Why white dwarfs can have unexpected colors

For most stars, color is closely linked to temperature. Cooler stars appear orange, stars like the Sun look white, and the hottest stars glow pale blue. The relationship is similar to heating iron, which changes from red to orange and eventually white as its temperature rises.

White dwarfs are the extremely dense remnants of stars similar to the Sun. Because the first white dwarfs discovered were unusually hot and appeared white or pale blue, scientists long believed these stellar remnants followed the same temperature-color pattern as other stars.

New research published in RAS Techniques and Instruments (RASTI) suggests that white dwarfs can display a much wider range of colors than previously expected—especially when viewed from close enough to distinguish their appearance.

“Most stars, including most white dwarfs, behave like ‘perfect radiators’, so their color is determined almost entirely by their temperature. But no one has seriously considered what else white dwarfs are capable of,” said Dr Mark Holland, Warwick Prize Research Fellow in the University of Warwick’s Astronomy and Astrophysics Group, who led the study. “We found that these incredibly dense stars can produce colors that no one expected, some of which were truly surprising.”

The explanation lies in the extreme density of a white dwarf’s atmosphere. In the coldest white dwarfs, atmospheric material can reach a density of about 1 gram per cubic centimeter—comparable to water and millions of times denser than the Sun’s visible surface, or photosphere.

Under these conditions, small amounts of elements such as carbon, calcium and sodium interact with the surrounding helium plasma. These interactions allow the elements to absorb a much wider range of light wavelengths than they would in less dense environments.

As a result, light escaping from a white dwarf can take on a color that does not match what its temperature alone would suggest. Instead of appearing orange, white or blue, some of these stellar remnants can display entirely different hues.

How astronomers calculated the true colors of white dwarfs

White dwarfs are generally too distant for astronomers to directly distinguish their colors with the human eye. To determine how the stars would appear, the researchers analyzed measurements of their starlight.

They examined each star’s spectrum, created by separating light into its component wavelengths in much the same way a prism splits sunlight into a rainbow. A spectrum acts as a unique fingerprint, revealing information about a star’s temperature and chemical composition.

The team then processed the measurements through a human color-vision model. This allowed them to calculate the colors people would actually perceive and showed that spectral data can reveal not only a star’s temperature and composition, but also its visual appearance.

WD J1330+6435: The first known green star

When the researchers examined white dwarfs across the sky, they found that the stars could be divided into several color groups according to the chemical composition of their atmospheres.

White dwarfs containing large amounts of carbon can appear pink or purple. Other white dwarfs that have accumulated material from destroyed asteroids, comets and the debris of former planetary systems tend to appear yellow.

Cool white dwarfs with atmospheres containing both hydrogen and helium can look cyan or pale blue. This effect occurs when hydrogen molecules collide with helium atoms under the enormous pressures found in these stars.

The most notable discovery is WD J1330+6435, a white dwarf with a temperature of about 4,000 degrees Celsius.

Over time, the star accumulated rocky debris from previous planetary systems. This introduced calcium, lithium and sodium into its atmosphere. According to the researchers’ calculations, these elements give WD J1330+6435 its distinctive chartreuse-green appearance.

As far as the research team knows, no other star has been reliably confirmed to be truly green, whether it is still undergoing fusion or has already died. That makes WD J1330+6435 the first known example of a green star in the Milky Way.

“The traces of sodium in the white dwarf’s atmosphere absorb the orange part of the spectrum, leaving behind a ghostly green glow. Imagine standing on top of the planet and looking up at a green star. It feels like something out of science fiction,” Hollands said.

WD J1011+2845 may be the reddest star ever discovered

At the opposite end of the color spectrum, the astronomers identified another unusual white dwarf that may be the reddest star ever recorded.

Known as WD J1011+2845, the stellar remnant is located just 48 light-years from Earth. A deep convective zone transports carbon from the star’s interior to its surface.

Once at the surface, carbon molecules dramatically alter the wavelengths of light emitted by the star. Almost all of the light shifts to wavelengths longer than 600 nanometers, producing an unusually deep salmon-pink appearance.

Its color is unlike those typically seen in normal stars, making WD J1011+2845 another striking example of how chemical reactions in a white dwarf’s atmosphere can transform its appearance.

How common are unusually colored white dwarfs?

To determine whether these colors are rare exceptions or part of a broader pattern, the researchers examined a representative sample of 151 white dwarfs within 20 parsecs, or about 65 light-years, of Earth.

Five of the stars had unusual colors: two appeared yellow and three appeared pink.

Based on these results, the researchers estimate that approximately 1 in 30 white dwarfs throughout the universe could display unexpected colors. Although that is a relatively small percentage, it still suggests that the universe contains a huge number of unusually colored stellar remnants.

The remaining 146 white dwarfs were analyzed using a measurement called color temperature. This describes how the color of an object at a particular temperature compares with the color of an ideal radiator. The same concept is used to describe household lighting, including “warm white” bulbs at 3,000K and “cool white” bulbs at 6,000K.

The researchers found that the white dwarfs in the representative sample had a median color temperature of about 6,800 Kelvin, corresponding to light that appears white to the human eye.

Why most white dwarfs appear white, not blue

The findings challenge the widespread impression that white dwarfs are usually blue. Hotter, bluer white dwarfs produce more light and remain visible over greater distances, making them disproportionately common in astronomical surveys that select stars by brightness.

When astronomers study a nearby and more representative population, they find that a typical white dwarf is white after all.

Professor Boris Gensik of the University of Warwick’s Department of Physics said: “One in 30 may not sound like a big number, but when you look at the universe’s entire population of white dwarfs, there are still a lot of strangely colored stars. Yet, when we take the median value of a representative population of white dwarfs, we find that the average white dwarf is indeed white.”