Researchers admit error: White dwarfs are not always white

Their name is misleading: White dwarfs do not always shine white. Researchers discover specimens in green, pink, cyan and violet. This is due to asteroids crashing into the dying stars.

White dwarfs - their name says it - glow white. That's what astronomers thought, at least so far. But this is a mistake, as now a research team from England and Germany shows. Impurities in the dense atmospheres of the dwarf stars can produce a variety of different colors, the scientists report in the journal "Royal Astronomical Society Techniques and Instruments".

'Most stars, including most white dwarfs, behave like perfect emitters, so their colour is almost entirely determined by temperature,' explains Mark Hollands of the University of Warwick in England. From space, without the influence of the Earth's atmosphere, our sun, which is 6,000 degrees hot on the surface, appears white. Stars with temperatures above 10,000 degrees glow blue, cooler stars with decreasing temperature yellow, orange and finally red.

According to this, white dwarfs, which are between 8,000 and 40,000 degrees hot, should shine white or bluish. White dwarfs are dying stars. At the end of their life cycle, when their nuclear fuel is consumed, stars like our Sun first inflate into a red giant star. Then they collapse into a white dwarf that is only about the size of the earth. Because nuclear fusion no longer takes place inside the white dwarfs, they cool slowly - very slowly: only after billions of years do they become dim and yellowish.

Hollands and his colleagues wanted to know in more detail: what would a white dwarf actually look like to an observer nearby? Not a single white dwarf is visible to the naked eye in the night sky. And photographs of large telescopes do not necessarily reproduce the colors that human eyes would perceive.

The actual color of a star can only be determined from its spectrum, i.e. from the splitting of its light into the rainbow colors. This shows how bright a star shines at different wavelengths of light. So Hollands and his colleagues took spectra of many white dwarfs and fed this data into a model of human color perception.

“It turned out that these stars can create colors that no one expected—and some of them really surprised me.” The team came across white dwarfs in the colors green, pink, cyan, yellow and violet. But why can white dwarfs glow in colors that are not found in other stars?

The researchers see the cause in the atmospheres of the stars, more precisely: in the so-called photosphere, that outer layer of a star from which its light reaches us. While the photosphere of the sun is about 400 kilometers thick, it often measures only a few meters in white dwarfs - but is millions of times denser. And this high density - equivalent to about one gram per cubic centimeter of water - causes even minor contaminants to stain the star's light.

And such contamination is common in white dwarfs - for example, by asteroids or debris from planets that fall into the star. “Sodium in the atmosphere absorbs the orange part of the spectrum, leaving a ghostly green glow,” explains Hollands. Carbon in the atmosphere, on the other hand, can color the star's light pink or violet.

Nevertheless, the term white dwarfs is not wrong, as the researchers point out. The team studied the spectra of all 152 white dwarfs within a radius of 65 light years. "The mean temperature of these dwarf stars is 6,500 degrees Celsius," says Hollands and his colleagues, "so on average the color of white dwarfs is actually white." Only about one in thirty white dwarfs has a different color due to pollution of its atmosphere.