Using Stellar Evolution to Generate Music Score: Part 1


As part of our upcoming performance Our Star Will Die Alone, we wanted to generate a music score inspired by the actual evolution of a star as it evolves off of the main sequence – as it “dies” so to speak.  To generate this, we used an open source software package called MESA (Modules for Experiments in Stellar Astrophysics) developed by a team of astronomers and utilized by an extensive community of researchers, teachers and students to study how a star evolves with time (the code will be used in Adam’s undergraduate and graduate stellar astrophysics courses at UCSD this year).

To examine the evolution of our star, we upped the mass of our model to 3 times the mass of the Sun for faster evolution and a more extensive post-main sequence track.  We then ran the code through 40,000 “calculation frames” (taking about 2 days on a little MAC laptop) to cover the full evolution, up to the point where the star has its last “gasp”, ejecting most of itself into the cosmos (to see an example of this happening right now, check out the star V838 Monocerotis).  The movies below show the outcome of these calculations, which Michael will be using to produce the score.  As you watch these, keep in mind that the video time step is based on the calculated frames and does not correspond to “real time” (the calculation requires more steps when things start to get interesting):

This movie displays the interior properties of the star over time. The large panel on the right displays the interior temperature-density profile of the star as it evolves, with line coloring indicating both energy transport and generation. The top left panel shows a classic HR diagram with luminosity versus effective temperature. The bottom right panel shows the evolution of core temperature and density.

The first few frames shows the star collapsing under its own weight, then hydrogen fusion turns on within the first 1 second of the movie.  3 seconds in, the star has fused all of the hydrogen in its center and is starting to fuse helium. 5 seconds into the movie it’s fused all the helium in its center, and now starts a internal dance between hydrogen and helium fusion in internal shells, a very unstable situation that  leads to the “wiggles” as one shell pushes against the other – these are the thermal pulses that happen at the very last stages of a stars life.  By 1:03, the star stalls out – it’s effectively dead (although calculations need to be continued beyond this to map the star to its core white dwarf stage).

This movie displays the chemical abundances (colored lines) through the interior of the star.  The center of the star is on the left, the surface is on the right. We start with a pretty uniform star, but pretty quickly in the movie both hydrogen (2 seconds) and then helium (5 seconds) are lost from the core as they fuse to heavier elements such as carbon and oxygen. The thermal pulses described above show up to mix elements between the various layers.

This movie displays the surface properties of the star – what we can measure from our outside perspective.  The top panel shows surface density and radius; second panel shows surface pressure and gravity; third panel shows luminosites of various energy sources and effective (surface) temperature; bottom panel shows the total mass and mass loss rate.  Things to notice: the very early minimum in surface density and pressure, and maximum in effective temperature around model step 1000 – that’s when the star is on the main sequence, happily fusing hydrogen as the Sun is now; the jagged pulses after model step 4000 are the unstable thermal pulses described above; and the drop off in mass toward the end of the simulation, which levels off right at the end – a sure sign of stellar death.

This movie displays the energy generation and transport in the interior the star. The center of the star is at the bottom, the surface is at the top, and time moves to the right. Regions that carry energy by convection (like boiling water) are indicated in grey; radiative regions are uncolored. Notice the “black bump” around model step 1000 (catch it in the first second!) – this is when the star is on the main sequence. Most other times the star is almost entirely convective outside its core, the result of a low temperature atmosphere and lots of heat from hydrogen and helium fusion trying to get out.  The thermal pulses show up again – the colored lines show energy from hydrogen and helium fusion going up and down as the star expands and contracts in this unstable phase.


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