For the first time, astronomers have spotted a rapidly spinning neutron star that is gravitationally bound to a helium star companion. The discovery of this unusual binary system helps confirm a long theorized—but rarely seen—cosmic process called common envelope evolution.

Binary star systems, or pairs of stars that orbit around each other, are very common. In fact, it’s estimated that 85% of stars in the universe have at least one companion. But this newly discovered pair is unlike any seen before. 

In this case, a helium star is bound to a millisecond pulsar: a fast-spinning neutron star that emits beams of radiation at regular intervals. These stars achieve their extraordinary rotation rates by siphoning matter from nearby stellar companions. 

In May 2020, a team of researchers led by Jin Lin Han, a radio astronomer at the National Astronomical Observatories and the Chinese Academy of Sciences in Beijing, used China’s FAST radio telescope to detect weak signals from a point deep within the Milky Way galaxy.  

A few months later, the researchers confirmed that these signals were radiation emissions from a pulsar. They then tracked these bursts for four and a half years, and their measurements revealed that this star isn’t alone. It’s actually part of a binary system, orbiting its companion every 3.6 hours. But for one-sixth of that orbit, the pulsar’s radiation is blocked—or eclipsed—by its companion.

“That’s a large part of the orbit,” Han told Gizmodo. “That’s strange, only a larger companion can do this.”

In binary systems, a millisecond pulsar is usually accompanied by a white dwarf: a hot, dense core left behind after a star like our Sun has exhausted its fuel. But the data Han and his colleagues collected indicated that this companion had to lie somewhere in between a compact object and a normal star, he said. 

Further investigation of this strange companion revealed that it is roughly as massive as our Sun, but it couldn’t be a normal star because it was undetectable in all wavelengths outside of the radio spectrum. This led the researchers to conclude that it’s a star stripped clean of its hydrogen, leaving behind a core primarily composed of helium. They published their findings today in the journal Science.

This type of binary system “has never been discovered before,” Han said. But it has long been theorized that such a pairing could form via common envelope evolution, and he and his colleagues believe that’s what happened here. 

“The process of common envelope evolution is slightly different to how stars like pulsars are often thought to interact in binary systems,” Duncan Lorimer, a professor of physics and astronomy at West Virginia University who was not involved in the study, told Gizmodo in an email.

Normally, a neutron star’s intense gravitational field pulls material from a companion star that has expanded, allowing its gaseous outer layers to be “eaten” by the neutron star, he explained. This process, called accretion, causes the neutron star to “spin up” and become a pulsar. 

But in common envelope evolution, “the companion star is so large that its outer layers engulf the neutron star as well,” Lorimer said. “This acts as a brake on the whole binary system.”

Inside the companion star’s outer layers—the envelope—friction causes the pulsar and the companion’s core to spiral toward each other, forming a highly compact binary system, like the one Han and his colleagues have now observed. With an orbital period of just 3.6 hours, this pulsar and its companion are circling each other very closely. 

Ultimately, the outer layers of the companion star are expelled, Lorimer said, which explains why this millisecond pulsar’s helium star companion has been stripped. 

“The evolutionary pathway that the authors set out, it’s not a surprising pathway,” Victoria Kaspi, a professor of physics at McGill University who was not involved in the study, told Gizmodo. “It’s one that has been recognized, identified, discussed in detail for many years.”

“The interesting question is, if you’re going to find 1,000 millisecond pulsars, what fraction of them will be like this one? It’s about one in 1,000—something like that. And they found it,” she said. 

Han and his colleagues believe there are more than a dozen other systems like this one in our galaxy, making them exceptionally rare. The fact that these researchers found one of them is a “great breakthrough,” Lorimer said.

“The more millisecond pulsars we find, the more likely we are to find examples of rare evolutionary outcomes. This system is an excellent example of that,” he said.