I found an article at,https://www.universetoday.com/52252/main-sequence/ , which talks about the HR diagram. Most of the dots on the Hertzsprung-Russell diagram is on a nearly diagonal, straight, line, which went faint and red to blue and bright. The discovery of the main sequence was required to wait until the distance to a minimum of a few hundred stars which can be reasonably estimated. This all happened early 20th century
Going back to the 19th century it would be impossible to answer the following questions. Why do most stars seem to lie on the main sequence? Why don’t we find stars all over the Hertzsprung-Russell diagram. They could not answer these question back then because the quantum theory was not created yet. Also they did not know about nuclear fusion or what powered the Sun.
As the answers became closer to us, the stars on the main sequence is just a sequence of mass. Stars on the Hertzsprung Russell diagram that were elsewhere, their positions reflect what nuclear reactions powered them. There are so many stars on the main sequence because stars spend a lot of their lives by burning hydrogen than they do in other ways producing energy.
Working out the details of stellar evolution took many decades. How some stars can live on long after it should be white dwarfs, how the size of a star reflects its internal composition and structure, what nuclear reactions for what exactly what composition and mass of a star, etc. There is still many unanswered questions in this day and age. My personal opinion of this article is over all good because I feel that I really learned a lot from it and I would recommend people should read this.I learned that cool stars on the main sequence are dim and small, most stars are on a line called the main sequence, giant stars are stars that are brighter than expected are large and are called giants or supergiants. This article connects to what I have learned in class because this article talks about the H-R diagram and I learned all about it in my class.