In human astronomy, stars were classified according to the temperature of their photospheres, accompanied by their spectral characteristics and the various properties derived from them. Wien's displacement law could use spectral measurements to gauge the surface temperature of an object, but this was very difficult for distant stars.
The Harvard University Observatory in the United States conducted in-depth spectral research on 500,000 stars. They classified them according to their differing spectral lines and found that these were also related to color.
Later, stellar spectra were divided into seven major categories: blue, blue-white, white, yellow-white, yellow, orange, and red.
Barnard's Star was a red "M"-type star and had an apparent magnitude of 4! Yet even though Barnard's Star was an M4 red dwarf, its distance made it appear much dimmer. Observers had to use a telescope to see it.
Many astronomical researchers in the past analyzed and estimated from stellar chronology that Barnard's Star was around 7 to 12 billion years old, far older than the Sun. Many astronomers even believed it might be the oldest star in the Milky Way Galaxy.
Barnard's Star had now lost a great deal of rotational energy. Periodic changes in its luminosity showed that it took 130 days to complete one rotation, whereas the Sun needed only 25 days.
Because Barnard's Star was considered an ancient star, it had long been assumed to be dormant. However, astronomers observed a powerful stellar flare more than twenty years ago! So Barnard's Star was actually still a flare star!
Flare stars were stars whose luminosity changed unpredictably. They could brighten dramatically within minutes, sometimes changing by several magnitudes in just a few minutes, then gradually return to normal after lasting from several minutes to several hours. Flare stars were usually rather complex.
"Four?" Chen Mengchuan froze.
"Yes, Mengchuan.