White dwarf stars may glow pink, yellow or green

Some white dwarf stars may not be white at all. Stars look hotter or cooler depending on their surface temperature. Hotter ones look blue or white, while cooler ones look red or orange. This is the blackbody principle: an object that absorbs all radiation glows with a color set only by its temperature. White dwarfs have much denser atmospheres than stars like the Sun, so they are normally expected to look white.

Colors driven by atmospheric elements

A team led by the University of Warwick used computer models to study 151 white dwarfs within 20 parsecs (about 65 light-years) of Earth. They tested whether elements such as carbon, calcium or sodium in the stars' very dense atmospheres could change their colors. The models showed that five of the 151, about 3.3 percent, could look pink or yellow instead of white. Two were yellow and three were pink, and the pink ones were linked to carbon. Separately, white dwarfs with debris from their original planetary systems tend to look green.

The findings suggest white dwarfs are not as simple as once thought. Dr. Mark Hollands, the study's lead author, said brown dwarfs may also break the blackbody rule. Brown dwarfs are too cool to fuse hydrogen, and their dense atmospheres can absorb light from sodium, potassium and titanium oxide. Some astronomers who have studied brown dwarfs have concluded that they can appear violet.