When astronomers observe some white dwarf stars, there can sometimes be small amounts of hydrogen trapped in the stars' upper layers. The traditional view is that these tiny stellar husks suck up diffuse interstellar hydrogen gas, but now a team of researchers suggest something else is going on: comets are likely raining down into the white dwarfs' ancient atmospheres from exo-Oort clouds, seeding them with hydrogen.
ANALYSIS: White Dwarfs Are Eating ‘Earth-like' Planets for Dinner
White dwarfs form after sun-like stars extinguish the majority of hydrogen fuel in their fusion cores. This causes heavier and heavier elements to fuse, turning the star from a once-quiescent state into a violent, churning red giant. Eventually, through powerful stellar convulsions, the bloated red giant is ripped apart, leaving a small, dense white dwarf in its wake.
A white dwarf's structure is not maintained by the outward pressure supplied by fusion reactions to counter gravity, like our sun. The quantum pressure supplied by electron degeneracy is what prevents its gravity from causing it to collapse in on itself. This balance between forces creates a very dense stellar object that may have a mass comparable to the sun, but collapsed into a diameter comparable to the Earth. As such, white dwarfs can continue to shine for many billions of years.