Science fiction has been painting alien skies with strange suns for decades, and now astronomers have found one that fits right in. A team led by Dr. Mark Hollands at the University of Warwick has identified what it believes is the first star ever found that would appear green to the human eye.
The star is a white dwarf called WD J1330+6435, and according to the team's models, it would glow a pale chartreuse green if you could get close enough to see its disc. The findings were published last week in the journal RAS Techniques and Instruments in a paper titled Any Colour You Like: The Diverse Chromaticities of White Dwarfs.
"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 in the university's announcement.
Why Stars Are Usually Orange, White, Or Blue
For most stars, color is basically a thermometer. Cooler stars look orange, stars like our Sun look white, and the hottest ones glow a pale blue. That is because most stars behave like what physicists call perfect radiators, where temperature does almost all the work in setting their color.
"But nobody had really looked into what else might be possible for white dwarfs," Hollands explained. "It turns out these incredibly dense stars can produce colours nobody expected, and some of them genuinely surprised me."
White dwarfs are the dense leftover cores of Sun-like stars that have reached the end of their lives. The coolest of them have atmospheres that reach roughly the density of water, millions of times denser than the visible surface of the Sun. In those extreme conditions, trace elements like carbon, calcium, and sodium absorb light across a much wider range of colors than they normally would, pushing the star's color away from the usual orange-white-blue range.
A Star Colored By Shredded Planets
WD J1330+6435 sits at roughly 4,000 degrees Celsius and has pulled in rocky material from its old planetary system. That debris has enriched its atmosphere with calcium, lithium, and sodium. In the paper, the team explains that the broad sodium absorption eats away at orange wavelengths while calcium absorbs at violet ones, leaving the star's light peaking around 500 nanometers, right in the green.
Speaking to CNN, Hollands said white dwarfs can act as a diagnostic tool for measuring what those destroyed planets were made of, since the shredded material mixes into the star's outer layers.
Since no white dwarf is close enough to see its color directly, the team worked it out from spectra, the light fingerprints astronomers already collect, and ran them through a model of human color vision. As far as the researchers know, no other star of any kind has ever been credibly reported as green.
Pink, Purple, And Cyan Stars Too
The green star is the headliner, but it has some colorful company. Carbon-rich white dwarfs tend to look pink or purple, debris-polluted ones lean yellow, and cooler white dwarfs with mixed hydrogen and helium atmospheres can take on a cyan tinge. One nearby white dwarf, WD J1011+2845, about 48 light years away, may be the reddest star ever recorded, with a deep salmon-pink glow.
The team also looked at the 151 white dwarfs within about 65 light years of Earth. Only five had unusual colors, which suggests roughly one in thirty white dwarfs shine with an odd hue. The rest came out to a median color temperature of about 6,800 Kelvin, meaning the average white dwarf really is white.
"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," said co-author Professor Boris Gansicke.
Hollands told CNN that upcoming spectroscopic surveys should deliver a tenfold increase in white dwarfs with spectra, and that the team might go from one or two green white dwarfs to 10 or 20. For writers looking to give their next alien world a sky that feels truly different, the universe just made the job a lot easier.
Would you rather look up at a green sun or a salmon-pink one on an alien world?
Let us know your thoughts in the comments below.