Rediscover the appearance of the Milky Way

Guo Shoujing Telescope in the Milky Way Galaxy. Photo by Chen Ying

Schematic diagram of the increase in the area of the silver plate. Courtesy of the National Astronomical Observatory
　　On August 7, the National Astronomical Observatory of the Chinese Academy of Sciences announced that the National Major Science and Technology Infrastructure Guo Shoujing Telescope (LAMOST), which is managed and operated by it, has successfully completed the first phase of spectral sky survey observation. The first phase of the survey released a total of spectral 9.01 million, including high-mass spectra (signal to noise ratio greater than 10) 7.77 million to determine the spectral parameters of 5.34 million groups of stars.
　　The number of spectra published by LAMOST is 1.8 times the sum of the spectra published by other sky survey projects in the world. The dataset (DR5) was released to domestic astronomers and international collaborators on December 31, 2017. In addition, since the first survey, 345 SCI papers have been published using LAMOST data and have been cited more than 3,000 times.
　　The boundaries of the galaxy have been greatly expanded, and the radius may reach about 100,000 light years
　　In the past few decades, astronomers have generally believed that the radius of the Milky Way is about 50,000 light-years. There is a rod-like structure near its center, with a disk-like structure outside (the Milky Way disk), surrounded by sparse stars (the Milky Way halo), and the sun is about 25,000 light-years away from the center of the Milky Way.
　　Previously, it was thought that the Milky Way disk had a clear boundary about 50,000 light-years from the center of the galaxy, at which the number of stars in the Milky Way disk plummeted, as if the Milky Way disk were cut off. But in recent years, some observations have found young stars belonging to the Milky Way disk beyond this boundary, which seems to suggest that the boundary of the Milky Way disk should be larger. However, to confirm this, more and more distant stars need to be counted.
　　At the end of 2017, researchers at the National Astronomical Observatory used LAMOST data to accurately count the number of stars in the outer reaches of the Milky Way galaxy by applying complex statistical methods, and successfully drew a cross-sectional map of the spatial structure of the outer reaches of the Milky Way disk.
　　Liu Chao, a researcher at the National Astronomical Observatory, said: "We can see from the figure that although the number of stars in the galactic disk decreases with the radius of the galactic disk, it does not stop at 50,000 light-years, but extends all the way to 62,000 light-year from the center. This is about a quarter larger than the radius quoted in textbooks."
　　In 2018, an international research team composed of researchers from the Canary Institute of Astrophysics in Spain and the National Astronomical Observatory of China further used the massive stellar spectra obtained from the LAMOST data and other related data to rewrite the size of the Milky Way disk again, and found that the disk-like structure containing most of the stars in the Milky Way may be much larger than astronomers previously thought, and the radius may reach about 100,000 light years.
　　"This study provides a new understanding of the star composition of the Milky Way disk and a more accurate measurement of the size of the galaxy," said Mr. Liu. "The constant rewriting of the size of the galaxy will prompt astronomers to re-examine the general laws of galaxy formation and cosmic evolution."
　　In addition, the researchers recalculated accurately based on the LAMOST data, rewriting the structural characteristics of the Milky Way halo and establishing a new structure of the inner flat and outer circle. Liu Chao said: "This clear evidence overturns the previous speculation that the stellar halo is a flat sphere with a constant axis ratio, and helps humans re-understand the formation history and evolution of the Milky Way stellar halo."
　　Measuring the orbital eccentricity and inclination of nearly 700 exoplanets for the first time, the solar system is not an exception in the universe
　　In this survey, researchers used LAMOST to measure the orbital eccentricity and inclination of nearly 700 exoplanets for the first time, and found that about 80% of the planetary orbits resemble the nearly circular orbits of the solar system.
　　Zhao Gang, a researcher at the National Astronomical Observatory, said: "This shows that the solar system is not a special case in the universe, but a representative one. To some extent, it enhances our confidence in finding another earth and extraterrestrial life."
　　Quasars are distant celestial bodies outside the Milky Way. Their energy comes from the huge attractive force released by the accretion of surrounding matter by the supermassive black hole at their center. They are important probes for studying the distant universe.
　　"More than 12,000 quasars have been found in the LAMOST spectrum, with an average redshift of 1.5 and a maximum redshift of 5," said Zhao Yongheng, a researcher at the National Astronomical Observatory. "In addition, the mass of the central black hole has been estimated based on these data. The discovery of these quasars will provide important help for the statistical study of large samples of quasars."
　　The sample of nearby host galaxies is a classic in the Large Scale Galaxy Spectroscopic Survey (LAMOST), which began in the 21st century. However, due to the effect of fiber collisions, the sample has a high degree of incompleteness on a small scale. These missing galaxies are observed as supplementary galaxy samples in the LAMOST survey.
　　Zhao Yongheng said: "Due to the wide range of the survey, the spectral measurements of nearly 10,000 supplementary galaxies were obtained in the first survey, and nearly 10,000 new pairs of close galaxies were confirmed. These close galaxies have important scientific value for studying the merger process of galaxies."
　　The researchers also used the LAMOST data to discover a new class of extrasolar planets – hot starfish, which share several hallmarks with hot Jupiters, providing key clues and new research directions for uncovering the origin of hot Jupiters and other short-period planets.
　　Dark matter accounts for more than 90% of the total mass of galaxies, but most of the distribution is very scattered, and the proportion of dark matter at the location of the sun is very low. This brings great difficulties to human direct detection of dark matter.
　　"We used the LAMOST data to re-estimate the density of dark matter near the sun, which is of great significance for finding dark matter particles and understanding the distribution of dark matter in the galaxy," Zhao Yongheng said.
　　In addition, using LAMOST data, researchers have also accurately estimated the age of millions of stars, increasing the number of star samples with accurate age by 1,000 times, providing basic data for the study of the evolution of the Milky Way; at the same time, measuring the magnetic activity index of nearly 6,000 sun-like stars, found that the sun has a level of magnetic activity comparable to that of super flare stars, confirming that the sun has the possibility of a super flare.
　　The discovery of a star with a lithium content about 3,000 times that of similar celestial bodies has doubled the international observation limit for lithium content
　　Old metal-poor stars, like cosmic "fossils," record the first history of the chemical evolution of the universe, and their analysis can enable "stellar archaeology" of the first generation of stars and the nature of the early universe.
　　Zhao Gang said: "We have found more than 10,000 metal-poor stars in the LAMOST spectrum with a metal content lower than one percent or even one ten thousandth of the sun, building the largest cosmic fossil sample in the world that is suitable for tracking and observation by existing large telescopes."
　　At the same time, the researchers also discovered a group of extremely rare small-mass metal-poor stars with lithium abundance exceeding the normal value by hundreds of times. Among them, one star has a lithium content about 3,000 times that of similar celestial bodies, which is the star with the highest lithium abundance known to mankind. The major discovery was published online in the international scientific journal Nature Astronomy on August 7 in the early morning of Beijing time.
　　Lithium is the key element connecting the Big Bang, interstellar matter and stars. Its evolution in the universe and stars has always been an important research object in the field of astronomy. However, the understanding of lithium in contemporary astronomy is still very limited. Giant stars rich in lithium are very rare, but they are of great significance in revealing the origin and evolution of lithium. Unfortunately, astronomers have only discovered a very small number of such celestial bodies in the past 30 years.
　　"This newly discovered lithium-rich star is located in the direction of Ophiuchus near the center of the Milky Way, about 4,500 light-years north of the Milky Way disk. After further study, we found that the lithium element of this star is likely to come from a special material exchange process inside the star." Dr. Yan Hongliang of the National Astronomical Observatory said. "This discovery changes human understanding of lithium in celestial bodies and doubles the limit of international observation of lithium content."
　　In addition, in the nearly 10 million spectra of LAMOST, researchers have found five hypervelocity stars in a haystack, and there are currently only more than 20 hypervelocity stars in the world.
　　Zhao Gang said: "They provide an important sample for further study of the formation mechanism of stars that are very fast and can eventually break free from the attractive forces of the Milky Way and’escape ‘the Milky Way."
　　(Originally published in People’s Daily 2018-08-08 12th edition)