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Astronomers once called them “faint white stars,” after the warm white glow of the first few they found. The name evolved into “white dwarf” and stuck. It turns out to be right on average and wrong at the edges. Seen from close enough to make out the disc, a handful of these dead stars would glow pink, cyan, butter yellow or a pale chartreuse green.
The green one is called WD J1330+6435. Mark Hollands, an astronomer at the University of Warwick in the U.K., and three colleagues worked out its color in a study published in the journal RAS Techniques and Instruments. They also flagged WD J1011+2845, a carbon-rich white dwarf about 48 light-years away whose deep salmon-pink hue likely makes it the reddest star known, in terms of the color a human eye would see. To the authors’ knowledge, no other star of any kind has been claimed to look green. That makes white dwarfs, so far, the only known way to get a green star.
White dwarfs are what’s left when most stars die. They’re the dense remnants that the majority of ordinary stars leave behind at the end of their lives. Ordinary stars get their color almost entirely from temperature, because they behave much like a blackbody, a physicist’s ideal radiator whose glow depends only on how hot it is. Cool ones look orange, sun-like ones look white and hot ones look pale blue. You can check this with your own eyes on a clear night: Betelgeuse and Antares look redder than their neighbors, while Sirius and Rigel lean slightly blue.
Their skies are as dense as water. The coolest white dwarfs can have atmospheres reaching about 1 gram per cubic centimeter, roughly the density of liquid water, which is absurd for the outer layer of a star. In that crush, traces of carbon, calcium or sodium absorb light across enormous stretches of the spectrum. A line that would be a thin dark stripe in the sun’s light can widen until it spans hundreds or even thousands of angstroms. Think of it as a pair of tinted glasses that block one big slice of the rainbow. Take out the orange, and what’s left no longer looks like a normal star.
Nobody can simply go and look. Apart from the sun, no star is close enough for its disc to be seen. Even through a telescope, faint stars don’t show their true colors, because the human eye loses much of its color sense in the dark. Most astronomical color pictures don’t help either. The Sloan Digital Sky Survey (SDSS), for example, maps three of its filters onto red, green and blue, though human vision only covers two of them. Let me explain what the team did instead. They took each star’s spectrum, the rainbow of its light broken down wavelength by wavelength, and ran it through the CIE 1931 color-matching functions, standard curves that describe how the eye responds to each wavelength. The result was then translated into sRGB, the color standard most computer screens use. Brightness was set aside on purpose: real stellar surfaces are far brighter than any monitor could ever show.
On average, white dwarfs really are white. To get a fair picture, the researchers took every known white dwarf within 20 parsecs of the sun, about 65 light-years, a complete local sample of 152 stars. The median color came out at about 6,800 kelvin in color temperature, very close to the 6,500-kelvin white point that screens use as their reference white. The common habit of drawing white dwarfs with a blue tint, the authors wrote, “to our amusement,” is unrepresentative. That blue image comes from hot, bright white dwarfs, which are easier to spot and so show up more often in surveys limited by brightness.
Only about one in 30 breaks the pattern. Of the 152 nearby stars, just five strayed far enough from blackbody colors to be set apart: two yellowish white dwarfs polluted with metals and three pink, carbon-rich ones. That works out to 3.4%. Everything else sat close to the normal run from orange to white to blue, with colors spanning roughly 3,000 to 25,000 kelvin.
Sodium turns one star green. WD J1330+6435 has a surface temperature of about 4,240 kelvin (roughly 7,170 degrees Fahrenheit), which on an ordinary star would mean an orange glow. But its atmosphere holds rocky debris from its old planetary system, including sodium, calcium and lithium. A strong, very broad sodium feature soaks up orange light, while calcium absorbs at violet wavelengths. What survives peaks around 500 nanometers, and the star comes out chartreuse green. “The traces of sodium in the atmosphere of this white dwarf absorb the orange part of the spectrum, leaving behind a ghostly green glow,” Hollands said in a statement. Two other stars in the study, WD J0851+1543 and WD J1351+2645, also look green, though much paler.
Carbon paints others pink and purple. In carbon-rich white dwarfs, carbon dredged up from the core forms C2 molecules. These absorb in wide bands, called Swan bands, right where the eye is most sensitive. Many of these stars come out in shades of pink, and some in pale purple. WD J1011+2845, estimated at about 4,300 kelvin, takes it furthest. Most of its visible light is restricted to wavelengths longer than 600 nanometers, while everything from 400 to 600 is heavily suppressed. The result is a deep salmon-pink, probably shaped by both its dense atmosphere and a strong magnetic field.
A cool star can even look blue. You know what’s stranger than a green star? A cold blue one. WD J2356-2054 sits at only about 4,000 kelvin, yet ultra-broad sodium absorption wipes out the middle of its visible spectrum and leaves light mostly between 450 and 500 nanometers. It comes out vivid blue, bluer than even an infinitely hot blackbody, so it can’t be given a color temperature at all. A different trick makes some stars cyan. In dense mixtures of hydrogen and helium, colliding molecules briefly absorb light they otherwise wouldn’t, a process called collision-induced absorption. It drains the red end, and stars such as WD J0039+3035 and WD J1403+4533 end up a clear cyan.
The team also painted portraits. Using model atmospheres, the researchers built color-accurate images of what several of these stars would look like from nearby. Stars look dimmer toward their edges, an effect called limb darkening, and in some white dwarfs the color shifts too. WD J1330+6435 is green at the center of its disc and turns orange in the outermost sliver. The authors expect such images to be useful for public outreach.
The palette comes with caveats. The unusual stars weren’t a random draw. The researchers hand-picked 15 cool white dwarfs with odd infrared behavior, 36 carbon-rich ones and 36 metal-polluted ones to show the widest range of colors, and they stress these groups aren’t representative. The spectrum of WD J1330+6435 is somewhat noisy, so its true color may differ slightly, though an older model of the star gave a similar green. Atmospheres this dense still challenge the best models, which is why the team used observed spectra for some groups, and one lithium-rich star, WD J1644+1234, couldn’t be modeled reliably and was left out. Finally, these are colors, not brightness. Even the faintest white dwarf in the study shines far brighter than any screen, and how the colors look on your device depends on its display settings.
More green stars may be waiting. WD J1330+6435 and WD J2356-2054 are close cousins: both are cool white dwarfs with helium atmospheres, and sodium dominates both spectra. The authors suspect a whole range of objects exists between them, which would mean other white dwarfs with vivid green, cyan and blue hues are out there for future surveys to find.
Source: Hollands, M. A., Gänsicke, B. T., Tremblay, P.-E., Koester, D. “Any colour you like: The diverse chromaticities of white dwarfs.” RAS Techniques and Instruments (2026). DOI: 10.1093/rasti/rzag069
