The helium burning shell is very thin and is somewhat unstable, producing bursts of energy known as thermal pulses . These bursts eventually eject the entire envelope of the star, forming a planetary nebula . Instead it slowly cools as a white dwarf . If a white dwarf increases in mass, it shrinks in size. White dwarfs cannot be more massive than about 1.4 solar masses, the Chandrasekhar mass .
Clusters of stars consist of stars which are all approximately the same age.
By finding the main sequence turnoff point , the age of the cluster can be calculated. Open clusters are loose, irregular clusters of stars in the disk of the Galaxy and are young. Globular clusters are giant, spherical clusters of stars in orbit around the Galaxy and are very old. Most stars in the Galaxy are in binary systems and their evolution can in some case be influenced by the transfer of mass from one star to another, particularly when one of the stars swells to overflow its Roche lobe . Novae are stars that occasionally undergo an enormous increase in luminosity before returning eventually to their original state.