Astronomers discover a white dwarf that appears green
White dwarf stars are usually associated with white or blue light, but astronomers at the University of Warwick have found that some can appear pink, purple, yellow, cyan, and even green. Their research identifies WD J1330+6435 as the first known star calculated to appear genuinely green to the human eye, revealing how the unusual chemistry of stellar remnants can transform their colour.
White dwarfs are the dense remains of stars that have exhausted their nuclear fuel. Although their colour has traditionally been understood largely in terms of temperature, a study published in RAS Techniques and Instruments shows that the composition of their atmospheres can make a substantial difference to the light they emit.
The explanation lies in the extraordinary conditions around these stars. In the coolest white dwarfs, atmospheric material can reach densities comparable to water, far greater than those found at the Sun’s visible surface. Under this pressure, elements including carbon, calcium, and sodium interact with the surrounding helium plasma, absorbing particular wavelengths of light and changing the colour that escapes into space.
Led by Dr Mark Hollands of the University of Warwick’s Astronomy and Astrophysics Group, the researchers analysed stellar spectra and used a model of human colour vision to calculate how the stars would appear to the naked eye.
Their most striking example, WD J1330+6435, has a temperature of around 4,000°C. Material drawn in from the remains of its former planetary system has introduced elements including calcium, lithium, and sodium into its atmosphere. The sodium absorbs much of the orange light, leaving the star with a distinctive chartreuse-green appearance.
The team also identified WD J1011+2845, just 48 light-years from Earth, as a candidate for the reddest star yet documented. Carbon brought to its surface changes the escaping light, producing an unusually deep salmon-pink colour.
To establish how common such unusual appearances might be, the researchers examined a representative sample of 151 white dwarfs within 20 parsecs of Earth. Five had unexpected colours: two were yellow and three pink. The results suggest that roughly one in 30 white dwarfs could display unusual hues.
Despite these exceptions, the typical white dwarf remains white. The sample had a median colour temperature of approximately 6,800 kelvin, challenging the impression that these stellar remnants are predominantly blue, which partly reflects the greater visibility of hotter stars in brightness-selected surveys.
The findings show that a star’s appearance depends on more than its temperature. Even long after a star has ceased nuclear fusion, the chemistry of its atmosphere can produce colours that would seem extraordinary in our own night sky.
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