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A large team of astronomers and astrophysicists affiliated with several institutions in China has discovered a binary star system, where one of the stars is a millisecond pulsar and the other is made mostly of helium. In their paper in the journal Science, the group describes how they discovered that a pulsar under study since 2020 had a companion star—one that was gravitationally bound to it.

Researchers on the team first spotted the pulsar back in May of 2020, and soon thereafter noticed that not only did it spin incredibly fast, but for one-sixth of its orbit, its radiation emissions were blocked. That suggested an object was passing between it and Earth. Over the next four years, the team studied the apparent binary system to learn more about its characteristics and confirm that there truly was a second star.

Pulsars are a type of neutron star that emit beams of radiation from their poles. They appear to pulse as viewed from Earth due to their spinning—the radiation signal can only be seen when one of the poles is pointed directly at the Earth.

In this new effort, the pulsar was found to be spinning so fast that it qualified as a millisecond pulsar, which means it rotates on its axis at least a hundred times a second. Even at that speed, the researchers were able to detect gaps in its pulse, suggesting something was situated between the pulsar and Earth.

In studying data from the Five-hundred-meter Aperture Spherical radio Telescope, the research team was able to see that the cause of the gap was another star—one approximately 1 to 1.6 times the mass of the sun and made mostly of helium.

The finding may mark the first observation of a pulsar that is gravitationally bound to a helium companion star. The researchers suggest the companion star once held a lot of hydrogen, but it was stripped away by its pulsar companion, which suggests they have a common envelope.

That common envelope, the team notes, explains why the two stars are so close together, approximately 50 times closer than Mercury is to the sun. That closeness means they have short orbital paths, resulting in them circling one another every 3.6 hours.

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