China's FAST telescope discovers rare evolving triple star system with pulsar

In June 2020, China's Five-hundred-meter Aperture Spherical Radio Telescope detected a pulsar in the southwestern mountains of Guizhou province—a discovery that would eventually reveal something far more unusual than a single spinning neutron star. A research team from the National Astronomical Observatories under the Chinese Academy of Sciences made the initial detection, but the real story emerged only after months of follow-up work. Colleagues at the Xinjiang Astronomical Observatory found that this pulsar was not alone. It orbited a white dwarf companion, and its rotation period was increasing at an unexpected rate: one nanosecond per year, roughly a hundred times faster than similar pulsars elsewhere in the galaxy.

That anomaly prompted deeper investigation. Han Jinlin, a researcher at the National Astronomical Observatories, led a team that combed through archival data from FAST and other observatories. What they uncovered was a third member of the system—a Sun-like star still actively evolving, with a mass nearly identical to our own sun. The three objects together formed something astronomers had rarely seen before: a primordial triple star system caught mid-evolution, with one star still burning while the other two had already exhausted their fuel and died.

The story of these three bodies stretches back billions of years. According to stellar evolution theory, all three were born together from the same cloud of gas and dust. The most massive of the three completed its life first, exploding as a supernova and collapsing into the pulsar that FAST detected. The second-most massive followed a similar path, becoming the white dwarf now orbiting the pulsar. The least massive star, positioned far enough from its companions to escape the worst effects of their violent deaths, survived intact. It continues its slow burn today, still in the middle chapters of its existence.

This configuration is extraordinarily rare. Before this discovery, astronomers knew of only one other pulsar triple star system, detected by the Green Bank Telescope in the United States. That system consisted of a pulsar and two white dwarfs—three objects all past their evolutionary prime. The Chinese discovery is unique because it preserves a living star alongside the dead ones, offering an unprecedented window into how triple systems evolve over cosmic time.

The three bodies interact through gravity in ways that are measurable and testable. The team noted that the system exhibits clear three-body gravitational interactions, making it an ideal natural laboratory for testing fundamental theories of gravity, including the strong equivalence principle. It also provides a rare opportunity to study the complex mechanics of how triple star systems form and change, and to observe astrophysical processes under the extreme conditions that exist near neutron stars and white dwarfs.

FAST itself has become a prolific pulsar hunter since it began formal operations in January 2020. The telescope, nestled in a naturally deep karst depression and spanning an area equivalent to thirty football fields, has discovered more than a thousand pulsars in its first few years of operation. This triple system represents one of its most scientifically valuable finds—not because it is the brightest or the most distant, but because it is still telling the story of its own transformation.