Astronomers found a star that a human would see as green: why this is nearly impossible

There are almost no green stars in space — the physics of starlight usually does not allow the human eye to see such a color. But the white dwarf WD J1330+6435 turned out to be an exception: calculations show that up close a human would see it as green, with a chartreuse tint.

The reason turned out to be unusual. Traces of sodium in the star's super-dense atmosphere absorb a significant portion of yellow, orange, and red light, leaving a spectrum that our vision would perceive as green.

Researchers from the University of Warwick call WD J1330+6435 the first known star that should actually look green to the human eye.

Why there are almost no green stars

The color of most stars primarily depends on their surface temperature.

Cooler ones look reddish or orange, stars like the Sun are white, and the hottest acquire a bluish tint.

At the same time, a heated star emits light over a wide range of wavelengths at once. Even if the maximum emission falls in the green part of the spectrum, neighboring colors mix, and the human eye perceives such light as white, not green.

That is why astronomers long believed that truly green stars should practically not exist.

What made WD J1330+6435 green

WD J1330+6435 is a cool white dwarf with a temperature of about 4000 degrees Celsius. Calcium, lithium, and sodium were found in its atmosphere.

According to the researchers, these elements reached the star together with rocky material — remnants of objects from a former planetary system.

The atmosphere of a cool white dwarf is so dense that atoms in it interact differently than in ordinary stellar gas. Because of this, sodium begins to absorb a much wider range of light.

In the case of WD J1330+6435, the yellow, orange, and red parts of the spectrum are especially strongly suppressed.

After that, the remaining light shifts into the region that the human eye would perceive as green with a yellowish tint — approximately chartreuse.

“Traces of sodium in the atmosphere of this white dwarf absorb the orange part of the spectrum, leaving behind a ghostly green glow,” explained study leader Mark Hollands from the University of Warwick.

But there is a nuance: astronomers did not see it green directly

Scientists did not observe WD J1330+6435 as a bright green dot through a telescope.

White dwarfs are too small and faint, and the distances to them too great, for the human eye to directly discern their actual color.

Therefore, the researchers measured the star's spectrum — that is, the distribution of its light across different wavelengths — and then calculated how this light would be perceived by the human visual system.

It was the modeling that showed that with sufficient brightness and distance, WD J1330+6435 would look green.

This is an important clarification: we are talking not about a green photograph of the star, but about a physically calculated color that the human eye would see.

White dwarfs turned out to be much more colorful than expected

The green star was the most unusual result of the study, but not the only one.

The team examined 151 white dwarfs within about 65 light-years of Earth and found five objects with noticeably unusual colors — including green, yellow, and pinkish tints.

According to the authors' estimate, roughly one in 30 white dwarfs may have a color noticeably different from the usual white.

The reasons vary. A large amount of carbon can make a star pink or purple, and features of atmospheric composition and density are capable of shifting the color into the blue-green region.

One of the objects, WD J1011+2845, turned out to be a candidate for one of the reddest known stars: the features of its carbon spectrum should give it a rich pinkish-salmon tint.

At the same time, most of the studied white dwarfs still remained truly white: pronounced color deviations were found only in a small part of the sample.

WD J1330+6435 is interesting precisely as a rare exception. It shows that the usual rule “temperature determines the color of a star” can be broken if the chemical composition and extreme density of the atmosphere strongly enough alter the light passing through it.

The study “Any colour you like: The diverse chromaticities of white dwarfs” is published in the journal RAS Techniques and Instruments.

Based on materials from: University of Warwick, RAS Techniques and Instruments.