On the 6th, it was learned from Xiamen University that Professor Gu Weimin's team from the School of Physical Science and Technology and Professor Li Zhiyuan's team from the School of Astronomy and Space Science at Nanjing University have collaborated to reveal the physical properties of the first ultra compact X-ray binary star outside the Milky Way, opening a new window for exploring the evolution of celestial bodies in distant galaxies. The relevant research results were published on the same day in the comprehensive academic journal "Innovation". The binary system discovered this time is located in the Andromeda Galaxy, 2.5 million light-years away from Earth. It is an extremely compact binary system composed of a stellar black hole and a white dwarf. Researchers have introduced that white dwarfs are tightly held by the gravitational "rope" of black holes and rotate at high speeds at extremely close distances. When the two are close enough, the strong gravity of the black hole will "strip" the material on the surface of the white dwarf, forming a hot rotating accretion disk. Matter is heated to millions of degrees before being engulfed by a black hole, emitting strong X-rays, which is also the key to being able to observe it. In the preliminary work, the team has identified the ultra short period signal of the source and proposed it as a candidate for ultra compact X-ray binary stars. This study advances this "candidate" into the first known extragalactic ultra compact X-ray binary through more systematic data analysis and physical modeling. Its orbital period is only 465 seconds, indicating that the two celestial bodies are extremely close, and the companion star cannot be an ordinary star, only a highly dense celestial body such as a white dwarf; The high X-ray luminosity and strong accretion activity exhibited by the system also led the research team to determine that the main star is more likely to be a black hole. This type of black hole white dwarf binary system is rare in the universe. It is not only the first time that humans have discovered ultra compact X-ray binary stars outside the Milky Way, but also the shortest orbital period, largest white dwarf mass, highest X-ray luminosity, and strongest gravitational wave radiation among similar systems. (New Society)