Researchers at the University of Warwick have identified a diverse range of colors among white dwarf stars, challenging the long-held assumption that stellar color is determined solely by temperature. The findings, published in RAS Techniques and Instruments, reveal that these dense stellar remnants can produce hues including green, pink, purple, yellow, and cyan—colors that would never be expected from typical stars based on their temperatures alone.
The explanation for these unusual colors lies in the extreme density of white dwarf atmospheres. In the coolest white dwarfs, atmospheric material reaches densities comparable to water, millions of times denser than the Sun’s visible surface. Under these extraordinary conditions, elements such as carbon, calcium, sodium, and lithium interact with surrounding helium plasma in ways that allow them to absorb a much broader range of light wavelengths than would occur in less dense environments. This unusual absorption fundamentally alters the light escaping from these stars, producing unexpected hues.
Since no white dwarf exists close enough for direct visual observation, researchers analyzed stellar spectra by separating light into component wavelengths and applying models of human color vision to determine perceived colors. Among their discoveries was WD J1330+6435, a star approximately 4,000 degrees Celsius in temperature that has accumulated rocky debris containing calcium, lithium, and sodium. According to their calculations, sodium in this star’s atmosphere absorbs orange wavelengths, leaving a distinctive chartreuse green appearance—making it the first known genuinely green star. Additionally, WD J1011+2845, located 48 light-years from Earth, exhibits an unusually deep salmon-pink color due to carbon transported from its interior to its surface.
To assess the prevalence of these unusual colors, the team examined 151 white dwarfs within approximately 65 light-years of Earth. Five displayed unexpected colors: two yellow and three pink. Based on this sample, researchers estimate that roughly one in every 30 white dwarfs across the Universe could exhibit an unexpected color, suggesting that enormous numbers of strangely colored stellar remnants exist throughout the cosmos despite representing a relatively small percentage of all white dwarfs.
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