Ask an astronomer for a green star and you will usually get a polite no. Stars come in reds, oranges, whites and blues, but nobody had ever credibly reported a green one. Researchers at the University of Warwick now say they have found an exception: a white dwarf called WD J1330+6435, some 280 light-years away.
The reason green stars are so rare is simple physics. A star pours out light across a broad band of wavelengths. Even one whose output peaks in green still emits lots of red and blue light as well, and the eye blends the mix into white. White dwarfs, the compact leftover cores of Sun-like stars, were assumed to play by the same rules.
What changes the picture is density. In the coolest white dwarfs the atmosphere can be about as dense as water, and in that crowded helium soup trace elements soak up far wider slices of the spectrum than they would elsewhere. WD J1330+6435 has swallowed rocky debris from its old planetary system, loading its skies with sodium, calcium and lithium. The sodium strips out much of the orange light, and what remains would look chartreuse.
No one can actually see it, of course. The team led by Mark Hollands worked out the colour from the star’s spectrum, feeding the data into a model of human vision. Hollands described the result as “a ghostly green glow” and invited readers to imagine such a star rising over an alien world.
The paper, which appears in the Royal Astronomical Society journal RASTI, finds a broader rainbow too: carbon-rich white dwarfs come out pink to violet, and those polluted by asteroid fragments lean yellow. In a census of 151 nearby white dwarfs, all closer than roughly 65 light-years, five showed odd hues, suggesting roughly one in thirty glows in something other than plain white.
