For most stars, the color of their light is directly linked to their temperature: cooler stars appear reddish or orange, stars like our Sun appear yellowish-white, and the hottest stars glow pale blue. White dwarfs, the burnt-out remnants of stars, usually shine whitish-blue due to their usually high temperatures – hence their name. “Most stars behave like ‘ideal radiators’: their color is determined almost exclusively by their temperature,” explains lead author Mark Hollands from the University of Warwick.
Are white dwarfs really white?
But now it turns out that white dwarfs can deviate from the usual color scheme. Hollands and his colleagues discovered this when they took a closer look at how white dwarfs of different compositions and temperatures would appear to our eyes. Although the light spectrum of many stars is known, they are too far away to be imaged directly. What color they would be for our eyes was therefore unknown.
For their study, the astronomers converted the spectral features of various white dwarfs into RGB colors that we can see. “Nobody has yet investigated whether white dwarfs can also have other colors,” says Hollands. “It turns out that these incredibly dense stellar remnants can produce colors that no one expected – some of which really surprised me.”
Color variants of white dwarfs. At the top left is the bluish star Sirius for comparison. — © Mark Hollands/University of Warwick
An apple-green white dwarf
The most surprising discovery is the white dwarf WD J1330+6435, located around 280 light-years away. This remnant of stars in the constellation of the Dragon is around 4000 degrees hot and should therefore appear yellowish. But the analysis of its spectrum revealed that this white dwarf is colored green instead. With a wavelength of around 500 nanometers, its color corresponds to that of a lime or a green apple.
“To our knowledge, no star with a green color has been discovered before,” report the astronomers. The white dwarf WD J1330+6435 is the first green star in our galaxy. Its color goes beyond the usual range of spectral classes and stellar colors. “Imagine standing on a planet and looking up at a green star,” says Hollands. “That sounds like a scenario from a science fiction story.”
Planetary debris as a color source
But why is this white dwarf so green? The reason for this is a combination of extreme density and metallic impurities in this white dwarf, as the astronomers explain: The spectral analyzes reveal that the shell of WD J1330+6435 is highly enriched in sodium, lithium and calcium – these metals probably arrived on the white dwarf with planetary debris. “The trace amounts of sodium in its atmosphere absorb the orange part of the spectrum, leaving behind a ghostly green glow,” explains Hollands.
The astronomers have discovered two other white dwarfs that have a slightly greenish glow – although not as pronounced as WD J1330+6435. One of them appears rather pale greenish blue. According to the spectral analyses, calcium ions and a relatively steep drop in light in red wavelengths are primarily responsible for the coloring of these white dwarfs.
White dwarfs also come in salmon pink, pink and yellow
But green is apparently not the only unusual color that white dwarfs can take on: the astronomers also discovered one of the reddest stars ever observed. The white dwarf WD J1011+2845, located around 48 light-years away, shines in a narrow spectral range around 600 nanometers wavelength. This corresponds to a deep salmon pink – such a color has never been observed in a star before.
The reason for the pink color in this stellar remnant is a strong convection current that carries diatomic carbon molecules (C2) transported from the core of the white dwarf to its surface. There the carbon molecules interact with the helium of the gas shell and produce a light spectrum in the pink range.
The cool white dwarf WDJ0902+2010 shines deep pink. The reason is carbon molecules that reach its surface through strong convection currents. — © Mark Garlick/University of Warwick
Every 30th white dwarf is “colorful”
To find out how common colored “deviants” are, the astronomers analyzed the light spectra of all white dwarfs within a radius of 65 light-years. Among these 152 stellar remnants, they identified five unusually colored specimens: two yellow and three pink white dwarfs. Accordingly, a visible deviation from the temperature-related blackbody scheme only occurs in one in 30 white dwarfs.
“One in thirty may not sound like a particularly large proportion, but if you look at all the white dwarfs in the universe, there are a lot of stars out there with unusual colors,” says co-author Boris Gänsicke from the University of Warwick. The typical color of white dwarfs is still white, but there are exceptions.
Source: Mark Hollands (University of Warwick, UK) et al., RAS Techniques and Instruments, 2026; doi: 10.1093/rasti/rzag069