The Stellar Evolution Construction Site
Note: This exercise is for advanced students.

Before trying this, you might want to have a look at the Zero Age Main Sequence model building page to see a little of how stellar models are constructed.

The form below will use a series of programs collectively known as "Starcode", written by Steve Ratcliff, to generate a "Zero Age Main Sequence" (ZAMS) stellar model. Such a model is a reasonably appropriate snapshot of the interior state of a star shortly after its formation, i.e., before it has had a chance to substantially alter its composition (and therefore structure) through nucleosynthesis.

The basic procedure consists of the same two steps as are done for the construction of a zero-age main sequence model:
1) Generating an opacity table for a specific composition.
2) Compute a ZAMS model which satisfies the equations of stellar structure.

But now there is also a third step where the ZAMS model is evolved by considering tiny changes in the interior composition (due to nucleosynthesis) and their effect on the structure of the star. This is of course why stars evolve in the first place - as the composition changes, so does the equation of state (the relation between pressure, temperature, and density). This forces the structure to change in order to maintain hydrostatic equilibrium. An excellent example of this is the increasing temperature of a main sequence star as H is converted in He in its core (you can look for this yourself using the softwate here).

To track the changes the idea is:
1) compute a ZAMS model with some initial composition throughout.
2) The model tells you what the temperatures, pressures, and densities are throughout the star. In doing this, the rates of nuclear reactions are also calculated, which tells you how fast the composition is changing. And if you know how fast the composition is changing, you can figure out what it will be some time later.
3) Use the reaction rates to predict the composition some time later and use this composition to generate a new model.
4) Return to step 2 and repeat.

The trick is of course to take time steps which are short enough that the changes between steps are small, but not so small that you spend the rest of your life wating for the results! How to actually do this is probably more than we have room for here, but it does lead to one of the limitations of this page - you cannot get to the helium core flash.

Output:

When you run the form below (which will take a few minutes), you'll eventually be presented with a large table of numbers.

The first three lines are basically a header to remind you of your choice of parameters:

The original grid of envelopes:
1.00000 0.05000 0.73000 0.03000 1.60000-0.310 3.650 0.050 0.010 40 40 UNIX results of model computations for M/MSUN = 1.000 X = 0.73000 Z = 0.03000 ALPHA = 1.600

Model NH TIME(Gyr) lg Teff log L lg Tc lg RHc XYc MXY0 Mconv MOBC DXM/L %HeIII L.fun lg DT9 DFLPT M(NG4)

This is followed by a column heading and then summaries of the model characteristics as they evolve. You'll probably be most interested in the first 8 columns of the table:

1. Model number
2. Number of depth points used in the model
3. Age (in billions of years)
4. The log of the star's effective temperature
5. The log of the star's luminosity
6. The log of the star's central temperature
7. The log of the star's central density
8. The abundance of H the star's core

Note:

• You can have intermediate models,complete with their structures also printed you by checking the appropriate box.
• The software is limited in that it cannot evolve stars beyond the He flash.

To compute a model & evolve it, you must supply the following information: