Want to "see" the Milky Way clearly and discover strange "superstars"?

Whenever night falls, the Guo Shoujing Telescope (LAMOST) located in the hinterland of Yanshan, Xinglong County, Hebei Province slowly opens its dome. In this spectroscopic survey telescope independently developed by China and currently the world's largest aperture, starlight from the distant universe passes through the reflection Schmidt correction plate MA, spherical main mirror MB and focal plane, and then is transmitted to the spectrometer through optical fiber.

In the past 7 years, LAMOST’s “Sky Survey” has acquired the largest spectral observation database of Milky Way stars in the world.

Since 2014, with the support of the National Natural Science Foundation of China's major project "LAMOST Galaxy Research", Chinese astronomers have made a series of progress on major scientific issues related to the Milky Way based on LAMOST survey data. .

Based on tens of millions of spectra

"Altair is so far away and so beautiful is the beautiful river" - the phenomenon that Altair and Vega are separated by the Milky Way has aroused the rich imagination of our ancestors. As a quiet "heavenly river", the Milky Way is a beautiful and poetic existence.

Different from the impression of a telescope, people do not directly take pictures of celestial objects through it, but obtain the spectrum of the visible light band of celestial objects. Scientists rely on spectral analysis to obtain information about distant celestial bodies. Just like people's fingerprints are different, different elements will leave their own unique imprints at specific wavelengths. As long as the light emitted by a celestial body can be obtained, we can know what material the celestial body is made of through spectral analysis.

Among them, the spectrum of galaxies can provide information such as distance, chemical composition, and radial motion, while the spectrum of stars can infer chemical composition, temperature, age, mass, and evolutionary history. Many strange celestial objects and phenomena can also be discovered from spectral observations of a large number of celestial bodies. All these will promote mankind's new understanding of the most basic physical laws such as the laws of universe evolution, material structure, and interaction.

LAMOST is the largest spectroscopic survey telescope in the world, and it is also a powerful tool for census of celestial bodies. Its completion has given Chinese astronomers great encouragement.

"We use the spectra obtained from the LAMOST survey to study some key scientific issues in the Milky Way." Researcher Zhao Gang, director of the LAMOST Operation and Development Center of the National Astronomical Observatory of the Chinese Academy of Sciences, told China Science News.

In 2013, the National Natural Science Foundation of China issued application guidelines for the major project "LAMOST Galaxy Research", and the application of the research team organized by Zhao Gang was approved. They condensed several key scientific issues for using the LAMOST survey to study the Milky Way, including Milky Way mass and dark matter distribution, dynamic models, young star clusters and star formation, and special celestial bodies and special physical processes in the Milky Way.

"We hope to make the most of the survey spectral data to study some key scientific issues in the Milky Way and promote the continuous development of Chinese astronomy." Zhao Gang said.

Uncovering the Mystery of the Milky Way

"There are tens of millions of spectra, study the first one." This is a sentence often said by researchers of this major project. What do you think? Emphasize the importance of conducting scientific research after obtaining spectral data.

On March 27, 2019, Zhao Gang showed a slide of the Milky Way at the closing meeting of this major project. The dark blue background is set against a flat yellow bright spot in the center. The bright spot is composed of countless celestial bodies of different colors. Multiple components and structures are displayed simultaneously on this picture, which shows the complexity and depth of the exploration of the Milky Way.

"Studying the structure, formation and evolution of the Milky Way requires separately studying the composition of different star populations in the Milky Way, and also requires closely combining various information such as the Milky Way's star formation process and initial mass function." Zhao Gang emphasized.

This is exactly the scientific goal of this major project. Over the past five years, scientists have achieved fruitful results around this scientific goal.

LAMOST’s huge amount of data has become a “rich mine” for mining strange celestial objects. A few years ago, in LAMOST's massive spectral data, researchers discovered a rare spectrum and determined that it came from a star with an abnormally high lithium abundance.

In fact, as early as 1981, astronomers used a small telescope to discover a special star for the first time. Its spectrum was very strange, with spectral lines appearing where there should not be. A strong lithium line. In order to clarify the formation of lithium-rich giants, scientists began to collect samples of such celestial bodies, but only a very small number of lithium-rich giants were discovered, which was difficult to provide sufficient evidence to solve scientific problems.

Fortunately, LAMOST found an object carrying rare spectral lines during sky surveys. This has aroused great interest among Chinese scientists.

After further follow-up observations, scientific researchers found that the mass of this peculiar star is less than 1.5 times that of the sun, and its radius is about 15 times that of the sun. It is a typical giant star. Then, they accurately measured its lithium abundance and found that the star's absolute lithium abundance was as high as 4.51, which is equivalent to 3,000 times the lithium content in the sun. It is the giant star with the highest lithium content known to mankind.

This discovery was published in "Nature Astronomy" in August 2018. "Lithium-rich giant stars are very rare and are of great significance in revealing the origin and evolution of lithium." Yan Hongliang, the first author of the paper and an assistant researcher at the National Astronomical Observatory of the Chinese Academy of Sciences, said that this lithium-rich star comes from a snake near the center of the Milky Way. In the direction of the constellation Virgo, it is located north of the Milky Way, about 4,500 light-years away from the Earth.

Subsequently, the researchers explained the source of the lithium element in this star. They finally confirmed that the lithium element in this star is likely to come from a special material exchange process inside the star - with the help of an asymmetric convection, the upward flow rate is much faster than the downward flow rate, resulting in many raw materials inside the star being carried away to the surface, eventually turning into lithium.

Zhao Gang said at the conclusion of this major project: "This discovery doubles the international observation limit of lithium abundance in lithium-rich giants, and at the same time provides insights into the synthesis of lithium and the evolution of existing stars. The theory puts forward a unique new perspective. ”

Another strange star was also discovered by LAMOST. Not long ago, an academic article published in Nature Astronomy proposed that Chinese and Japanese scientists discovered a star with a particularly high content of heavy metals such as silver, europium, gold, and uranium in the galactic halo.

With the support of this major project, researchers conducted an in-depth study of the source of this star. The corresponding author of the paper, Zhao Gang, explained in an interview with the media that because the dwarf galaxies "eaten" by the Milky Way have a similar mass distribution to the current "surviving" dwarf galaxies, their member stars also have similar chemical characteristics. "So by studying the chemical composition of the stars in the dwarf galaxies near the Milky Way, we can learn the chemical characteristics of the stars in the dwarf galaxy family, and thereby screen out the stars from the dwarf galaxies in the Milky Way just like DNA identification."

By comparing the contents of elements such as magnesium, silicon, calcium and titanium in stars in the galactic halo and members of dwarf galaxies nearby the Milky Way, they determined that the star belonged to the "foreign population" and was abducted by the Milky Way when it swallowed the dwarf galaxy. of "immigration". In fact, this confirms the hypothesis about the formation of the Milky Way: the original Milky Way continuously devoured nearby dwarf galaxies, eventually forming today's Milky Way.

In the five years since this major project was implemented, researchers have made a series of significant progress and representations in the fields of the evolution of the galactic halo and the distribution of dark matter, the dynamics of the Milky Way, the star formation process in the Milky Way, and special stars in the Milky Way. Sexual results. These results have been published in more than 280 papers in internationally influential SCI journals.

Fully demonstrate the characteristics of "big science"

Currently, the scope, scale, and complexity of many scientific problems are constantly expanding, and global scientific research has entered the "big science era." In this regard, Li Haining, one of the members of this major project and an associate researcher at the National Astronomical Observatory of the Chinese Academy of Sciences, has deep feelings: "It has become inevitable for scientists from different fields and countries to cooperate."

In the implementation of this During the major project, scientists based on LAMOST's galaxy research, fully demonstrated the characteristics of the "big science" of today's astronomy in multi-dimensional cooperation.

First, it is a collaborative effort between data scientists and astronomers. "The amount of LAMOST spectral data is very large. Data scientists need to first process the spectral data into data that scientists can use, and then astronomers can interpret and study it." Li Haining said, "Almost every team has scientists who specialize in data mining. . ”

At the same time, interdisciplinary collaboration is also helpful. For example, when the lithium-rich giant star was discovered, researchers such as Yan Hongliang, Zhao Gang, and Shi Jianrong collaborated with scientists from the China Institute of Atomic Energy, Beijing Normal University and other universities to conduct in-depth multidisciplinary research on this peculiar star. Research.

Subsequently, they also combined the high-resolution spectrum of the American "Automated Planet Searcher Telescope" and the latest atomic data of the China Institute of Atomic Energy to reproduce the changes experienced by its interior through simulations, thereby understanding the planet. The star's lithium abundance provides a reasonable explanation.

According to researchers, it is a series of collaborative studies that support the conclusion that lithium is produced inside stars.

In the research to discover the aforementioned "foreign immigrants" in the Milky Way, Chinese scientists collaborated with Japanese scientists. "LAMOST plays the role of a 'census'. Once interesting targets are searched for, it is necessary to use telescopes with higher resolution capabilities to carry out targeted and precise research." Li Haining told China Science News.

At that time, in order to confirm the origin of this heavy metal "excessive" star, Chinese scientists and Japanese scientists applied for observation time from the National Astronomical Observatory of Japan's 8-meter optical telescope, and Chinese scientists led a high-resolution Spectral joint observation study.

In addition, Chinese scientists have established long-term and stable cooperative relationships with many world-renowned astronomical institutions such as the Max Planck Institute for Astronomy in Germany, Heidelberg University, and the University of Munich.

Zhao Gang pointed out that with the support of this major project, observation and theory in the field of galaxy research have been fully intertwined and integrated. “Using LAMOST’s massive spectral data, Chinese astronomers have been able to uncover many unsolved mysteries of the Milky Way.

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