Warwick Astronomers Discover White Dwarf Stars Can Glow in Unusual Colors Like Green and Pink
University of Warwick astronomers discovered that dense white dwarf stars can glow in unexpected colors like green, pink, and cyan, identifying the galaxy's first known green star.
Astronomers from the University of Warwick have discovered that white dwarf stars can glow in an extraordinary variety of colors, including the first star ever found that appears green to the human eye. For most stars, color serves as a measure of temperature. Cooler stars appear orange, sun-like stars appear white, and the hottest stars emit pale blue light. Previously, it was believed that white dwarfs—the dense remnants left behind after sun-like stars reach the end of their lives—obeyed the same rule. They were named because the earliest discovered examples were surprisingly hot and glowed in white or light blue.
However, a study published in the journal RAS Techniques and Instruments shows that certain white dwarf stars could glow in colors such as green, pink, cyan, yellow, and purple. "The color of most stars, including white dwarfs, is determined almost entirely by their temperature. But no one has closely examined what other possibilities might exist among white dwarfs," said Dr. Mark Hollands, a research fellow in the astronomy and astrophysics group at the University of Warwick, who led the research. "It turns out that these stars, which have immense density, are capable of displaying colors that no one expected—and some of them truly surprised me."
The key lies in density. The atmospheres of the coldest white dwarfs can reach a density of about 1 gram per cubic centimeter—roughly the density of water, and millions of times the density of the sun's photosphere. Under these conditions, trace elements that may be found in the atmosphere—such as carbon, calcium, and sodium—react with the surrounding dense helium plasma and absorb light across a wider color range than they would at lower density. This process causes the star's color to deviate from the orange, white, and blue hues characteristic of a perfectly radiating body.
Revealing True Stellar Colors
No white dwarf is close enough for its color to be seen directly, so the research team calculated this from data routinely gathered by astronomers: a stellar spectrum, the "fingerprint" created when starlight splits by wavelength, much like a prism splitting sunlight into a rainbow. Running these spectra through a model of human color vision revealed what the light color of each star would look like to the human eye.
By applying this method to white dwarfs in the sky, the researchers discovered that their colors fall into distinct groups, each related to what is found in the star's atmosphere. Carbon-rich white dwarfs are generally pink or purple, while those enriched with materials from shattered asteroids and comets tend toward yellow. Cooler white dwarfs with mixed hydrogen and helium atmospheres frequently acquire a cyan or pale blue hue, caused by the collision of hydrogen molecules with helium atoms in their exceptionally dense atmospheres.
The most prominent example is WD J1330+6435. The star's temperature is about 4,000 degrees Celsius, and it has accreted rocky material from its planetary system. This process enriched its atmosphere with calcium, lithium, and sodium, giving it a greenish-yellow appearance in the models created by the researchers. To the best of the researchers' knowledge, no other star—living or dead—has ever been reported with a green color, making it the known green star in the galaxy.
Unprecedented Spectral Extremes
In contrast, at the other end of the scale, the study's authors identified what may be the reddest star ever recorded. WD J1011+2845 is a nearby white dwarf located just 48 light-years from Earth, with wavelengths exceeding 600 nanometers. The researchers examined a representative sample of 151 white dwarfs within about 65 light-years of Earth, finding that five exhibited unusual colors—two appeared yellow and three appeared pink. This finding suggests that roughly one in every thirty white dwarfs in the universe emits light in an unusual color.
"One in thirty might not sound like a particularly high proportion, but considering the entire population of white dwarfs in the universe, that is still a large number of stars with unusual colors," said Prof. Boris Gänsicke from the physics department at the University of Warwick.
