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

White dwarfs, the extremely dense remains of stars similar to our Sun, were long believed to follow this same pattern. In fact, they received their name because the first examples discovered were unusually hot and appeared white or pale blue. However, scientists have now found that these stellar remnants can produce a much broader spectrum of colors.

The findings, published in RAS Techniques and Instruments (RASTI), suggest that some white dwarfs would look dramatically different from ordinary stars if they could be viewed from close enough.

"Most stars, including most white dwarfs, behave like 'perfect radiators,' so their color is set almost entirely by temperature. But nobody had really looked into what else might be possible for white dwarfs," said Dr. Mark Hollands, Warwick Prize Fellow in the Astronomy and Astrophysics Group, University of Warwick, who led the work. "It turns out these incredibly dense stars can produce colors nobody expected, and some of them genuinely surprised me."

Why Some Dead Stars Glow in Unexpected Colors

The explanation lies in the extraordinary density of white dwarf atmospheres. In the coolest of these stars, atmospheric material can reach densities of approximately 1 gram per cubic centimeter, comparable to water and millions of times denser than the Sun's visible surface, known as the photosphere.

Under these extreme 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, the light escaping from a white dwarf can take on colors that would not normally be expected based on temperature alone. Instead of appearing orange, white, or blue, some of these stars develop entirely different hues.

Because no white dwarf is close enough for astronomers to directly distinguish its color with the human eye, the researchers used measurements of starlight to determine how each star would appear.

They analyzed stellar spectra, which are produced by separating light into its component wavelengths, much as a prism divides sunlight into the colors of a rainbow. Each spectrum acts as a distinctive fingerprint, revealing information about a star's temperature and chemical composition.

By processing these measurements through a model of human color vision, the team calculated the colors that people would actually perceive. This approach demonstrated that spectral data can reveal not only what a star contains and how hot it is, but also its true visual appearance.

Astronomers Identify the First Known Green Star

When the researchers examined white dwarfs throughout the sky, they found that the stars could be divided into several color groups based on the chemical makeup of their atmospheres.

White dwarfs containing large amounts of carbon often appear pink or purple. Others that have accumulated material from destroyed asteroids and comets, remnants of planetary systems that once surrounded them, tend to look yellow.

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

Perhaps the most remarkable discovery was WD J1330+6435, a white dwarf with a temperature of approximately 4,000 degrees Celsius.

Over time, this star has drawn in rocky debris from its former planetary system, introducing calcium, lithium, and sodium into its atmosphere. According to the researchers' calculations, these elements give the star a distinctive chartreuse green appearance.

As far as the team knows, no other star, whether still undergoing nuclear fusion or already dead, has ever been credibly identified as genuinely green. This makes WD J1330+6435 the first known example of a green star in the Milky Way.

Dr. Hollands added: "The traces of sodium in the atmosphere of this white dwarf absorb the orange part of the spectrum, leaving behind a ghostly green glow. Imagine standing on a planet and looking up at a green star. It feels like something out of science fiction."

A Dead Star That May Be the Reddest Ever Found

At the other end of the color spectrum, the astronomers identified another extraordinary white dwarf that may be the reddest star ever documented.

Known as WD J1011+2845, this stellar remnant is located just 48 light-years from Earth. It has a deep convection zone that transports carbon from its interior to its surface.

Once there, carbon molecules dramatically alter the wavelengths of light escaping from the star. Nearly all of its light is shifted toward wavelengths longer than 600 nanometers, producing an unusually deep salmon-pink appearance.

The resulting color is unlike anything typically observed in ordinary stars and makes WD J1011+2845 another striking example of how the unusual chemistry of white dwarfs can reshape their appearance.

How Common Are These Unusually Colored Stars?

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

Among these stars, five had unusual colors. Two appeared yellow, while three were pink.

Based on these results, the researchers estimate that approximately one in every 30 white dwarfs throughout the Universe could display an unexpected color. Although this represents a relatively small percentage, it suggests that enormous numbers of strangely colored stellar remnants exist across the cosmos.

The remaining 146 stars were examined using a measurement known as color temperature. This describes how the color of an object compares with that of an ideal radiator at a particular temperature. The same concept is commonly used to characterize household lighting, such as bulbs labeled ('warm white' at 3,000K, 'cool white' at 6,000K).

The researchers found that the median color temperature of these white dwarfs was approximately 6,800 Kelvin, corresponding to light that appears white to human eyes.

This finding also challenges the widespread impression that white dwarfs are usually blue. Hotter, bluer white dwarfs produce more light and remain visible across greater distances, making them disproportionately common in astronomical surveys that select stars based on brightness.

When astronomers instead examine a more representative nearby population, the typical white dwarf turns out to be white after all.

Professor Boris Gänsicke, Department of Physics, University of Warwick, said: "One in thirty might not sound like a huge fraction, but across the entire population of white dwarfs in the Universe, that is still a lot of oddly colored stars out there. Yet if we take the median across a representative population of white dwarfs, we found that the average white dwarf really is white."