“Weighing” + “Census” are countless stars in the sky, we racked our brains

  In my childhood impression, the stars in the sky are densely packed, dazzling, like the sand on the river beach, countless. People often use dotted lines to describe a large number, which is really apt. So, how many stars are there in the sky? From ancient times to the present, people who are curious have counted it over and over again, and mankind’s understanding of the universe is getting deeper and deeper in the process of counting the stars.
  Looking Yelan thousands of stars
  in ancient China to observe the stars and explore a long history, as early as the Warring States period, there have been the first catalog of the world – “Shih catalog.” During the Three Kingdoms period, Chen Zhuo, the Taishi of Wu, sorted out and regulated the three star scriptures of Gan De, Shi Shen, and Wu Xian that were popular at that time, and established 283 star officials, including 1,464 stars. The history is called “Chen Zhuo Ding Ji”. . During the Sui and Tang dynasties, a hermit named Dan Yuanzi created “Bu Tian Ge”, which summarized all star officials into three gates and twenty-eight constellations, which became the “standard” Chinese astrological system passed down to later generations.
  After the western learning spread to the east in the late Ming and early Qing dynasties, officials and missionaries of the Qin Tianjian made a relatively large expansion of the traditional astrological system. In the “Yixiang Kaocheng Sequel” compiled during the Daoguang Period of the Qing Dynasty, the total number of stars published reached 3240. The Chinese star names we now use are all based on this.
  In modern times, astronomical surveys have done several rounds of “big census” of brighter stars throughout the sky. The results show that the total number of relatively bright stars that can be seen directly by the naked eye is about 9,000.
  At different latitudes, the starry sky area seen is different. The higher the latitude, the smaller the starry sky area seen. For example, on the equator, the entire sky can be seen. But at the north and south poles, only half of the starry sky can be seen, and the other half of the stars never rise. In the area of ​​30 degrees north latitude, the total number of stars that can be seen is about 8,100. It’s just that in the night of the city, we basically don’t see more than a few dozen stars. They were almost flooded by lights.
  Hundreds of billions of stars shape the galaxy
  All the stars that we can see with the naked eye are just the sun’s immediate neighbors, just the tip of the iceberg of the Milky Way. How many stars does the Milky Way contain? Many astronomers have made arduous efforts to solve this problem. The most famous of these was Herschel, the British astronomer who discovered Uranus. By 1785, he had counted 117,600 stars with a self-made telescope. In 1922, the Dutch astronomer Captain summarized the photographic data of him and his colleagues. By counting the number of stars and analyzing their distribution, he constructed a physical model of the Milky Way, containing 47.4 billion stars!
  However, it is impossible to know the number of stars in the Milky Way just by counting. This is not only because the farther stars look darker, beyond our detection limit, but also because the interstellar space between stars is filled with extremely thin but massive interstellar gas and dust. They will absorb and obscure the light of stars in the distance, making the method of counting stars no longer accurate.
  So astronomers thought of a way to “weigh” the Milky Way: After finding the total mass of the Milky Way, the number of stars can be estimated based on the average mass of the stars. The latest observation data from the Hubble Space Telescope and the Gaia satellite in 2019 show that the total mass of the Milky Way is approximately equal to 1.54 (+0.75/-0.44) trillion times the mass of the sun. However, as much as 84% ​​of its mass is composed of dark matter, which is completely different from ordinary matter that composes stars and planets. It neither emits light nor participates in electromagnetic interaction.
  In 16% of the luminous matter, stars account for only 4%, and the other 12% is interstellar gas and dust. In this way, we can estimate that the total mass of stars in the Milky Way is about 60 billion times the mass of the sun. But you must know that the size and mass of stars vary greatly. The smaller the mass, the greater the number of stars. The number of stars less massive than the sun is much larger than that of more massive stars. After considering this correction, the number of stars in the Milky Way is estimated to be around 200 to 400 billion. If they are “distributed” to people on earth, on average, each person can “distribute” hundreds of them.
  A corner of trillions of galaxies
  In the entire universe, the Milky Way is only a drop in the ocean. On a larger scale, giants like galaxies are nothing more than grains of sand. They are just the basic units that make up the universe. In 2016, astronomers estimated that there are approximately 2 trillion galaxies in the observable universe based on the Hubble Space Telescope’s deep exposure images. If the Milky Way is taken as the average value, then the total number of stars in the universe is probably on the order of 1024, which is 1 followed by 24 0s! It may be much more than the total amount of sand on the earth.
  This is not the end. The “observable universe” mentioned earlier is only a small part of the real universe. We know that the age of the universe is about 13.77 billion years. Even the earliest celestial body was born, its light has only gone for less than 13.77 billion years. The distance traveled by light in a year is a “light-year”. However, as the light spreads, the space of the universe is also expanding, which makes the farthest distance we can see exceeds 13.77 billion light-years, reaching about 46.5 billion light-years. The spherical space with this radius is the limit range that we can observe. We don’t know how big the space outside this is, and we don’t know how many stars there are.
  Finally, the universe we live in may be just one of the numerous “cosmic bubbles” formed by inflation after the Big Bang. The other bubbles are “parallel universes” that are not connected to us. The physical parameters in the parallel universe are different from ours, and there will be a different evolutionary picture, and the number of stars in it cannot be estimated.
  After studying so deeply, I didn’t expect that “how many stars are there in the sky”-the question that this kid has almost asked, turns out to be so complicated that it is close to “metaphysics”. However, to understand our Milky Way galaxy by measuring the mass and counting the stars, and then to understand the large-scale structure of the universe, this is an inevitable course of understanding nature, and it will definitely help us reveal more cosmic mysteries.