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Most stars in our universe come in pairs. While our own sun is a loner, many stars like our sun orbit similar stars, while countless other exotic star-globular pairs fill the universe. Black holes, for example, often orbit each other. One pairing that turns out 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 Karim El-Badri 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. By themselves, they are extremely weak and usually cannot be detected directly. But when a neutron star orbits a sun-like star, it pulls on its companion, causing the star to move 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 fluctuations of more than a billion stars, so the chances are good to find even very rare objects,” said El-Badri, assistant professor of astronomy at Caltech and assistant scientist at the Max Planck Institute for Astronomy in Germany .

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

While neutron stars have previously been discovered orbiting stars like our sun, these systems have all been more compact. With a small distance separating the two bodies, 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 brightly in X-rays or radio waves. In contrast, the neutron stars in the new study are much further 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 corpses are too far away from their partners to steal material from them. Instead, they are quiet and dark. “These are the first neutron stars detected solely because of their gravitational effects,” says El-Badri.

The discovery comes as somewhat of a surprise because it is not clear how an exploded star ends up in a star like our sun.

“We still don’t have a complete model of how these binaries are formed,” explains El-Badri. “In principle, the progenitor of the neutron star must have become massive and interacted with the solar-type star during its late evolution.” The massive star would have knocked the small star down, possibly temporarily engulfing it. The neutron star progenitor would later explode in a supernova, which the models say should have released the binary systems, sending the neutron stars and sun-like stars hurtling in opposite directions.

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

Gaia was able to find the unlikely moons 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 oscillation will be too small to detect,” says El-Badri. “With Gaia, we’re more sensitive to wider orbits.” Gaia is also most sensitive to binaries that are relatively nearby. Most of the newly discovered systems are within 3,000 light-years of Earth—a relatively short distance compared, for example, to the 100,000 light-year diameter of the Milky Way galaxy.

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

El-Badri is also interested in finding invisible dormant black holes orbiting sun-like stars. Using data from Gaia, he has discovered two of these silent black holes hidden in our galaxy. One, called Gaia BH1, is the closest known black hole to Earth at 1,600 light years.

“We also don’t know for sure how these black hole binaries formed,” says El-Badri. “There are clearly gaps in our models of 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.”