For decades astronomers repeated a simple fact: there are no green stars. That changed this week, when a team led by Dr Mark Hollands of the University of Warwick reported the first star that would appear green to human eyes.

According to CNN, lead author Hollands explained that normal stars are limited to orange, white or blue, but white dwarfs in special cases show big deviations in their spectrum. Study published in RAS Techniques and Instruments (RASTI).

Star color is normally set almost entirely by temperature, so the stellar palette was thought to run from red through white to blue and nothing more. '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,' Hollands said, adding that some of the colors 'genuinely surprised me.' The key is density: the coolest white dwarf atmospheres reach about the density of water, millions of times denser than the Sun's photosphere.

At those pressures, trace elements like sodium absorb a far wider range of light — sodium alone absorbs the orange part of the spectrum, 'leaving behind a ghostly green glow,' Hollands said, a scene he likened to science fiction.

No white dwarf is close enough to observe its color directly, so the team worked from spectra — the 'fingerprint' produced when starlight is split by wavelength, much as a prism splits sunlight into a rainbow.

According to BBC Sky at Night Magazine, the researchers ran each star's spectrum through a model of human color vision to determine what the light would look like to the eye.

Hollands surveyed the 151 known white dwarfs within about 65 light-years of Earth, and CNN reports the green star's main elements are calcium, sodium and lithium.

Agnès Kim, an astrophysicist at Penn State Scranton who was not involved, told CNN the study is 'a cool intersection of physics and biology.'

The color is also a record of planetary destruction. CNN reports that when an asteroid ventures too close to a white dwarf, it is 'shredded up into dust' that falls onto the star and mixes into its outer layers, letting astronomers measure the chemistry of the rocky material.

'The white dwarf is taking apart the planets, and we're able to see what was actually inside that planet,' said Adam Burgasser of UC San Diego, who was not involved in the study. 'It is a crime scene, and we're kind of piecing through the evidence.'

The green star's sodium- and calcium-rich debris suggests it recently consumed the remains of its own planetary system.

The green star is not the only oddball. The study also identified what BBC Sky at Night Magazine calls possibly the reddest star ever recorded: WD J1011+2845, just 48 light-years away, whose deep convection zone lifts carbon from its core to the surface, producing a deep salmon-pink glow.

Carbon-rich white dwarfs tend toward pink or purple, debris-fed ones lean yellow, and cooler hydrogen-helium hybrids show cyan or pale blue tinges. Still, out of the 151 white dwarfs surveyed only five showed unusual colors — two yellow and three pink — meaning roughly one in thirty may shine oddly.

'Across the entire population of white dwarfs in the Universe, that is still a lot of oddly colored stars out there,' co-author Boris Gänsicke said, noting the average white dwarf really is white.

Hollands plans to keep analyzing white dwarf spectra, and the outlook is expanding fast.

'We'll be getting a tenfold increase in the number of white dwarfs that have spectra,' he told CNN, pointing to new spectroscopic surveys about to deliver data — and possibly going 'from one or maybe two green white dwarfs to 10 or 20.'

Burgasser told CNN that the study's classification scheme by temperature and atmospheric properties could help the field organize why certain colors appear and whether the stars are compositionally related.

For now, the finding stands as a reminder that the dense husks of dead stars can paint the galaxy in colors nobody expected.