Astronomers discover what could be 21 neutron stars orbiting sun-like stars

Most stars in our universe come in pairs. While our sun itself is lonely, many stars like our sun orbit similar stars, while a host of other exotic pairings between stars and cosmic orbs make up the universe. Black holes, for example, are often found orbiting each other. One pairing that has proven to be quite rare is that between a sun-like star and a type of dead star called a neutron star.

Now, 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. On their own, they are extremely faint and usually cannot be detected directly. But as a neutron star orbits a sun-like star, it pulls on its companion, causing the star to shift back and forth across the sky. Using the European Space Agency’s Gaia mission, astronomers were able to capture these telltale oscillations to reveal a new population of dark neutron stars.

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

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

While neutron stars have previously been discovered orbiting stars like our sun, those systems have all been more compact. With little distance separating the two bodies, a neutron star (which is more massive than a sun-like star) can steal mass from its partner. This mass transfer process causes the neutron star to glow brightly at X-ray or radio wavelengths. In contrast, the neutron stars in the new study are much farther from their partners—on the order of one to three times the distance between Earth and the Sun.

This means that the star’s young corpses are too far away from their partners to steal material from them. They are in a quiet and dark place. “These are the first neutron stars discovered purely because of their gravitational effects,” says El-Badry.

The discovery comes as somewhat of a surprise, because it’s not clear how an exploding star wraps itself around a star like our sun.

“We still don’t have a complete model of how these binaries form,” explains El-Badry. “In principle, the progenitor of the neutron star should have become massive and interacted with the solar-type star during its late-stage evolution.” The large star would have knocked the small star around, likely engulfing it temporarily. Later, the neutron star’s progenitor would have exploded in a supernova, which, according to models, should have unbound the binary systems, sending the neutron star and the sun-like star hurtling in opposite directions.

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

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

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

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

El-Badry also has an interest in finding unseen dormant black holes orbiting sun-like stars. Using Gaia data, he has found two of these quiescent black holes hidden in our galaxy. One, called Gaia BH1, is the closest known black hole to Earth at 1,600 light-years away.

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