White dwarfs — the remnants of stars like the Sun — may glow not only white or bluish, but also pink, yellow, turquoise, and even green. That is the conclusion reached by astronomers at the University of Warwick. They found that the color of these stellar “remains” depends not only on temperature, but also on the chemical composition of their dense atmospheres.

One example is the white dwarf WD J1330+6435, located about 326 light-years from Earth. It is accreting material from a rocky celestial body, and the buildup of chemical elements, including sodium, calcium, and lithium, may have given it a greenish tint. If the researchers’ calculations are correct, this is the first known case of a star with such a color, Space.com reports.

Why white dwarfs can change color

The color of stars is usually linked to their temperature. Relatively cool stars appear orange or reddish, the Sun is perceived as white, and the hottest stars have a bluish hue. White dwarfs were thought to follow the same rule.

White dwarfs form when stars comparable in mass to the Sun exhaust the fuel for thermonuclear reactions. After that, their cores collapse under their own gravity, leaving behind an extremely dense stellar remnant.

The matter in a white dwarf is packed so tightly that its properties differ markedly from those of matter in ordinary stellar atmospheres. Under such conditions, chemical elements can interact with light differently — for example, by absorbing certain parts of the spectrum and thereby changing the star’s visible color.

A team led by Mark Hollands of the University of Warwick modeled how the composition of white dwarf atmospheres affects their color. The results showed that these objects may have a much more diverse palette than previously thought.

How dead stars become green

One unusual mechanism is linked to the accretion of material from planets, asteroids, and comets that survived the death of their parent star. The debris enters the white dwarf’s atmosphere and changes its chemical composition.

According to the researchers’ model, this is exactly what happened to WD J1330+6435. Traces of calcium, lithium, and sodium were found in its atmosphere, likely appearing as a result of accreting rocky material.

Sodium plays a particularly important role: it absorbs the orange part of the spectrum. As a result, the star’s light may appear greenish.

“Imagine standing on a planet and looking at a green star. It sounds like science fiction,” Hollands noted.

At the same time, this does not mean the star is literally covered in green material. Its color changes because of how chemical elements in the dense atmosphere interact with radiation.

White dwarfs have other unusual shades too

Green is only one of the possible colors. According to the modeling, white dwarfs with high carbon content may appear pink or violet. If the remnants of destroyed asteroids and comets accumulate in their atmospheres, the hue may shift toward yellow.

Cooler white dwarfs with atmospheres containing hydrogen and helium may display turquoise or pale blue shades. These are linked to interactions between hydrogen molecules and helium atoms in the dense atmosphere.

The researchers also identified a possible record holder for a red-pink color — the white dwarf WD J1011+2845, located just 48 light-years from Earth. According to calculations, a deep convective zone transports carbon from the star’s interior to the surface. Carbon molecules alter the light passing through the atmosphere, giving the object a rich salmon-pink hue.

Unusual stars are rare

Despite the variety of possible colors, most white dwarfs, according to scientists’ estimates, remain white. The researchers studied 151 such objects within about 65 light-years of Earth and found unusual coloration in only five.

Extrapolating the results to the entire white dwarf population, the team suggested that roughly one in 30 such stars has an unusual color. This means there may be quite a few objects with unexpected coloration in the Universe, although each individual type remains relatively rare.

The study was published in the journal RAS Techniques and Instruments. It shows that the color of stellar remnants can tell scientists not only about their temperature, but also about their chemical history — including what planetary systems they may have consumed after their death.

In the distant future, the Sun will also turn into a white dwarf. If rocky debris still remains around it by then and the star begins accreting it, its color could theoretically change. But this is only one possible scenario: the current results do not allow scientists to claim that the Sun’s remnant would necessarily become green.