Astronomers discover first 'green' star among white dwarf remnants

For the first time, astronomers have identified a white dwarf star that would glow green if a human observer could see it from Earth. The discovery, led by Mark Hollands, an astrophysicist at the University of Warwick, emerged from analysis of stellar spectra—the light signatures that reveal a star's composition when split across wavelengths, much as a prism divides sunlight into a rainbow.

White dwarfs are the dense, cooling remnants of sun-like stars after they have exhausted their fuel and shed their outer layers. What makes them unusual is their capacity to display colors far beyond the narrow palette available to ordinary stars. Normal stars, Hollands explained, appear only in shades of orange, white, or blue, their hues determined entirely by temperature. White dwarfs, by contrast, can produce an astonishing range of colors because of exotic physics occurring in their thin outer atmospheres. When helium in those layers interacts with trace elements—iron, magnesium, sodium, and other materials—the resulting spectrum deviates sharply from what temperature alone would predict.

The source of these foreign elements tells a story of cosmic violence. Rocky material from nearby planetary systems, or fragments of asteroids drawn too close, gets torn apart by the white dwarf's gravity. This debris—dust and fragments—falls onto the star's surface and mixes into its outer layers. By studying the composition of this accreted material, astronomers can reverse-engineer what the destroyed planets were made of, effectively using the white dwarf as a forensic tool. Adam Burgasser, a professor of astronomy at the University of California San Diego, described the process as reading a crime scene: piecing together evidence left behind to reconstruct the planet's composition and formation history.

For this study, Hollands examined 151 known white dwarfs within 65 light-years of Earth. The green star's distinctive color arises from the presence of calcium, sodium, and lithium in its atmosphere. Another notable object appeared almost salmon pink, colored by an unusually high density of carbon molecules. None of these white dwarfs can be observed directly with the naked eye from Earth; Hollands calculated their apparent colors by analyzing their spectral data and determining what wavelengths would reach a human observer's eye.

The finding has drawn interest not only for its novelty but for what it reveals about how astronomers can bridge the gap between abstract data and human perception. Hollands noted that the work adds a tangible dimension to studying objects so distant that researchers can feel disconnected from them. Agnès Kim, an astrophysicist at Penn State Scranton, praised the study's approach as a compelling intersection of physics and biology—the translation of spectrum into what human eyes would perceive.

Burgasser emphasized that the color classification scheme, organized by temperature and atmospheric properties, offers a practical framework for future research. By grouping white dwarfs by their visual appearance, astronomers can ask whether objects of similar color share compositional or evolutionary relationships, potentially organizing existing data in a more intuitive way.

The work points toward a significant expansion of this research. Hollands is planning to analyze more white dwarf spectra as new spectroscopic surveys come online, with technological advances enabling observations of stars even farther from Earth. He expects the number of white dwarfs with available spectral data to increase tenfold. If that projection holds, the single green white dwarf discovered so far may soon be joined by ten to twenty others, opening new avenues for understanding how planetary systems form and die.