White dwarf stars, the dense remnants of Sun-like stars, have long been thought to follow a simple color-temperature relationship, appearing white or pale blue when hot. But a new study led by Dr. Mark Hollands of the University of Warwick, published in RAS Techniques and Instruments (RASTI), reveals that these stellar corpses can produce a surprising range of hues including green, pink, purple, yellow and cyan.
The explanation lies in the extraordinary density of white dwarf atmospheres. In cool white dwarfs, atmospheric material can reach densities comparable to water, millions of times denser than the Sun's visible surface. Under these extreme conditions, small amounts of elements such as carbon, calcium and sodium interact with surrounding helium plasma, absorbing a much wider range of light wavelengths than normal. This alters the escaping light, creating colors not predicted by temperature alone.
The most remarkable find is WD J1330+6435, a white dwarf that has drawn in rocky debris from its former planetary system, introducing calcium, lithium and sodium into its atmosphere. "The traces of sodium in the atmosphere of this white dwarf absorb the orange part of the spectrum, leaving behind a ghostly green glow," said Dr. Hollands. "Imagine standing on a planet and looking up at a green star. It feels like something out of science fiction."
At the other extreme, the team identified WD J1011+2845, located 48 light-years from Earth, which may be the reddest star ever documented. Its deep convection zone transports carbon to the surface, shifting nearly all its light toward longer wavelengths and producing a deep salmon-pink appearance.
To assess how common such unusual colors are, the researchers examined a representative sample of 151 white dwarfs within about 65 light-years of Earth. Five had unexpected colors: two yellow and three pink. Based on this, they estimate that roughly one in 30 white dwarfs worldwide could display an unusual hue. "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," said Professor Boris Gänsicke, also of the University of Warwick.
The researchers caution that no white dwarf is close enough to see its color directly with the naked eye. Instead, they used stellar spectra and a model of human color perception to determine how each star would appear. The study also challenges the impression that most white dwarfs are blue, finding that when a representative nearby sample is examined, the typical white dwarf is white, with a median color temperature of about 6,800 Kelvin.