The Green Star
Stars come in a short list of colors: red, orange, yellow, white and blue. For most of them, color is set almost entirely by temperature. Cooler stars lean red. Hotter ones lean blue.
Green has never been on the list. As far as the researchers know, no star had ever been credibly reported as green.
Now astronomers at the University of Warwick, in a study led by Mark Hollands and published this week in the journal RAS Techniques and Instruments, have found one. They calculated that a white dwarf called WD J1330+6435, a few hundred light-years away, would look green, a shade coverage has called a ghostly chartreuse.
What a white dwarf is
A white dwarf is what remains after a star like the Sun reaches the end of its life. The outer layers are gone, and what's left is a small, extremely dense core.
WD J1330+6435 has cooled to about 4,000 °C. The atmospheres of the coolest white dwarfs are so compressed that they reach about a gram per cubic centimeter, roughly the density of water and millions of times denser than the visible surface of the Sun.
That density turns out to matter for its color.
Why this one is green
The color comes from what the star took in. At some point, WD J1330+6435 pulled rocky material from its own planetary system into its atmosphere, which now carries calcium, lithium and sodium.
Sodium is the key. In an ordinary star it absorbs a narrow slice of orange light. In this star's dense atmosphere it absorbs a much wider band, taking out yellow, orange and some red. What's left reaches the eye as green. Hollands put it plainly: the traces of sodium soak up the orange part of the spectrum.
In other words, the first green star is green because of what it ate.
How do you see a color no one has seen?
No one has actually looked at this star and seen green. White dwarfs are too far away to view up close, so the team calculated the colors instead.
They started with spectra, the detailed breakdown of a star's light that astronomers already collect as a matter of routine. Then they ran those spectra through a model of human color vision to work out what a person would see.
Behind it, Hollands has said, was a simple curiosity: what would these stars look like if you could stand near one?
A whole palette, mostly white
Green wasn't the only surprise. A white dwarf called WD J1011+2845, about 48 light-years away, may be the reddest star ever recorded. Carbon dredged up from its core gives it a deep salmon pink.
The models also predict pink or purple for carbon-rich white dwarfs, yellow for ones polluted by debris from asteroids and comets, and cyan or pale blue for some cooler ones. Sirius B, the white dwarf nearest the Solar System, comes out pale blue.
Odd colors are rare. Of 151 white dwarfs within about 65 light-years of Earth, five came out unusual: two yellow and three pink. The team estimates that about one in 30 white dwarfs is oddly colored. Co-author Boris Gänsicke noted that the typical one really is white.
Where to see them
You can't see WD J1330+6435's green for yourself, but you can see the team's version. Warwick's announcement includes color-accurate simulated images of the stars, side by side, and the full paper is in RAS Techniques and Instruments.
The timing helped the story travel: Space.com called the star a cosmic zombie, just in time for Halloween.
So the list grows by one: red, orange, yellow, white, blue and, for one star with an unusual diet, green.
