A green star among the stellar remnants
Astronomers studying white dwarfs have identified what they say is the first star that would appear green to human eyes, according to a new study. White dwarfs are the dense remnants of sun-like stars that have died.
Lead author Mark Hollands, an astrophysicist at the University of Warwick in England, spoke about the findings on Thursday. He told CNN that ordinary stars come in a narrow palette. "The colors that normal stars can have are pretty much only orange, white or blue," he said.
Most white dwarfs follow that pattern too, Hollands said. But some special cases have outer layers where the physics is strange enough to cause big deviations in their spectrum.
Why some white dwarfs look so different
According to the report, the unusual colors come from the interaction between helium in a white dwarf's atmosphere and traces of other elements. Those traces vary from star to star, depending on what material has interacted with the star over its lifetime.
Iron, magnesium and sodium, which are common in rocks, can show up on a white dwarf's surface. Hollands said that could be evidence of a nearby former planetary system, or of "something like an asteroid that ventures too close to the white dwarf." Such an object can be shredded into dust and debris that falls onto the star and mixes into its outer layers.
That makes the star useful for science. Hollands said white dwarfs can serve as a diagnostic tool for measuring the chemistry of planetary material. Adam Burgasser, a professor of astronomy and astrophysics at the University of California San Diego, who was not involved in the study, described it as a crime scene in which researchers piece together what a planet was made of and how it formed.
How the colors were determined
Hollands examined the 151 known white dwarfs within 20 parsecs of Earth, or around 65 light years. For the green star, the main elements present are calcium, sodium and lithium, he said.
No white dwarf can be seen directly with the naked eye from Earth. Hollands instead calculated each star's color from its spectrum data. A statement from the researchers defines a spectrum as the "fingerprint" produced when starlight is split apart by wavelength, much like a prism splits sunlight into a rainbow.
Another star in the sample was "almost like a salmon pink," Hollands said, owing to a high density of carbon molecules. The study was published in the journal RAS Techniques and Instruments, known as RASTI.
Reactions from outside researchers
Burgasser said the green object had drawn his interest because of how much material is falling onto it. He said such extreme objects can excite both the public and scientists.
Agnès Kim, an astrophysicist and professor of physics at Penn State Scranton, who was not involved in the work, said the observations rely on existing data and may not yield new understanding of white dwarfs. Still, she called the approach intriguing, writing in an email that going from spectrum to what the human eye would see is "a cool intersection of physics and biology."
Burgasser added that the study's classification scheme, based on temperatures and atmospheric properties, could help the field. He said it offers a simplified way of organizing information that already exists in other forms.
What comes next
Hollands plans to keep analyzing white dwarf spectra. Several spectroscopic surveys are about to begin delivering data, he said, which will open up a new group of stars that are even farther away.
He expects a tenfold increase in the number of white dwarfs with spectra. "Maybe if we're lucky, we'll go from one or maybe two green white dwarfs to 10 or 20," Hollands said.
