What is a White Dwarf?
Before diving into this colourful discovery, it's helpful to understand what a white dwarf is. When a star similar to our Sun runs out of fuel, it sheds its outer layers. What's left behind is its incredibly hot, dense core: a white dwarf. These objects
pack about half the Sun's mass into a sphere roughly the size of Earth, making them extraordinarily dense. For a long time, scientists believed their colours, like those of most stars, were determined almost entirely by their temperature—from pale blue for the hottest to orange for the coolest. While they are called 'white' dwarfs, most do indeed appear white to us. This new research, however, shows that chemistry can sometimes paint them with a much more exotic brush.
The Case of the Green Star
A team of researchers led by Dr Mark Hollands from the University of Warwick in the UK didn't see the green star directly; white dwarfs are too far away for their true colour to be observed with the naked eye. Instead, they analysed the 'fingerprint' of light, known as a spectrum, from over 150 white dwarfs. By splitting the starlight into its component wavelengths, much like a prism makes a rainbow, they could determine its chemical composition and calculate what colour it would appear to the human eye. One star, named WD J1330+6435, stood out. Located about 280 light-years away, its unique atmospheric conditions give it a distinct chartreuse-green glow. This finding was published in the journal RAS Techniques and Instruments, rewriting a small but fundamental rule of the stars.
An Unexpected Chemical Cocktail
So, why is this star green? The answer lies in its atmosphere. Unlike most stars, whose colour is dictated by heat, the hue of some white dwarfs is the result of unusual elements polluting their surfaces. In the case of WD J1330+6435, its atmosphere is rich in calcium, lithium, and sodium. These elements are believed to be the rocky debris from a surrounding planetary system, perhaps from an asteroid that strayed too close and was shredded by the white dwarf's immense gravity. The sodium atoms in the star's dense atmosphere absorb light in the yellow, orange, and red parts of the spectrum. With those colours removed, the light that is left behind gives the star its greenish appearance. This process turns the white dwarf into a diagnostic tool, allowing astronomers to measure the chemical makeup of the planets it has consumed.
A Rainbow of Stellar Remnants
The green star wasn't the only surprise. The study revealed that white dwarfs can exhibit a whole spectrum of unexpected colours. Researchers also identified a white dwarf, WD J1011+2845, that would appear salmon pink. This particular star’s colour is caused by a high concentration of carbon molecules in its atmosphere. Other white dwarfs showed potential yellow or cyan tinges based on the mix of elements on their surface. A yellow hue can indicate the presence of remnants from old planetary systems, while a mix of hydrogen and helium can create a pale blue or cyan colour. However, the researchers stress that these colourful examples are the exception. Most white dwarfs, about one in 30 according to the study, have an unusual colour, while the vast majority live up to their name and appear white.
