In a First, Astronomers Identify a Star That Might Glow Green. New Research Highlights the Strange Hues White Dwarfs Can Take on

The color of most stars indicates their temperature: Cool ones glow orange, warmer ones look white and the hottest appear blue. It turns out, however, that white dwarf stars—technically dense remnants of dead stars—don’t always follow that rule. For the first time, astronomers say they’ve spotted one that would look green to our eyes.

White dwarf (WD) J1330+6435, located some 280 light-years from Earth, seems to have accumulated rocky material from its surrounding planetary system. The acquisition has enriched its atmosphere with calcium, lithium and sodium, giving the stellar object a chartreuse hue. The findings, published in the journal RAS Techniques and Instruments on October 15, show that changes in chemistry can make some white dwarfs emit rare colors.

But there are some special cases where the physics in the outer layers of these stars are sufficiently strange that it causes actually quite big deviations in terms of their spectrum.” White dwarfs often contain roughly half the sun’s mass but are just a little bigger than Earth. That means the stellar remnants are around 200,000 times denser than Earth. In the study, Hollands and colleagues looked at previously collected data on more than 150 white dwarfs.

Because these stars are too far from us for their colors to be observed directly, the researchers calculated their hues by using their spectra, the different wavelengths within their light. It’s like the rainbow produced when white light passes through a prism. A star’s spectrum is influenced by its temperature, chemical composition and other factors.

The team ran the white dwarfs’ spectra through a model of human vision to find out how the objects might look to the human eye. A handful of them had unusual coloring, including yellow, purple and pink, thanks to the interactions between the gases in their atmospheres and other trace elements. White dwarfs have incredibly dense atmospheres, up to about a gram per cubic centimeter, comparable to the density of liquid water.

That means even a tiny amount of an element, like calcium or sodium, in a white dwarf’s atmosphere interacts more with the surrounding helium plasma and absorbs more wavelengths of light than it would at lower densities, according to a statement. The property allows colors to shift in unusual ways. In the case of WD J1330+6435, “the end result is that the sodium atoms no longer absorb one very narrow wavelength of orange—they absorb the yellows, oranges and some of the red ​wavelengths,” Hollands tells Will Dunham at .