Astronomers Discover White Dwarfs Glow in Unexpected Colors Including Green

For centuries, astronomers have relied on a simple rule: a star's color tells you its temperature. The coolest stars glow orange. Stars like our Sun appear white. The hottest burn pale blue, the way iron shifts from red to orange to white as a blacksmith's forge grows hotter. White dwarfs—the ultra-dense remnants left when stars like our Sun die—were thought to follow this same principle. They got their name because the first ones discovered were surprisingly hot and glowed white or pale blue. But a team at the University of Warwick has now shown that this rule breaks down in the most extreme stellar environments.

The reason lies in density. A white dwarf's atmosphere can reach about one gram per cubic centimetre, roughly the density of water and millions of times denser than the Sun's outer layers. In these crushing conditions, trace elements present in the star—carbon, calcium, sodium—interact with the surrounding dense helium plasma in ways they never could elsewhere. They absorb light across a wider range of wavelengths than they would under normal pressure, shifting the star's color away from the expected orange, white, and blue palette. The researchers, led by Dr. Mark Hollands, a Warwick Prize Fellow in the Astronomy and Astrophysics Group, published their findings in the journal RAS Techniques and Instruments. "Most stars, including most white dwarfs, behave like perfect radiators, so their colour is set almost entirely by temperature," Hollands said. "But nobody had really looked into what else might be possible for white dwarfs. It turns out these incredibly dense stars can produce colours nobody expected, and some of them genuinely surprised me."

No white dwarf is close enough for its true color to be seen directly from Earth. Instead, the team extracted the information from data astronomers already collect routinely: a star's spectrum, the unique fingerprint produced when starlight is split apart by wavelength, like a prism creating a rainbow. They ran these spectra through a model of human color vision to calculate what each star's light would actually appear as to the human eye. The method revealed that white dwarf colors fall into distinct groups, each linked to what fills the star's atmosphere. Carbon-rich white dwarfs typically glow pink or purple. Those enriched with debris from shattered asteroids and comets—the remnants of old planetary systems—tend toward yellow. Cooler white dwarfs with mixed hydrogen and helium atmospheres often take on a cyan or pale blue tinge, caused by hydrogen molecules colliding with helium atoms in their ultra-dense environments.

The most striking discovery is WD J1330+6435, a star roughly 4,000 degrees Celsius that has accreted rocky material from its planetary system. This enrichment of calcium, lithium, and sodium in its atmosphere produces what the team's models show as a chartreuse green appearance. As far as the researchers are aware, no other star—living or dead—has ever been credibly reported as green, making this the first known genuinely green star in the Galaxy. "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. "Imagine standing on a planet and looking up at a green star. It feels like something out of science fiction." At the opposite extreme sits WD J1011+2845, a nearby white dwarf just 48 light years from Earth. It 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 nanometres, producing a deep salmon-pink glow unlike anything seen in an ordinary star.

But these vivid colors are not the norm. The team studied a representative sample of 151 white dwarfs within 20 parsecs—about 65 light years—of Earth. Only five showed unusual coloring: two yellow and three pink. This suggests that roughly one in thirty white dwarfs in the Universe will shine with an unusually colored light. For the remaining 146, the team measured the color temperature, a standard metric that compares an object's color to that of a perfect radiator and is commonly used to describe light bulbs. The median color temperature of the sample was approximately 6,800 Kelvin, which humans perceive as white. This finding corrects a widespread misconception in astronomy that white dwarfs are typically blue. The reason for the confusion is straightforward: hotter, bluer white dwarfs are brighter and can be seen from further away, so they are over-represented in surveys that rely on brightness alone.

Professor Boris Gänsicke, from the Department of Physics at Warwick, noted the broader implications. "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 coloured stars out there," he said. "Yet if we take the median across a representative population of white dwarfs, we found that the average white dwarf really is white." The research opens a new lens for understanding stellar remnants. By calculating the colors white dwarfs would display if viewed up close, astronomers can now infer information about their atmospheres and the planetary systems that once orbited them. A green star signals sodium absorption. A salmon-pink glow reveals carbon enrichment. Each unusual color is a window into the violent histories of these dense, ancient objects.