Main sequence explained to kids

The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell. Stars on this band are known as main-sequence stars or dwarf stars. After a star has formed, it generates thermal energy in the dense core region through nuclear fusion of hydrogen atoms into helium. During this stage of a star's life, it is located on the main sequence at a position determined primarily by its mass, but also its chemical composition and age. All main-sequence stars spend the vast majority of their lives within a few hundred parsecs of the Sun. The Sun, with a mass of 1.0 solar mass, is the most massive main-sequence star, and is located at an absolute magnitude of +4.83. A star of slightly lower mass, 0.85 solar mass, such as Procyon A, has a lifetime of 10 billion years and an absolute magnitude of +5.23. The most-massiver known main-sequence star, R136a1, has a mass 265 times that of the Sun, and has an absolute magnitude of −5.54. It has a lifetime of just 2 million years. The Hertzsprung–Russell diagram shows that the main sequence occupies a diagonal band from the top left to the bottom right of the diagram. This band is often subdivided into smaller regions. The upper end of the main sequence, where stars are the most luminous, is called the giant branch. The lower end of the main sequence, where stars are the least luminous, is called the dwarf branch. In between these two extremes are subgiants and main-sequence turnoff stars. The main sequence turnoff point is the brightness level at which stars exhaust their core hydrogen and begin to expand off of the main sequence. The Sun will reach this point in around 5 billion years. At this point, it will have a luminosity of around 4 times its current value and a radius of around 1.2 times its current value. The main sequence band is often further divided into spectral types. These are distinguished by the strength of certain absorption lines in the star's spectrum. The earliest spectral type, O, represents the hottest stars, while the latest spectral type, M, represents the coolest stars. The Sun is of spectral type G.