Astronomers say they’ve discovered the first green star

Astronomers analysing white dwarf stars have for the first time found a star that would appear green to human eyes, according to a new study.

While for most stars, color is temperature-dependent and limited to a few hues, white dwarfs, which are the dense remnants of sun-like stars that have died, can glow in an incredible range of colors, said lead study author Mark Hollands, an astrophysicist at the University of Warwick, England, on Thursday.

“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,” he said.

These deviations are due to the interaction between the helium in their atmosphere and traces of other elements, which vary between white dwarfs, depending on what material has interacted with the star over the course of its lifetime.

For instance, iron, magnesium and sodium, elements commonly found in rocks, can be found on the surface — potential evidence of a nearby former planetary system, or from “something like an asteroid that ventures too close to the white dwarf,” Hollands explained.

“It gets shredded up into dust, and then those rocky bits of dust and debris fall onto the white dwarf,” he said.

“And then that gets mixed into the outer layers, and then you can actually use that to measure what the rocky stuff was made off,” Hollands said.

“So we can actually use the white dwarf as a sort of diagnostic tool to actually measure the chemistry of planetary material,” he added.

“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,” said Adam Burgasser, a professor of astronomy and astrophysics at the University of California San Diego. “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 in history.”

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Human connection

For this study, Hollands looked at the 151 known white dwarf stars that are within 20 parsecs, or around 65 light years, of Earth.

As for the green star, the main elements present are calcium, sodium and lithium, Hollands said.

“It kind of stimulated interest in me in looking more at that object and why it has so much material being accreted onto it,” said Burgasser, who was not involved in the study. “It can excite the public as well as the scientists in learning more about some of the more, I would say, extreme objects in this sample.”

Another notable star was “almost like a salmon pink,” due to the presence of a high density of carbon molecules, Hollands added.

While no white dwarf stars can be seen directly from Earth, Hollands was able to calculate their color using star spectrum data, which a statement from the researchers defines as “the ‘fingerprint’ produced when starlight is split apart by wavelength, much like a prism splits sunlight into a rainbow.”

Hollands explained that the research has also brought these space objects much closer to home.

“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,” he said. “But then you can sort of add a human element into it and it feels a lot more tangible.”

The observations about the green star are based on existing data and may not shed new understandings about white dwarfs, but the study does offer an intriguing approach and view of these celestial bodies, said Agnès Kim, an astrophysicist and professor of physics at Penn State Scranton, who was not involved in the work.

“Doing a study that goes from spectrum to what the human eye would see is a cool intersection of physics and biology to me,” Kim wrote in an email. “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.”

Burgasser added that the study’s categorisation scheme using temperatures and atmospheric properties can be useful for the field moving forward.

“Some things are kind of purplish pink, and some other things were greenish and blue — and that kind of classification, which by the way we do all the time in astronomy, is classify things, allows us to focus on: why are these types of objects these colors, and are they related to each other based on their composition?” Burgasser said. “We have that information in other forms, but I think this is maybe a simplified way of organising that data.”

Next, Hollands is planning to keep analysing more white dwarf star spectra, with advances in scientific techniques meaning that researchers will be able to look at a new tranche of stars that are even farther away from Earth.

“We know of white dwarfs that we don’t have spectra for,” he said, “but that’s all about to change because there’s quite a few spectroscopic surveys” that are going to start delivering data.

“We’ll be getting a tenfold increase in the number of white dwarfs that have spectra,” Hollands said. “And so 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, or RASTI.