mention a few variations in luminosity color?
Cepheid variables, Delta Scuti variables, and Beta Cephei variables
Main sequence
A nearly vertical region of the HR diagram, known as the instability strip, is occupied by pulsating variable stars known as Cepheid variables. These stars vary in magnitude at regular intervals, giving them a pulsating appearance. The strip intersects the upper part of the main sequence in the region of class "A" and "F" stars, which are between one and two solar masses. Pulsating stars in this part of the instability strip that intersects the upper part of the main sequence are called Delta Scuti variables. Main-sequence stars in this region experience only small changes in magnitude and so this variation is difficult to detect. Other classes of unstable main-sequence stars, like Beta Cephei variables, are unrelated to this instability strip.
do all stars spend the same amount of time on the main sequence The total life span can be estimated, to first approximation, as the total energy produced divided by the star's luminosity. what is main sequence? Continuous and distinctive band of stars that appears on plots of stellar color versus brightness. describe a few of its properties? Majority of stars on a typical HR diagram lie along the main-sequence curve and temperature of a star determines its spectral type
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Main sequence [SEP] Luminosity-color variation
As non-fusing helium ash accumulates in the core of a main-sequence star, the reduction in the abundance of hydrogen per unit mass results in a gradual lowering of the fusion rate within that mass. Since it is the outflow of fusion-supplied energy that supports the higher layers of the star, the core is compressed, producing higher temperatures and pressures. Both factors increase the rate of fusion thus moving the equilibrium towards a smaller, denser, hotter core producing more energy whose increased outflow pushes the higher layers further out. Thus there is a steady increase in the luminosity and radius of the star over time.