In author and computer engineer Liu Cixin’s science fiction hit, The Three-Body Problem, three suns throw a doomed world into chaos. Chinese astronomers say they have found a real three-star system that behaves much better. Using FAST, the world’s largest single-dish radio telescope in southwest China, researchers found two dead and one “living” stars locked in a constant cosmic dance about 7,000 light-years from Earth. One of the dead stars is a pulsar that spins more than 300 times per second, about as fast as the blades of a kitchen mixer. It orbits its companion, a white dwarf (the dark, burnt core of a smaller star) every eight days. Meanwhile, a star very similar to the Sun orbits the pair from afar, taking 73 and a half years to complete each revolution. The team, led by Han Jinlin of the National Astronomical Observatories in Beijing, reported the finding in The Astrophysical Journal Letters earlier this month. It is only the second such system known, after one found by a US-led team in 2014 in which all three stars were already dead. “The detailed evolution of triple star systems is complicated and poorly understood,” the researchers wrote in their paper. The newly identified system, they added, was “unique” for studying how those star families changed over time. Triple star systems are fairly common, but those containing a millisecond pulsar (the rapidly spinning core when a massive star explodes at the end of its life) are “extremely rare,” according to the team, because the star’s explosion can easily tear the system apart. These systems give astronomers a rare opportunity to test theories of gravity. For example, this could help test a key idea of Einstein’s theory of gravity, known as the strong equivalence principle: that everything falls the same way, whether it’s a feather or a dead star the size of a city and heavier than the sun. Pulsars are some of the most reliable clocks in the universe. As they spin, they sweep beams of radio waves through space like a lighthouse, and the pulses reach Earth with such regularity that they can rival atomic clocks. That’s why astronomers were baffled when this one, first detected by FAST in 2020, appeared to be running slowly. Five years of monitoring by a team at the Xinjiang Astronomical Observatory showed that its spin was slowing about 100 times faster than other pulsars of its type. In a paper published in Nature Astronomy in April, the Xinjiang team said the finding challenged a standard theory about how such pulsars formed. They also noticed hints that something else might be lurking in the system. The researchers behind the new study went searching. Combining FAST data with observations from other telescopes, including 18 years of gamma-ray data from NASA’s Fermi Space Telescope, they found the culprit: a Sun-like star hiding in plain sight. As the star slowly rotates around the pair, its gravity pulls the pulsar back and forth, making it appear as if the pulsar’s spin is slowing down. The three stars were probably born together from the same gas cloud, like triplets, the team said. Because they started out with very different masses, the brothers have aged at very different rates. The largest, about 20 times the mass of the Sun, burned up first and exploded, leaving the pulsar behind, while the second faded into the white dwarf. The smallest, about the mass of the Sun, is far enough away to survive the death of its siblings and still shines. Eventually, it will also burn up, researchers say, leaving behind a pulsar and two white dwarfs, much like the system found in 2014. A European team, with scientists from Germany, the Netherlands, France and Britain, independently reached the same conclusion in a paper published online in July. Both teams found that the Sun-like star will move closer to the pair in 2036. As it does, its pull on the pulsar will change rapidly, allowing scientists to measure the system much more precisely. — South China Morning Post
