Chinese astronomers have identified an exceptionally rare three-star system that offers new opportunities to test fundamental theories of gravity. Using data from the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in Guizhou Province, the team discovered a system located approximately 7,000 light years from Earth, consisting of two dead stars—a pulsar and a white dwarf—and a living, sun-like star.
The pulsar, a dense neutron star spinning over 300 times per second, orbits the white dwarf, the cooled core of a formerly smaller star, with an orbital period of about eight days. The third star, similar in size and characteristics to the sun, orbits this compact pair at a much greater distance, completing a revolution every 73.5 years.
Reported this month in The Astrophysical Journal Letters, the system is only the second of its kind ever identified. The first, discovered by a US-led team in 2014, consisted of three dead stars. The new discovery is notable for its stability despite the extreme conditions surrounding the pulsar and its companion. Researchers led by Han Jinlin at the National Astronomical Observatories in Beijing highlighted the system’s uniqueness and its potential to deepen understanding of how triple-star systems evolve, a process that remains poorly understood.
Triple-star systems occur with some frequency in the galaxy, but those containing a millisecond pulsar—a neutron star formed from a supernova explosion—are extremely rare. The violent birth of pulsars typically disrupts their stellar neighborhoods, making stable triple arrangements difficult to maintain. This system’s enduring gravitational balance has also allowed astronomers to consider its role as a laboratory for testing Einstein’s strong equivalence principle, which posits that gravitational acceleration is uniform regardless of an object's composition or size.
One intriguing aspect of the system relates to the pulsar’s unexpected spin behavior. Initially detected by FAST in 2020, the pulsar appeared to be slowing its rotation at a rate about 100 times greater than similar pulsars. Researchers from the Xinjiang Astronomical Observatory, after five years of observation, suggested this anomaly challenged existing formation theories and hinted at other influences within the system.
Subsequent analysis combining FAST’s data with other telescopes, including 18 years of gamma-ray observations from NASA’s Fermi space telescope, revealed the presence of the sun-like star. Its gravitational pull causes subtle shifts in the pulsar’s movement, creating the illusion of a spin slowdown.
The team believes all three stars formed together from a single cloud of gas but aged differently due to their varied masses. The most massive star, around 20 times the sun’s mass, exploded first to form the pulsar, while the medium-sized star evolved into a white dwarf. The smallest, comparable to the sun, remains active and stable for now. Eventually, it too will exhaust its fuel, likely evolving into a white dwarf, resulting in a system of one pulsar and two white dwarfs, analogous to the previously identified triple system discovered in 2014.