Astronomers Discover What May Be 21 Neutron Stars Orbiting Sun-Like Stars

Most stars in our universe come in pairs. Our Sun is a solitary star, but many stars like ours orbit similar stars, while a multitude of other exotic pairs of stars and cosmic objects dot the universe. Black holes, for example, often orbit each other. One pair that has proven quite rare is that between a Sun-like star and a type of dead star called a neutron star.

Astronomers led by Caltech’s Kareem El-Badry have discovered what appear to be 21 neutron stars orbiting stars like our Sun. Neutron stars are the dense, burned-out cores of massive stars that have exploded. In isolation, they are extremely faint and usually cannot be detected directly. But when a neutron star orbits a Sun-like star, it tugs on its companion, causing the star to shift back and forth across the sky. Thanks to the European Space Agency’s Gaia mission, astronomers were able to detect these telltale wobbles and reveal a new population of dim neutron stars.

“Gaia is constantly scanning the sky and measuring the wobbles of more than a billion stars, so the chances are good of finding even very rare objects,” says El-Badry, an assistant professor of astronomy at Caltech and an adjunct scientist at the Max Planck Institute for Astronomy in Germany.

The new study, which includes a team of co-authors from around the world, was published in The Open Journal for AstrophysicsData from several ground-based telescopes, including the W.M. Keck Observatory on Maunakea, Hawaii, the La Silla Observatory in Chile, and the Whipple Observatory in Arizona, were used to follow up on Gaia’s observations and learn more about the masses and orbits of hidden neutron stars.

Although neutron stars have been detected orbiting stars like our Sun before, those systems were all more compact. Because the distance between the two bodies is small, a neutron star (which is heavier than a Sun-like star) can steal mass from its partner. This mass transfer process causes the neutron star to glow in X-ray or radio wavelengths. In contrast, the neutron stars in the new study are much farther away from their partners, on the order of one to three times the distance between Earth and the Sun.

This means that the newly discovered stellar bodies are too far away from their partners to steal matter from them. Instead, they are quiet and dark. “These are the first neutron stars discovered solely through their gravitational effects,” El-Badry explains.

This discovery is somewhat surprising, because it is not known exactly how an exploded star ends up next to a star like our Sun.

“We don’t have a complete model yet for how these binaries formed,” El-Badry says. “In principle, the neutron star’s progenitor star would have become enormous and interacted with the sun-like star during its late evolution.” The giant star would then have jostled the smaller star, probably temporarily engulfing it. Later, the neutron star’s progenitor star would have exploded in a supernova, which, according to the models, would have broken the binary systems apart, sending the neutron stars and sun-like stars in opposite directions.

“The discovery of these new systems shows that at least some binaries survive these cataclysmic processes, even if models cannot yet fully explain how,” he says.

Gaia was able to find these unlikely companions because of their wide orbits and long periods (Sun-like stars orbit neutron stars with periods of six months to three years).

“If the bodies are too close, the wobble will be too small to detect,” El-Badry says. “With Gaia, we are more sensitive to wider orbits.” Gaia is also more sensitive to relatively close binaries. Most of the newly discovered systems are within 3,000 light-years of Earth, a relatively small distance compared to, say, the Milky Way’s 100,000 light-year diameter.

The new observations also suggest how rare these pairs are. “We estimate that about one in a million solar-type stars orbits a neutron star in a wide orbit,” he notes.

El-Badry is also interested in searching for invisible dormant black holes orbiting Sun-like stars. Using Gaia data, he discovered two such silent black holes hidden in our galaxy. One of them, called Gaia BH1, is the closest black hole to Earth, located 1,600 light-years away.

“We also don’t know for sure how these black hole binaries formed,” El-Badry says. “There are clearly gaps in our models of binary star evolution. Finding more of these dark companions and comparing their population statistics to predictions from different models will help us understand how they form.”