The Mysteries and Inner Workings of Massive Stars

Hands-on illustration of mixing in stellar interiors at the KITP residence. From left to right: Thomas Rivinius (ESO), Stan Owocki (Delaware), Natasha Ivanova (Alberta), Rich Townsend (Wisconsin), Ed van den Heuvel (Amsterdam).

On September 14, 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected the mergers of two black holes in a distant galaxy, ushering in the era of gravitational-wave astrophysics. Since then, two additional mergers of binaries containing pairs of black holes have been observed by LIGO. These events, and the surprisingly high mass of the black holes (more than 30 times the mass of the Sun in one case) has opened up a large range of questions that were addressed over the 11 weeks of the “The Mysteries and Inner Workings of Massive Stars” program in Spring 2017.

Gravitational waves are only efficient in driving black hole binaries to merger if the black holes are very close, a fraction of the distance from the Earth to the Sun. One scenario for reaching such high black hole masses in merging binaries relies on keeping the radii of the progenitor stars small, allowing the stars to avoid mass transfer in a close binary. This could be achieved by mixing the star while it is fusing hydrogen into helium. Efficient mixing powered by stellar rotation can bring nuclear fusion products out of the cores of stars to their surface, and fresh fuel into the cores; this allows for the formation of these black holes without the progenitor star becoming very large. Meanwhile, the efficient transport of angular momentum while the star is evolving, together with the details of the final collapse to the black hole will determine the spin (or rotation rate) of the black hole that remains. The masses and spins are being measured by gravitational-wave observations, which can therefore inform our understanding of the progenitor channels and evolutionary physics. New papers on this topic inspired by the KITP program have already begun to appear, and many more are in the works.

Improving understanding of stars by getting closer to them in the Los Padres mountains. From left to right: Selma de Mink (Amsterdam), Chris Belczynski (Warsaw), Thierry Foglizzo (CEA), Mathieu Renzo (Amsterdam).

Other important issues discussed during the program, both from a modeling perspective and using the latest observational constraints, included stellar winds and the associated mass loss rates, which again determine the mass of the black hole left behind at the end of the star’s life. Mass transfer between massive stars in binaries was another area of active investigation: how much of the gas lost by one companion ends up on the other, and when mass transfer may become dynamically unstable (the so-called ‘common envelope’ phase), are key factors determining the outcomes of isolated binary evolution.

Of course, the observational signatures and impact of massive stars are not limited to gravitational-wave sources. We discussed the modeling and observations of topics as diverse as supernovae, X-ray binaries, and asteroseismology. The program benefitted from a very stimulating conference on "Phenomena, Physics, and Puzzles Of Massive Stars and their Explosive Outcomes,” as well as a special short program for High School physics teachers on “Exploring the universe with LIGO.” The vibrant discussions, freely shared ideas, and new collaborations formed at KITP will continue to drive development in this field for years to come.

Ilya Mandel is a Professor of Theoretical Astrophysics at the University of Birmingham, UK, and was in residence at KITP for this program.
KITP Newsletter, Fall 2017