University of Warwick astronomers have shown that white dwarf stars can glow in a remarkable range of colours, including the first star ever found that would appear green to the human eye.
For most stars, colour is a guide to temperature. Cooler stars look orange, stars like our Sun look white, and the hottest glow pale blue, much like a blacksmith’s iron glowing red, then orange, then white-hot as it heats up. White dwarf stars, the dense remnants left behind when Sun-like stars reach the end of their lives, were thought to follow the same rule. They earned their name because the earliest examples found were surprisingly hot and glowed white/pale blue. But are orange, white and blue the only colours a star in the night sky can be?
The answer, according to new research from the University of Warwick, is no. Published in the journa RAS Techniques and Instruments (RASTI) , the new study shows that if we could view them up close, certain white dwarf stars would shine in colours such as green, pink, cyan, yellow and purple.
“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,” said Dr Mark Hollands, Warwick Prize Fellow in the Astronomy and Astrophysics Group, University of Warwick, who led the work. “It turns out these incredibly dense stars can produce colours nobody expected, and some of them genuinely surprised me.”
Why white dwarfs break the rule
The key is density. The coolest white dwarf atmospheres can reach around 1 gram per cubic centimetre, roughly the density of water and millions of times denser than the Sun’s photosphere. In these conditions, any trace elements that may be present, such as carbon, calcium and sodium, interact with the surrounding dense helium plasma and absorb light across a wider range of colours than they would at lower densities. This shifts a star’s colour away from the orange, white and blue of a perfect radiator.
No white dwarf star is close enough for its colour to be seen directly, so the team calculated it instead from data astronomers already collect routinely: a star’s spectrum, the ‘fingerprint’ produced when starlight is split apart by wavelength, much like a prism splits sunlight into a rainbow. Running these spectra through a model of human colour vision revealed what colour each star’s light would appear as to the eye. It is a reminder of just how much a spectrum can reveal: not just a star’s temperature or its chemical makeup, but its true colour as well.
Any colour you like
Applying this method to white dwarfs across the sky, the team found that their colours fall into distinct groups, each linked to what is in the star’s atmosphere. Carbon-rich white dwarfs are typically pink or purple, while those enriched with debris from shattered asteroids and comets, the remains of old planetary systems, tend towards yellow. Cooler white dwarfs with mixed hydrogen and helium atmospheres often take on a cyan or pale blue tinge, caused by hydrogen molecules colliding with helium atoms in their ultra-dense atmospheres.
The most striking example is WD J1330+6435, which is roughly 4,000 degrees Celsius and has accreted rocky material from its planetary system. This has enriched its atmosphere with calcium, lithium, and sodium, giving it a chartreuse green appearance in the team’s models. As far as the researchers are aware, no other star, living or dead, has ever been credibly reported as green, making this the first known genuinely green star in the Galaxy.
Dr Hollands added: “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.”
At the opposite extreme, the team identified what may be the reddest star ever recorded. WD J1011+2845, a nearby white dwarf just 48 light years from Earth, has a deep convection zone that has lifted carbon from its core to its surface. The resulting carbon molecules 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 unlike anything seen in an ordinary star.
The average is still white
The team also studied a representative sample: the 151 white dwarfs within 20 parsecs (about 65 light years) of Earth. Five showed unusual colouring, two yellow and three pink. This suggests that roughly one in thirty white dwarfs in the Universe will shine with an unusually coloured light.
For the remaining 146, the team measured the colour temperature, which compares the colour of an object with that of a perfect radiator and is commonly used to describe light bulbs (‘warm white’ at 3,000K, ‘cool white’ at 6,000K). The median colour temperature of the sample was approximately 6,800 Kelvin, which we perceive as white. This corrects a common misconception in astronomy that white dwarfs are typically blue: hotter, bluer white dwarfs are brighter and can be seen from further away, so they are over-represented in surveys that rely on brightness alone.
Professor Boris Gänsicke, Department of Physics, University of Warwick, said: “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. Yet if we take the median across a representative population of white dwarfs, we found that the average white dwarf really is white.”
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