One Star Is Orbiting Inside Another In This Never-Before-Seen Binary System

Source: Found here

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.

"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 .