The First Green Star: White Dwarfs Glow Pink, Purple, Cyan, and Salmon

One of the simplest rules in astronomy is that a star's color reveals its temperature. Cool stars glow orange, stars like the Sun look white, and the hottest burn pale blue, the same progression as a blacksmith's iron heating from red to white-hot. White dwarfs, the dense stellar corpses left behind when Sun-like stars exhaust their fuel, earned their name because the first ones discovered glowed white. A new study suggests that was only the opening chapter.

Astronomers at the University of Warwick have shown that white dwarfs can shine in green, pink, cyan, yellow, and purple. The team ran the light spectra of white dwarfs through a model of human color vision to calculate what each star would actually look like to the eye, and found at least one that would glow green: the first credibly green star ever reported, living or dead. Published in the journal RAS Techniques and Instruments, the work suggests the most common stellar remnants in the sky are far more colorful than anyone suspected.

The first green star

The standout is WD J1330+6435, a white dwarf roughly 4,000 degrees Celsius (7,232 degrees Fahrenheit) that has been eating rocky material from its own planetary system. The accreted debris has enriched its atmosphere with calcium, lithium, and sodium, and in the star's crushing atmosphere the sodium absorbs light across broad bands of yellow, orange, and red. What survives the absorption is a chartreuse green glow. Dr. Mark Hollands, the Warwick Prize Fellow who led the work, described the effect as ghostly: sodium eating the orange part of the spectrum and leaving green behind. As far as the researchers are aware, no other star, living or dead, has ever been credibly reported as green.

Why density breaks the rule

The key is density. The coolest white dwarf atmospheres reach about 1 gram per cubic centimeter, roughly the density of water and millions of times denser than the Sun's photosphere. In those conditions, trace elements such as carbon, calcium, and sodium interact with the surrounding dense helium plasma and absorb light across a much wider range of colors than they would at lower densities. Temperature stops being the whole story.

No white dwarf is close enough for its color to be seen directly, so the team worked from data astronomers already collect routinely: spectra, the fingerprints produced when starlight is split apart by wavelength, much like a prism splits sunlight into a rainbow. Running each spectrum through a model of human color vision revealed the chromaticity, the precise color quality of the light as the human eye would perceive it.

The sodium in the atmosphere absorbs the orange part of the spectrum, leaving behind a ghostly green star. No star has ever looked like this.

Enriched from above, enriched from below

The colors fall into distinct groups, each tied to a different way a white dwarf's atmosphere gets polluted. WD J1330+6435 is enriched from above: rocky debris from a shattered planetary system rains down and seasons the atmosphere with metals. White dwarfs polluted by the wreckage of asteroids and comets tend toward yellow, with the exact hue depending on what fell in and how hot the star is.

Others are enriched from below. WD J0902+2010 has a deep convection zone in its helium-dominated outer envelope that has reached all the way down to the white dwarf's carbon core, dredging carbon to the surface where it forms diatomic carbon molecules. These C2 molecules absorb light across the optical range, including blue, green, and yellow wavelengths, leaving a pink glow. Carbon-rich white dwarfs are typically pink or purple. Still others, cooler white dwarfs with mixed hydrogen and helium atmospheres, take on a cyan or pale blue tinge, caused by hydrogen molecules colliding with helium atoms in their ultra-dense atmospheres.

The reddest star on record

At the opposite extreme from the green star, the team identified what may be the reddest star ever recorded. WD J1011+2845, a white dwarf just 48 light-years from Earth, has a deep convection zone that has lifted carbon from its core to its surface. The resulting carbon molecules distort the star's light so heavily that almost all of it is squeezed into wavelengths beyond 600 nanometers, producing a deep salmon-pink glow unlike anything seen in an ordinary star.

The average is still white

To see how common these oddballs are, the team studied a representative sample: the 151 white dwarfs within 20 parsecs (about 65 light-years) of Earth. Five showed unusual coloring, two yellow and three pink. That suggests roughly one in 30 white dwarfs in the universe shines with an unusually colored light. For the remaining 146, the team measured the color temperature, the comparison against a perfect radiator familiar from light-bulb packaging (warm white at 3,000 K, cool white at 6,000 K). The median came out at approximately 6,800 kelvin, which we perceive as white.

That result also corrects a widespread misconception in astronomy: the idea that white dwarfs are typically blue. Hotter, bluer white dwarfs are brighter and can be seen from farther away, so they are overrepresented in surveys that rely on brightness alone. Take a fair census of the neighborhood and the average white dwarf really is white, just as Professor Boris Gänsicke of Warwick noted, even if one in 30 glows some other color entirely.

Seeing what spectra have been hiding

The study doubles as a demonstration of how much information a spectrum carries. Beyond temperature and chemical makeup, a spectrum encodes a star's true chromaticity, and the team used its spectral models to generate color-accurate images of white dwarf discs as they would appear up close. Sirius B, the nearest white dwarf to the solar system, with its hot hydrogen atmosphere, comes out pale blue. WDJ1147-7449, one of the coolest nearby white dwarfs with a featureless spectrum, comes out orange.

The paper is titled "Any colour you like: The diverse chromaticities of white dwarfs," and it lands as both a census and a correction. Most white dwarfs behave like perfect radiators, so their color is set almost entirely by temperature. But in the densest stellar atmospheres in nature, chemistry gets a vote too, and sometimes that vote is green.