University of Warwick astronomers reported new colors of white dwarf stars, including the first star ever found that would appear green to the human eye. Dr Mark Hollands led the work and said the result came from stars so dense that they can look very different from the white, blue or orange often expected.
He said: "Most stars, including most white dwarfs, behave like ‘perfect radiators,’ so their colour is set almost entirely by temperature. 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." The study was published in RAS Techniques and Instruments.
WDJ1330+6435 and color
The team calculated the colors from spectra and a model of human color vision because no white dwarf star is close enough for its color to be seen directly. That approach let the researchers sort white dwarfs into green, pink, cyan, yellow and purple, rather than stopping at the older temperature-based rule.
White dwarf atmospheres can reach around 1 gram per cubic centimetre in the coolest cases. In those conditions, trace elements such as carbon, calcium and sodium can interact with dense helium plasma and absorb light across a wider range of colors, shifting what the star would look like to a human observer.
University of Warwick spectra
The study also links each color to a different atmospheric mix. Carbon-rich white dwarfs are typically pink or purple, white dwarfs enriched with debris from shattered asteroids and comets tend toward yellow, and cooler white dwarfs with mixed hydrogen and helium atmospheres often take on a cyan color.
The clearest outlier is WDJ1330+6435. The white dwarf has accreted sodium-rich rocky material from its planetary system into its atmosphere, and the sodium absorbs light over a broad range of yellow, orange and red wavelengths. That leaves WDJ1330+6435 with a ghostly green appearance, making it the first green star ever identified for the human eye.
For readers following the finding, the practical takeaway is straightforward: white dwarf color is not fixed by temperature alone. The next question left by the study is how common green-appearing white dwarfs are across the sky.
