Introduction
LAMOST (Guo Shoujing Telescope) is a spectral survey telescope with both large aperture and large field of view independently developed by the mainland. Since 2011, LAMOST Sky Survey has released more than 17 million celestial spectra, building the world's largest database of astronomical spectra. Sky Survey data from LAMOST has driven important advances in many frontiers in astronomical research, in particular by expanding our understanding of the Milky Way and its various types of objects (Figure 1).
Figure 1 Graphic summary
Astronomy is an observation-based science. 400 years ago, Galileo first pointed a telescope at the night sky, and then the astronomical telescope became a powerful weapon for human understanding of the universe. At that time, astronomical observations were mostly for a certain celestial body or a certain region, and with the development of astronomical telescopes and human curiosity about the universe, astronomers were no longer satisfied with only studying specific targets or regions, but ambitiously wanted to survey a large area, or even all the celestial bodies in the entire night sky. This form of astronomical observation is called a "sky survey."
It is not difficult to imagine that the sky survey needs a telescope with both a large field of view and a large aperture, that is, to "see widely" and "to see far". In this demand, LAMOST was born. LAMOST's full name is the Large Sky Area Multi-Object Fiber Spectroscopy Telescope, which is the first major national scientific and technological infrastructure in the field of astronomy. After its completion, it was also named the Guo Shoujing Telescope (Guo Shoujing was a famous astronomer in the Yuan Dynasty of the mainland). Figure 2 shows lamost's structure and optical path system. The light of the celestial body is incident from the night sky to the active corrective mirror Ma, and then is reflected to the spherical main mirror Mb, and finally imaged on the focal surface.
Figure 2 Schematic diagram of the structure and optical path of LAMOST
LAMOST is a reflective Schmidt telescope, which applies thin deformation mirror active optics plus spliced mirror active optics technology, and the field of view can reach a wide field of view of 5 degrees with an effective aperture of 4 meters. It can observe nearly 4,000 targets simultaneously, making it one of the telescopes with the highest efficiency in spectral acquisition. Figure 3 shows the spectrum of different types of celestial bodies taken by LAMOST and the range of "footprints" of the annual sky survey. At the time of the eighth data release (DR8), LAMOST has released more than 17 million spectra, which has long become the world's largest celestial spectral database.
Figure 3 LaMost spectroscopy example with sky survey footprint
Most of LAMOST's spectrum comes from celestial bodies in the Milky Way. The Milky Way is the home galaxy of human beings, but our understanding of it is actually very limited, which is a true portrayal of "not knowing the true face of Lushan Mountain, only being in this mountain". At the same time, the Milky Way is one of the few galaxies that can be "anatomically" studied at the level of a single star, and it is also a reference for us to study other galaxies, just like the sun is to stars.
The most basic and core scientific value of LAMOST is the more than 17 million spectra it has observed so far, as well as the basic information of various celestial bodies derived from it (such as temperature, surface gravity, elemental abundance, etc.). They are the basis for astronomers to study the properties of various celestial bodies, and it is this data that gives us a new understanding of the Milky Way. Figure 4 shows the most common sets of physical parameter information for all stars in LAMOST DR8.
Figure 4 Stellar physical parameter information disclosed by LAMOST's eighth data release (~6 million stars in the figure)
Thanks to the spectral data from the sky survey, we now know that the Milky Way Disk (Silver Disk) is not a flat disk (Figure 1), but rather has been distorted and perturbed throughout its evolutionary history. The merger with numerous smaller galaxies (later known as dwarf galaxies) was thought to be a key reason for the formation of the Disk of the Milky Way as we see it today. The material of these dwarf galaxies is attracted and stripped away by the Milky Way, eventually becoming a star stream distributed around the Milky Way. The light blue star stream in Figure 1 is one of the historical sites of such an event, the remains of the Sagittarius dwarf galaxy. We found or studied many similar relics through LAMOST data.
Let's take another example of LAMOST helping us better understand the Milky Way. For quite some time, it was thought that the Milky Way was only about 100,000 light-years in diameter, but with the help of LAMOST data, it is now clear that the Milky Way is at least 200,000 light-years old.
In addition to changing the perception of the Milky Way, the LAMOST massive spectrum is also like a treasure trove of cosmic wealth, allowing astronomers to discover a large number of interesting objects. For example, LAMOST discovered a star orbiting the Void and eventually found a monster in the Void that could devour everything – a black hole (Figure 5); found the star with the highest lithium abundance, which contains more than 3,000 times more lithium than the Sun; and discovered a new group of exoplanets , Hot Starfish " , providing new clues to the theory of planet formation. Of course, its discovery is far more than this, there are "living fossils of the universe" metal-poor stars, "Bolt of the Milky Way" supervelocity stars, quasars that will "change their faces", "wandering" stars from other galaxies, etc., which will not be expanded one by one due to space limitations.
Figure 5 Schematic diagram of a stellar-level black hole discovered by changing the velocity of the line of sight
LAMOST has been surveying the sky for ten years, looking through the galaxy, and a few thousand words cannot describe its scientific discoveries. Nearly a thousand scientific papers have been published based on LAMOST data, many of which have changed our understanding of the universe. These achievements, even if they are 10,000 words long, are difficult to cover, but they are a meaningful thing after all.
Summary and outlook
The LAMOST sky survey has entered the next phase. Since 2018, it has successfully launched a new sky survey mode - medium resolution sky survey. The medium-resolution spectrum (R≡λ Δλ 7500) reveals finer spectral line structures, providing higher precision information about the physical parameters, chemical abundance, and apparent speed of celestial bodies. The medium-resolution data enables LAMOST to conduct better "time domain astronomy" research on the basis of higher precision, especially in the hot fields of binary star evolution, variable stars, star formation regions, emission line nebulae, and compact objects. In the near future, LAMOST will be further upgraded to enter a new era of high-precision and ultra-large-scale data volume to better expand future sky survey observation and scientific goals.