First Truly Green Star Found: A Dead Sun Glowing Chartreuse

Under those conditions, trace elements like carbon, calcium, and sodium interact with the surrounding helium plasma. In plain terms, that's a soup of electrically charged helium gas so squeezed that atoms behave differently. These interactions let the elements absorb a much wider range of light wavelengths than they could in a thinner atmosphere.

The light that escapes can then carry colors temperature alone would never predict.

Nobody has ever seen a white dwarf's color directly — none sits close enough for the human eye to judge it. So the team worked from stellar spectra: measurements made by splitting starlight into its component wavelengths, the way a prism divides sunlight into a rainbow. Each spectrum is a fingerprint of a star's temperature and chemistry. The researchers ran that data through a model of human color vision to calculate what observers would actually perceive.

Which colors turned up?

Sorting white dwarfs across the sky by atmospheric chemistry produced distinct color groups:

• Carbon-rich white dwarfs often appear pink or purple.
• Stars that have swallowed debris from destroyed asteroids and comets tend to look yellow.
• Cooler stars with hydrogen and helium atmospheres can turn cyan or pale blue, triggered by collisions between hydrogen molecules and helium atoms under immense pressure.

Then there's the green one. WD J1330+6435 pulled in rocky debris from its former planetary system, loading its atmosphere with calcium, lithium, and sodium. The sodium is the key.

"The traces of sodium in the atmosphere of this white dwarf absorb the orange part of the spectrum, leaving behind a ghostly green glow. Imagine standing on a planet and looking up at a green star. It feels like something out of science fiction," Hollands said.

As far as the team knows, no other star — living or dead — has ever been credibly identified as genuinely green. That makes this object the first known green star in the Milky Way.

Is there an opposite extreme?

Yes. WD J1011+2845, a white dwarf just 48 light-years from Earth, may be the reddest star ever documented. It has a deep convection zone — a churning layer that carries carbon from the star's interior to its surface. Carbon molecules there shift nearly all escaping light to wavelengths longer than 600 nanometers, producing a deep salmon-pink hue unlike anything in ordinary stars.

How common are oddly colored white dwarfs?

The team checked a representative sample of 151 white dwarfs within 20 parsecs — about 65 light-years — of Earth. Five had unusual colors: two yellow, three pink.

The remaining 146 were analyzed by color temperature, the same measure used to label light bulbs as "warm white" at 3,000K or "cool white" at 6,000K. Their median color temperature came out around 6,800 Kelvin, which the eye sees as white.

That result corrects a survey bias. Bluer, hotter white dwarfs shine brighter and stay visible across greater distances, so they dominate brightness-selected catalogs and created the impression that white dwarfs are usually blue. In a fair, nearby sample, the typical white dwarf is white after all.

"One in thirty might not sound like a huge fraction, but across the entire population of white dwarfs in the Universe, that is still a lot of oddly colored stars out there," said Professor Boris Gänsicke of Warwick's Department of Physics. "Yet if we take the median across a representative population of white dwarfs, we found that the average white dwarf really is white."

The colors are calculated, not observed with the eye, and the one-in-30 figure is an extrapolation from a small nearby sample. Even so, the message is simple: dead stars have a far wider wardrobe than astronomers thought.