ASTRONOMERS IDENTIFY GREEN WHITE DWARF

Astronomers at the University of Warwick say they have identified a white dwarf that appears green to the human eye. They say the star took on its chartreuse colour after swallowing rocky material from its planetary system. The findings, published in RAS Technique and Instruments, suggest that planetary debris can change the visible colour of white dwarfs. Mark Hollands, who led the study, said: “It turns out these incredibly dense stars can produce colours nobody expected, and some of them genuinely surprised me.”

White dwarfs form when stars such as the Sun exhaust their nuclear fuel and shed their outer layers, leaving dense cores. Their colour usually depends chiefly on temperature, with hotter stars emitting more light at shorter, bluer wavelengths. Hollands said most white dwarfs behave like “perfect radiators”, whose colour temperature largely determines. The study examined how material in a white dwarf’s atmosphere could alter that pattern.

The researchers say the atmospheres of the coolest white dwarfs can reach the density of water. At those densities, trace elements including carbon, calcium and sodium interact with dense helium plasma. The interaction causes the elements to absorb a wider range of colours than they would at lower densities, according to the study. The findings suggest that white dwarfs containing planetary debris could also appear pink or purple when viewed up close.