Astronomers analysing stellar spectra have discovered a white dwarf located 280 light-years away in the constellation Draco that glows with an unusual chartreuse-green hue, challenging long-held assumptions about stellar coloration.
For decades, the color of a star in the night sky served as a reliable astronomical thermometer. Cool stars burned orange, sun-like stars appeared white, and the hottest stellar bodies glowed pale blue. Astrophysics textbooks treated white dwarfs — the dense stellar remnants left behind when stars of low or intermediate mass exhaust their nuclear fuel — as followers of this same rule. New research published in the journal RAS Techniques and Instruments upends that premise, revealing that stellar remnants can take on unexpected hues when their extreme internal densities alter how elements absorb light.
Planetary Debris and Sodium Collisions in WD J1330+6435
Researchers first focused on the object because planetary material had fallen onto its surface, enriching its outer layers with calcium, lithium, and sodium. While normal stars possess atmospheres where sodium absorbs only a very narrow band of orange light, the physical reality of a white dwarf changes that mechanism entirely.
Because these compact stellar remnants pack roughly the mass of the Sun into a sphere comparable in size to Earth, their atmospheric densities reach extraordinary extremes. Surrounding helium atoms constantly bombard the sodium atoms, shifting their energy levels. Instead of absorbing just a single narrow wavelength, the altered sodium captures yellow, orange, and portions of red light, leaving behind a distinctive spectral signature.
Imagine standing on a planet and looking up at a green star. It feels like something out of science fiction.
Mark Hollands, astrophysicist at the University of Warwick
Modelling Human Colour Vision and Finding Salmon-Pink Stars
By passing these spectral fingerprints through mathematical models simulating human colour perception, the investigators calculated what each star would look like to the eye.
While WD J1330+6435 provides the first documented instance of a green star, another notable discovery emerged at a much closer distance. In that star, a deep convection zone lifts carbon from the core to the surface, creating carbon molecules that 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.
Atmospheric Composition Drives Diverse Stellar Hues
The study reveals that white dwarf colours fall into clear, chemically defined categories. The research team was led by Mark Hollands, an astrophysicist at the University of Warwick in England, with Boris Gänsicke, a Warwick professor, co-authoring the study published on Thursday, October 14, in the journal RAS Techniques and Instruments.
| White Dwarf | Distance (Light-Years) | Observed Colour | Primary Chemical Driver |
|---|---|---|---|
| WD J1330+6435 | 280 | Chartreuse-green | Sodium and helium interactions |
| WD J1011+2845 | 48 | Salmon-pink | Core carbon brought to surface |
Despite these unusual chromatic variations, researchers emphasize that white remains the most common appearance for the population as a whole. Extrapolating from their local sample, the team estimates that roughly one in thirty white dwarfs features an unconventional colour, leaving many oddly tinted stellar remnants scattered across the universe.