivalent to higher temperature and higher pressure. Thus the pressure to stabilize the remnant core is provided not by fusion reactions but by degeneracy. The star is now a white dwarf and is no longer generating energy.

This entire Sequence can be graphically summarized like the (poor) figure below:

In Class Exercise:

Go here to start: HR Diagram Simulator (make sure this fills your screen)

1. Hit the 100 button twice to add 200 stars to the main sequence; Clicking on any of the stars will bring up information on that star.

   Click on any point (star) in the HR diagram to bring up information for that star. using this table record the mass of the star, the luminosity and the main sequence lifetime for stars of mass 0.5, 1.0, 2.0, 4.0, 8.0, 10.0 and 12 solar masses.

   note that the units of luminosity are in scientific notation. So, for instance, a star of mass approximately 4 solar masses will have a luminosity of approximately 2.5 x 10² which is 250. Hence, please publish this value as 250 not 2.5 x 10². Lifetimes are in units of millions of years and you can just publish your results in those units.

   Publish to global view when your done.

   Now using the published data, let's examine the following questions:

   What is the ratio of lifetimes between a 10 solar mass star and a 1 solar mass star? What about between 2 solar masses and 1 solar mass? What about between 1 solar mass and 0.5 solar masses?

   What is the ratio of luminosities of a 10 solar mass star and a 1 solar mass star? What about between 2 solar masses and 1 solar mass? What about between 1 solar mass and 0.5 solar masses?

2. Now hit the step button a few times. Notice in the lower left hand corner an age readout. Hit the step button or the evolve/stop button until the following ages are reached:
   10 million years
   100 million years
   1 billion years
   2 billion years
   5 billion years
   10 billion years
   20 billion years
   

   Use this table to record the time step and the mass of the most massive star that is still on the main sequence?

   Publish to global view when your done.