 Very low mass stars , with masses below 0 @.@ 5 M ☉ , are fully convective and distribute helium evenly throughout the whole star while on the main sequence . Therefore , they never undergo shell burning , never become red giants , which cease fusing and become helium white dwarfs and slowly cool after exhausting their hydrogen . However , as the lifetime of 0 @.@ 5 M ☉ stars is longer than the age of the universe , no such star has yet reached the white dwarf stage . 
