Main sequence is a band of stars that appears on plots of stellar color versus brightness. Stars on this band are known as main-sequence stars or dwarf stars. These are the most numerous true stars in the universe, and include the Earth’s Sun. Main sequence is sometimes divided into upper and lower parts based on the dominant process that a star uses to generate energy.
About Main sequence in brief
In astronomy, the main sequence is a band of stars that appears on plots of stellar color versus brightness. Stars on this band are known as main-sequence stars or dwarf stars. These are the most numerous true stars in the universe, and include the Earth’s Sun. After condensation and ignition of a star, it generates thermal energy in its dense core region through nuclear fusion of hydrogen into helium. Main sequence is sometimes divided into upper and lower parts, based on the dominant process that a star uses to generate energy. After the hydrogen fuel at the core has been consumed, the star evolves into a supergiant, red giant, or directly to a white dwarf. In 1933, Bengt Strömgren introduced the term Hertzsprung–Russell diagram to denote a luminosity-spectral class of stars. This reflected the development of this technique by this time. The same technique also showed that there were very few faint stars that were very very very bright. The reverse is true of dwarf stars, and the true star class is a red giant. In the early part of the 20th century, information about the types and distances of stars became more readily available. The spectra of stars were shown to have distinctive features, which allowed them to be categorized. Annie Jump Cannon and Edward C. Pickering at Harvard College Observatory developed a method of categorization that became known as the Harvard Classification Scheme, published in the Harvard Annals in 1901. In Potsdam in 1906, the Danish astronomer Ejnar HertzSprung noticed that the reddest stars could be divided into two distinct groups.
These stars are either much brighter than the Sun, or much fainter. In 1913, Henry Norris Russell published the first plots of color versus luminosity for these stars, which he named the Main Sequence. These plots showed a prominent and continuous sequence of stars, that he called the Main sequence. The plots also showed the same relationship between the spectral types of stars and their actual brightness as corrected for distance—their absolute magnitude. This allowed real star to be predicted with reasonable accuracy. Of the red dwarf stars observed by Hertz Sprung and Russell, the dwarf stars also followed the relationship discovered by Russell, but the red giant stars are brighter than red dwarfs and so do not follow the same pattern. The main sequence also includes the very faint white dwarfs, which are very very faint and do not have great surface density or great surface-brightness. In general, the more massive a star is, the shorter its lifespan on the mainsequence. The stars on this sequence are located at a position determined primarily by its mass, but also based upon its chemical composition and age. Above this mass, in the upper main sequence, the nuclear fusion process mainly uses atoms of carbon, nitrogen and oxygen as intermediaries in the CNO cycle that produces helium from hydrogen atoms. With decreasing stellar mass, the proportion of the star forming a convective envelope steadily increases. When core convection does not occur, a helium-rich core develops surrounded by an outer layer of hydrogen.