Chinese and foreign scientists have recently discovered a binary star system consisting of a white dwarf and a hot sub dwarf. These two stars are orbiting each other at record breaking speeds and can generate strong gravitational wave radiation in the millihertz frequency band. The gravitational wave signal is expected to be detected by future space gravitational wave observatories. The important astronomical discovery made by Professor Wang Xiaofeng's research team from the Department of Physics at Tsinghua University and domestic and foreign collaborators using the Tsinghua University Ma Huateng Sky Survey Telescope located at the Xinglong Observatory of the National Astronomical Observatory was published online on the 9th in the international authoritative astronomical journal Nature Astronomy. Wang Xiaofeng introduced that the research team has discovered a binary star system (code named TMTS J0526) that is approximately 2760 light-years away from Earth and rapidly orbits at a period of 20.5 minutes. This binary system consists of a dense white dwarf with a mass of approximately 0.74 times that of the Sun and a hot sub dwarf with a mass of approximately 0.33 times that of the Sun. "A white dwarf is a dense celestial body formed by the burning of an ordinary star's core over a long period of time. A spoonful of material from a white dwarf is usually comparable in weight to two adult elephants. After about 5 billion years, our sun will become a dense white dwarf with a volume similar to that of Earth. At this point, its internal nuclear reaction has stopped and can only rely on its own waste heat to emit light outward," said Lin Jie, a core member of the team. He said that a thermal dwarf star refers to a type of high-temperature, low-mass star whose outer hydrogen shell is mostly stripped off and whose interior undergoes stable helium fusion combustion. Most hot sub dwarfs have a mass of about 0.45 times that of the Sun and are formed through intense ignition of helium nuclei; Another type of smaller thermal sub dwarf star (with masses as low as 0.32 to 0.36 times the mass of the Sun) can be formed by binary stars undergoing two ejections of common envelope material, without the need for intense burning of helium nuclei. "The thermal dwarf star discovered by our team, which is 0.33 times the mass of the Sun, belongs to the latter. It is worth mentioning that the radius of this thermal dwarf star is only about 7 times that of Earth, only three-quarters the size of the previously recognized smallest star, representing the smallest volume star discovered by astronomers to date." Lin Jie said. Wang Xiaofeng said, "This discovery has determined an important wave source system for future space gravitational wave experiments. More importantly, it has verified the theoretical prediction of the formation of such special binary system proposed by Han Zhanwen, an academician of the Yunnan Observatory of the Chinese Academy of Sciences, in 2003, and helped us further understand and improve the theory of star evolution, which is the cornerstone of our understanding of the evolution of the universe." According to the introduction, The Tsinghua University Ma Huateng Sky Survey Telescope is a uniquely designed fully automatic multi tube optical survey equipment built by Wang Xiaofeng's team with the support of Ma Huateng Fund and Tsinghua University. Since its official operation in 2020, the system has been observing the cosmic sky in the northern hemisphere at a frequency of 1 minute. As of the end of 2023, the project has accumulated over 27 million densely sampled optical data from stars, including a large number of rapidly changing sources with important scientific value. TMTS J0526 is one of the celestial bodies with the fastest photometric changes. (Lai Xin She)