Astronomers at the University of Warwick have discovered that white dwarf stars can display a variety of unexpected colors, including green, pink, purple, yellow, and cyan. The findings challenge the long-held assumption that white dwarfs, like most stars, follow a predictable color-temperature relationship.
White dwarfs are the extremely dense remains of stars similar to our Sun. Researchers led by Dr. Mark Hollands published their findings in RAS Techniques and Instruments.
The explanation lies in the extraordinary density of white dwarf atmospheres. In the coolest white dwarfs, atmospheric material reaches densities of approximately 1 gram per cubic centimeter, millions of times denser than the Sun's visible surface. Under these extreme conditions, elements such as carbon, calcium, and sodium interact with surrounding helium plasma in ways that absorb a much wider range of light wavelengths than would occur in less dense environments.
The team analyzed stellar spectra and processed measurements through a model of human color vision to calculate how each star would appear to the human eye.
WD J1330+6435 emerged as the most remarkable discovery. This white dwarf, with a temperature of approximately 4,000 degrees Celsius, has accumulated 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 has ever been credibly identified as genuinely green.
Dr. Hollands stated: "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 second remarkable white dwarf, WD J1011+2845, located 48 light-years from Earth, appears to be among the reddest stars ever documented. A deep convection zone transports carbon from its interior to its surface, producing an unusually deep salmon-pink appearance.
Among a representative sample of 151 white dwarfs within 20 parsecs of Earth, five displayed unusual colors. Two appeared yellow and three were pink. The researchers estimate that approximately one in every 30 white dwarfs throughout the universe could display an unexpected color.
Professor Boris Gänsicke noted: "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."