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Chinese scientists have, for the first time worldwide, made an important astronomical discovery in stellar observation, which provides a new method for studying the formation and evolution of the Milky Way. 

The research team from the Changchun Observatory, National Astronomical Observatories, Chinese Academy of Sciences directly observed evidence that when stars leave the main sequence and entering the “turnoff point” follow the law of angular momentum conservation. The scientists then discovered a correlation between angular momentum and isochrone age, opening up a new avenue for studying the star-forming environment in the Milky Way’s history, the Guangming Daily reported on Monday.

Stars like the sun, which derive their energy from hydrogen fusion in the core, are known as main sequence stars. When the hydrogen fuel in the core is no longer sufficient to sustain stable burning, the star leaves the main sequence and enters a critical transitional period called the “turnoff point” stage, serving as the intermediate phase for the stellar evolution from the main sequence to the giant stage, Shen Yufu, special research assistant of the research team, told the Global Times on Tuesday. 

According to Shen, the sun’s expansion into a giant star is a gradual process, beginning at the “turnoff point” when the sun passes its most stable stage and begins evolving toward a giant star. The film The Wandering Earth was based on the concept that the sun will one day expand into a giant star and engulf the Earth. 

After entering the “turnoff point” stage, a star’s radius suddenly increases. Due to the angular momentum conservation, which states that angular momentum is conserved in the absence of forces external to the system, the sun’s rotation speed correspondingly slows down – much like a ballerina adjusting her spin by pulling in or extending her arms, Shen explained. 

According to Shen, the concept of angular momentum conservation is not uncommon in daily life. For example, a bicycle staying upright or a spinning top maintaining stability are both manifestations of angular momentum conservation. 

Based on the phenomenon observed that when the stars leave the main sequence and enter the “turnoff point” stage, and conserve angular momentum, the research team has, for the first time, confirmed that for stars with masses above a certain threshold but less than about twice the sun’s mass retain their birth angular momentum, making them akin to “living fossils” of the universe. Their rotational angular momentum preserves information about the properties of the molecular clouds in the galaxy at the time of their formation. 

This finding once again demonstrates that the laws of physics apply universally – on both small and large scales, near Earth as well as in distant regions of the cosmos. By studying such stellar samples, scientists are able to trace the evolutionary patterns of molecular clouds throughout the long history of the Milky Way, opening up new pathways for uncovering the Milky Way’s formation and evolution.