Study Reveals Lithium's Secret Life in Sun-Like Stars

Are high-content lithium elements rare in sun-like stars? Do solar-like stars produce Li? At what stage did it evolve? The international team led by researcher Zhao Gang of the National Observatory of the Chinese Academy of Sciences and Dr. Kumar, using China's major scientific and technological infrastructure, the Guo Shoujing Telescope (LAMOST) spectral data and international GALAH sky-watching data, found that sun-like stars produce Li after helium flash generally, the discovery unravels the above puzzles.

On July 6, 2020, this research was published in the internationally renowned astronomical journal Nature Astronomy. Lithium is commonly used in modern communications equipment and transportation industries. Mobile phones, tablet computers, electric cars, etc. are powered by Li batteries. In addition, Li is also widely used in aerospace, defense, and military industries. But have you ever wondered where Li comes from?

The origin of the vast majority of Li can be traced back to the same event, that is, the Big Bang, the origin of the universe, about 13.8 billion years ago. Li is one of the three elements known to be produced in the early Big Bang (the other two are hydrogen and helium). Li has always been the key element connecting the Big Bang, interstellar matter and stars, and the study of Li is an important topic in the evolution of the universe and stars.

The Li 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 Li. Unlike other elements, researchers generally believe that the Li element will gradually disappear in the stars.

The evolution of the lithium element in the star from the main sequence through the red giant, helium flash (upper side of the red giant) to the burning phase of the helium nucleus of the red cluster. The red symbol band represents the helium nuclear burning stage of the red cluster star.

This is because Li participates in the nuclear reaction at a relatively low temperature inside the star (2.5 million degrees, which is several times a million degrees), and then mixes with the external atmosphere, the initial Li will disappear in the life cycle of the star. For example, the constituent elements of the sun and the earth are highly similar and are thought to form almost simultaneously, but the Li content in the sun is 100 times lower than that in the earth.

With the advancement of observation technology, people have found that the solar content of some solar-like stars (about 1/100 in the Milky Way) is very high, and in some cases, it is even 100,000 times higher than the theoretical model predicts. What causes abnormally high lithium content in sun-like stars? This problem has plagued researchers for the past 40 years.

With the help of GALAH, LAMOST, and GAIA survey data, the research team discovered that solar-like stars generally produce Li after helium flashes. Dr. Kumar, the first author of the paper, said: The team systematically studied the abnormal increase in Li abundance in late-stage solar stars.

Surprisingly, the phenomenon of abnormally increased lithium abundance after solar-like stars flashed through helium. Helium flash is an iconic event in solar-like stars, which accumulates helium at its core late in the star's evolution and causes temperature and pressure to continue to rise. The giant helium nucleus was eventually ignited, burning it out of control, like detonating a helium bomb inside a star, releasing the equivalent of the entire Milky Way in minutes.

Observations found that the average Li content of these stars was more than 200 times higher than the theoretically predicted value, indicating that sun-like stars produced new Li elements through helium flashes. Since helium flash is an inevitable process in the evolution of solar-like stars, solar-like stars generally produce Li after helium flash. LAMOST data play an important role in the identification of helium flash stars.

What's more, the study for revealing Li's secret life in sun-like stars also proposed a new standard to identify objects known as Li-rich giants, which have been discovered over the past 40 years as just the tip of the iceberg in the universe. According to researcher Zhao Gang, the leader of the research team and co-author of the study: For us, the key to the next step is to understand the nuclear fusion of lithium between helium and mixing mechanisms, which still contain many mysteries.

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