MESA: Virtual Stars for the People

by Richard Townsend | Associate Professor, UW-Madison
Posted Mar 02, 2018

How might we perform an experiment on a star — say, alter its composition to see how its size, brightness and color will change? On the face of it, this proposition seems impossible; stars are too distant for us to visit (with the possible exception of our own Sun), and in any case so immense that we lack the ability to make any significant changes to them. However, with the advent of computers it became possible to construct ‘virtual stars’ via programs that simulate the internal structure and time evolution of real stars.

For a long time, these programs were closely guarded secrets, available only to the few select research groups that had invested the significant time, effort and expertise required to create them.  However, in recent years the status quo has been up-ended with the advent of MESA (Modules for Experiments in Stellar Astrophysics).  MESA differs from existing proprietary programs in a number of notable ways.  It is freely available to anyone and everyone with no strings attached, running on Linux, Mac OS and Windows operating systems (visit if you want to try it yourself!).  Its source code is open and modular, so that users can poke around in its internals to see exactly how it works, and integrate pieces of functionality into their own software. And, it is extremely general — as well as run-of-the-mill stars like the Sun, MESA can construct exotic objects such as binary stars, white dwarfs, neutron stars and gas-giant planets; it can simulate dramatic evolutionary phases like novae and supernovae; and it can be used to analyze observations of seismic activity (star quakes) in real stars, giving us unique insights into the stars’ internal structure in a manner analogous to the way geologists study terrestrial quakes to probe the Earth’s interior.

Rich Townsend, Associate Professor in the UW-Madison Department of Astronomy, became involved in the MESA project soon after its creation in 2011 by software-developer-turned-astrophysicist Dr. Bill Paxton (University of California-Santa Barbara). Since then, he has been one of the leading contributors toward the project’s continued growth. Recently, in collaboration with Prof. Lars Bildsten (UC-Santa Barbara) and Prof. Frank Timmes (Arizona State University), Rich secured a $2M grant from the National Science Foundation’s Office of Advanced Cyberinfrastructure.  Approximately one-third of this award is earmarked for UW-Madison, to support undergraduates, graduate students and postdoctoral researchers in their ongoing involvement in the MESA project.

In February 2018, the fourth in a series of ‘instrument papers’ — describing MESA’s new features and demonstrating its application to key problems in stellar astrophysics — was published in the Astrophysical Journal Supplement Series (Paxton et al. 2018, ApJS, 234, 34).  Rich’s primary contribution, made in close collaboration with Dr. Anne Thoul (Université de Liège), was a significantly improved treatment of mixing by convection, which is already proving to have far-reaching implications for our understanding of how stars evolve. In previous instrument papers, Rich led the development of MESA’s seismic analysis capabilities, drawing on his two-decades’ expertise in studying star quakes.

MESA continues to go from strength to strength. There are currently over 850 users globally — professors, researchers, students and even amateur astronomers — distributed across many institutions and counties (see map above).  These users comprise a vibrant, diverse and still-growing community that asks and answers questions through a centralized MESA Users mailing list (with approximately 100 messages a month); gains hands-on experience with MESA in annual summer schools held at UC-Santa Barbara (taught by leading scientists from around the world); shares supporting tools, tutorials and input files from published research papers via a curated marketplace; and deploys MESA on in the classroom to teach stellar astrophysics to undergraduates and graduate students.

Taken together, the four MESA instrument papers have already garnered over 1,700 citations in the scientific literature, with approximately 50 new citations being added each month; these numbers underscore MESA’s success in driving new research into stars. But the project is also bringing a less-tangible but equally important benefit: MESA is challenging historical norms of closed/proprietary science, by demonstrating how an open source and open knowledge ethos makes winners of us all. This is the clearest message that MESA is sending to the global science community, and — from a Badger perspective — it is a message that resonates well with the Wisconsin Idea writ large.


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