A green star may well exist. And it belongs to a family whose name gives no hint of it: white dwarfs.
Usually, a star's color depends mainly on its temperature. The coolest stars tend toward orange or red, while the hottest appear white or bluish. The Sun emits largely white light. It is our atmosphere that gives it the slightly yellow hue we know, sometimes tending toward orange when the thickness of the atmosphere the light passes through is significant, for example near the horizon.
White dwarfs are the extremely dense cores left behind by stars after they have lost their outer layers. Within a volume comparable to Earth's, a white dwarf can concentrate a mass close to that of the Sun. Its matter is therefore subject to conditions very different from those inside an ordinary star.
Astronomers from the University of Warwick studied the light emitted by these stars. They broke it down into different colors. They then calculated the hue the human eye would perceive if we could observe these objects from sufficiently close by.
The result holds a few surprises. Some carbon-rich white dwarfs could appear pink or purple. Others, whose atmospheres contain remnants of rock from ancient planets or asteroids, take on yellowish, bluish or even green hues.
The most surprising example is WD J1330+6435. This dead star has accumulated rocky matter from its former planetary system, enriching its atmosphere with elements such as calcium and sodium. These elements absorb a large part of certain colors. The remaining light would give it a pale green appearance, close to chartreuse green.
Another white dwarf, WD J0902+2010, should appear pink. In this case, carbon brought up from the depths of the star absorbs certain colors of light. The phenomenon is different, but the result is just as unexpected for a star.
It should be made clear that no one has directly photographed these stars as green or pink disks. They are too far away for that. The colors presented by the researchers come from measurements of their light and calculations that take our vision into account. They are therefore scientific reconstructions of their appearance.
Most white dwarfs nevertheless remain white. In a sample of 151 nearby stars, only five had truly unusual colors. These rare exceptions show that a star's color is not determined solely by its temperature: the matter surrounding it can also profoundly transform the light it sends us.