For most stars, color comes down to temperature, and the menu is short: orange, white or blue. But a new survey of dead stars near Earth has found that white dwarfs — the dense, burned-out cores left behind when sun-like stars die — can break those rules in startling ways, including one that would look distinctly green to the human eye.
Astrophysicist Mark Hollands of the University of Warwick led the study, which examined the 151 known white dwarfs within 20 parsecs — about 65 light-years — of Earth. No white dwarf is close enough to see with the naked eye, so Hollands calculated each star's true color from its spectrum, the "fingerprint" produced when starlight is split apart by wavelength.
Rocky Debris, Unusual Colors
One star, cataloged as WD J1330+6435 and sitting a few hundred light-years away in the constellation Draco, stood out: calcium, sodium and lithium in its atmosphere absorb light in the orange part of the spectrum, giving the star an overall green cast. Hollands said that material likely came from rocky debris — the remains of planets or asteroids that strayed too close and were torn apart by the star's gravity, then mixed into its outer layers.
"The colors that normal stars can have are pretty much only orange, white or blue," Hollands told CNN. "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." A second star in the sample looked "almost like a salmon pink," Hollands said, colored by carbon molecules that absorb blue, green and yellow light instead. The team estimates that roughly one in 30 white dwarfs shows an unusual color.
Adam Burgasser, an astronomer at the University of California San Diego who was not involved in the research, said the approach amounts to forensic work on dead planetary systems. "It is a crime scene, and we're kind of piecing through the evidence that we have left available to us," he said, "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." Agnès Kim, an astrophysicist at Penn State Scranton who also was not involved, called the method "a cool intersection of physics and biology," translating raw spectra into what a human eye would actually perceive.
Hollands said the discovery also made the distant, hard-to-picture objects he studies feel more immediate. "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," he said. With new spectroscopic sky surveys coming online, he expects the catalog of oddly colored white dwarfs to grow. "We'll be getting a tenfold increase in the number of white dwarfs that have spectra," he said. "Maybe if we're lucky, we'll go from one or maybe two green white dwarfs to 10 or 20."
The study was published in the journal RAS Techniques and Instruments.
