China News Service, Beijing, July 7th (Reporter Sun Zifa) In solar-like stars, are high-content lithium elements rare, do solar-like stars produce lithium elements, and at what stage of the star's evolution... as it is currently known One of the earliest three elements produced by the universe, lithium has long received considerable attention in astronomical research.

  The international team led by Researcher Zhao Gang of the National Astronomical Observatories of the Chinese Academy of Sciences and Dr. Kumar used the Guo Shoujing telescope (large-area area multi-target fiber spectroscopy telescope, English abbreviation LAMOST) spectral data and international GALAH survey data. The latest research reveals the mystery between Tai-like stars and lithium: Solar-like stars generally produce lithium after helium flashes.

Schematic diagram of helium nuclear combustion. (The picture is from a documentary-How the universe works)

  This important astronomical research paper was published on the night of July 6, Beijing time in the international authoritative astronomical journal Nature. Astronomy. Zhao Gang, the co-corresponding author of the paper, told the media, "For us, the key to the next step is to understand the nuclear fusion of lithium between the helium flash and the mixing mechanism. There are still many unsolved mysteries here."

  According to the National Astronomical Observatory of the Chinese Academy of Sciences, lithium, hydrogen, and helium are the earliest three elements in the Big Bang that are known to occur about 13.8 billion years ago. Lithium has always been the key element connecting the Big Bang, interstellar matter, and stars. The study of lithium is an important subject of the evolution of the universe and stars.

  The lithium content increased slightly during the Big Bang, mainly because high-energy cosmic rays bombarded the heavier nuclei in the interstellar medium, such as carbon and oxygen, and split them into smaller atoms, such as lithium. Unlike other elements, the astronomical community generally believes that the lithium element will gradually disappear in stars. This is because lithium participates in the nuclear reaction at a relatively low temperature (2.5 million degrees) inside the star, and then after mixing with the external atmosphere, the initial lithium will disappear in the life cycle of the star.

  Scientists say, for example, that the constituent elements of the sun and the earth are highly similar and are thought to form almost simultaneously, but the lithium content in the sun is 100 times lower than that in the earth. With the advancement of observation technology, people have discovered that some solar-like stars (about 1/100 in the Milky Way) have a very high lithium content in the atmosphere, and in some cases, even 100,000 times higher than the theoretical model predicts .

  What causes the abnormally high lithium content in solar-like stars? This problem has plagued researchers for the past 40 years. The research team recently discovered that solar-like stars can generally produce lithium after helium flash, and finally solved this mystery.

  Kumar, the first author of the results paper, said that the research team systematically studied the phenomenon of abnormally high lithium abundance in late solar-like stars, and found that the phenomenon of abnormally high lithium abundance after solar-like stars undergoing helium flash is extremely common. Helium flash is an iconic event in solar-like stars. In the late stage of star evolution, the core of helium continues to accumulate helium, which causes the temperature and pressure to continue to rise. "This huge helium nucleus was finally ignited, and a violent uncontrolled nuclear burning occurred, just like a helium atomic bomb was detonated inside the star, releasing energy equivalent to the entire galaxy in a few minutes."

  He pointed out that the theoretical model predicts that the lithium content of stars undergoing this stage should be very low, but in fact, observations have found that the average lithium content of these stars is more than 200 times the theoretically predicted value, which indicates that solar-like stars have been produced by helium flash New lithium element. Since helium flashes are a process that must be experienced during the evolution of solar-like stars, solar-like stars generally produce lithium after helium flashes. LAMOST data plays an important role in the identification of helium flash stars.

  In addition, the study also proposed a new standard to identify objects called lithium-rich giant stars. According to this standard, the lithium-rich giant stars discovered in the past 40 years may be just the tip of the iceberg in the universe. (Finish)