Black Widow Pulsars

Black widows are pulsars (rapidly spinning magnetized neutron stars) that irradiate and gradually devour a companion star on a Keplerian orbit by overcoming its internal gravity.

The companion begins its life as a regular hydrogen-burning main-sequence star (similar to the Sun), and over the course of billions of years it is reduced to only a few percent of its original mass.

If the companion star has already fused some of its hydrogen into helium before being devoured by the pulsar, it may evolve into either very short or very long orbital periods, depending on the helium concentration.

See Conrad-Burton, Shabi & Ginzburg (2023) for details.

Evolutionary tracks of black widow companions that gradually lose their mass m. The tracks bifurcate as a function of the amount of helium fused by nuclear reactions, explaining both the shortest and longest observed orbital periods.