Astronomers have discovered a rare type of binary star system where one star orbits inside its partner. The system, known as PSR J1928+1815, is about 455 light-years from Earth and consists of two stars: a millisecond pulsar and a helium star. The pulsar has lost most of its outer layers, leaving behind a core made up mostly of neutrons.
The researchers used the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in China to detect the companion star, which is about 1-1.6 times the size of our sun. The two stars are only about 50 times closer than Mercury is to the sun and complete an orbit around each other every 3.6 hours.
The discovery challenges previous research that suggested binary systems may experience a “common envelope” phase, in which one star orbits within the outer layers of its companion. However, scientists have never detected such exotic binaries before.
Using computer models, the researchers estimate that only 16 to 84 counterparts of PSR J1928+1815 and its companion may exist in our galaxy. The discovery provides new insights into the formation and evolution of binary star systems and offers a rare opportunity for scientists to study this unique phenomenon.
Source: https://www.space.com/astronomy/stars/scientists-find-rare-double-star-system-where-one-star-orbited-inside-the-other