Variable Stars Stellar evolution involves change over thousands, millions and billions of years. These changes are usually too small to be observed in the short time that astronomers have been watching the skies. We have already mentioned pulsations, but flares and spots connected with magnetic fields on the stellar surface can be observed in cool stars, such as the BY Draconis and RS Canis Venaticorum variables. Nuclear explosions on the surfaces of accreting white dwarfs are seen as dwarf novae. Puffs of ejected dust obscuring the normally visible star are seen in R Corona Borealis stars.
Binary stars vary because of their orbital motion and eclipses.
A hot star can heat its cool companion so that it becomes hot on one side and cool on the other. New types of variability are discovered almost every year. Each one helps astronomers to learn more about the real physics going on in stars, and hence to build better models of stellar evolution. The end After hydrogen burning, a star passes through many transformations. Understanding how a star evolves allows us to see physics in action on a huge scale.