This story is the delightful tale of two stars, likely born together, that are entering their final stages, side-by-side while enjoying a not entirely healthy companionship. Discovered as a bright, mysterious gamma-ray source by the Fermi Space Telescope, this system, cataloged 4FGL J1120.0-2204, was observed with the SOAR Telescope. 

In visual light, observers found two interacting stars: a very young pulsar and an almost white dwarf. The pulsar would have formed through the death of a massive star that went supernova, and the soon-to-be white dwarf formed from a star smaller than the Sun. These two sizes of star – massive and smaller than the Sun – shouldn’t be dying at the same time, but the smaller star appears aided by the neutron star pulling off its atmosphere. With customary dark humor, astronomers refer to the neutron star that is eating its companion as a spider.

What got me about this story was the time scales. In textbooks, we simply say “Sun-like stars will puff off their outer atmospheres to form a  planetary nebula as their core collapses into a WD.” Lead astronomer Samuel Swilhart explains: Currently, [the smaller star is] bloated and is about five times larger in radius than normal white dwarfs with similar masses. It will continue cooling and contracting and, in about two billion years, it will look identical to many of the extremely low mass white dwarfs that we already know about.

That “and it collapses into a white dwarf” takes more than two billion years.

The time scales of our universe are long, and every white dwarf we see represents billions of years of evolution, even when that evolved star got helped along by a hungry spider of a neutron star.

More Information

NOIRLab press release

“4FGL J1120.0-2204: A Unique Gamma-ray Bright Neutron Star Binary with an Extremely Low Mass Proto-White Dwarf,” Samuel J. Swihart et al., to be published in The Astrophysical Journal (preprint on arxiv.org)