Chinese astronomers have found a rare evolving triple-star system involving a pulsar, using the Five-hundred-meter Aperture Spherical Radio Telescope (FAST). 

The system, called J0435+3233, gives astronomers a natural lab to study how stars evolve and pull on each other, and test fundamental theories of gravity, including the strong equivalence principle. Its discovery also shows the value of combining observations across different wavelengths. The findings were published in The Astrophysical Journal Letters on Friday.

Scientists believe all three stars formed from the same cloud of gas and dust. The heaviest exploded and became the pulsar. The second evolved into a white dwarf. The lightest survived both stages and is still evolving. Eventually, it too will become a white dwarf, leaving a stable system with one pulsar and two white dwarfs.

A pulsar is a fast-spinning neutron star that fires beams of radio waves. Analyzing five years of FAST observations, scientists found this one spins once every 3.2 milliseconds and orbits a white dwarf – the dense core of a dead star – once every eight days.

Researchers combined the radio data with publicly available infrared, optical and gamma-ray data to identify a third star, with a mass similar to the Sun's. It orbits the inner pulsar-white dwarf binary once every 73.5 years on an elongated path.

The third star also explains a strange FAST reading: the pulsar's spin period is increasing by about one nanosecond a year, roughly 100 times faster than comparable pulsars. The pull of the distant companion is behind this change.