Astronomers have identified a white dwarf star that would appear green to human eyes, with unusual atmospheric chemistry producing a colour that differs from those typically associated with stars.
The findings, published in the journal RAS Techniques and Instruments (RASTI), show how interactions between helium and trace elements in the outer layers of white dwarfs can alter their observed spectra, says CNN.
"The colours that normal stars can have are pretty much only orange, white or blue. What we found is, for most white dwarfs, that's the same as well. But there are some special cases where the physics in the outer layers of these stars are sufficiently strange that it causes actually quite big deviations in terms of their spectrum," said Mark Hollands, an astrophysicist at the University of Warwick and lead author of the study.
White dwarfs are dense remnants of stars similar to the Sun. Their unusual colours can result from material accreted from nearby debris, including asteroids and the remains of planets that venture too close to the stars and are torn apart by their gravity.
The green hue in the identified star is primarily associated with calcium, sodium and lithium in its outer layers.
"It gets shredded up into dust, and then those rocky bits of dust and debris fall onto the white dwarf. And then that gets mixed into the outer layers, and then you can actually use that to measure what the rocky stuff was made of," Hollands said.
The process allows astronomers to study the composition of planetary material that has been destroyed by a white dwarf.
"The white dwarf is taking apart the planets, and we're able to see what was actually inside that planet — the proportion of the different elements — and that gives us clues in how these planets were formed. It is a crime scene, and we're kind of piecing through the evidence that we have left available to us, and that's allowing us to kind of piece the story of what the planet was and how it was made way, way, way back in history," said Adam Burgasser, a professor of astronomy and astrophysics at the University of California, San Diego.
Hollands examined 151 known white dwarfs within 20 parsecs, or roughly 65 light-years, of Earth. The researchers calculated how the stars would appear to human eyes using spectral data, which records the wavelengths of light emitted by the stars.
The study also identified a white dwarf that would appear "almost like a salmon pink", a colour attributed to a high density of carbon molecules.
"Doing a study that goes from spectrum to what the human eye would see is a cool intersection of physics and biology to me. I love that some would look green to us if they were close enough for us to see them, and even more fun that some would look pink," said Agnès Kim, an astrophysicist at Penn State Scranton.
Hollands said translating spectral data into visible colours could also help researchers relate to objects that are otherwise difficult to visualise.
"It gives you a nice human connection to these very distant things that are so far away that sometimes even as a researcher you feel quite disconnected from these objects that you study. But then you can sort of add a human element into it and it feels a lot more tangible," he said.
Upcoming spectroscopic surveys are expected to increase the amount of white dwarf spectral data available to astronomers tenfold, potentially allowing researchers to identify more stars with unusual colours.
"We'll be getting a tenfold increase in the number of white dwarfs that have spectra. And so maybe if we're lucky, we'll go from one or maybe two green white dwarfs to 10 or 20," Hollands said.