Astronomers have identified the first known green star, a dense stellar remnant called a white dwarf that took on a ghostly chartreuse hue after swallowing rocky debris from planets that once orbited it. The object, designated WD J1330+6435, lies about 280 light-years from Earth in the constellation Draco, within the Milky Way. A light-year is the distance light travels in a year, roughly 5.9 trillion miles.
The finding comes from a project assessing the colors of white dwarfs as human eyes would see them. White dwarfs are the leftover cores of low- to medium-mass stars that exhausted their nuclear fuel and shed their outer layers. Typically about half the mass of the sun is packed into a body the size of Earth, making them extraordinarily dense. Most appear white, and their color is not governed by temperature in the way it is for ordinary stars, where cooler stars look orange, sun-like stars look white, and the hottest ones look pale blue.
In this case, planetary material falling onto the star enriched its outer layers with calcium, lithium and, in particular, sodium. In normal stars, sodium absorbs a very specific wavelength of orange — the same orange that gives sodium street lamps their characteristic color. But the white dwarf's density is so high that surrounding helium atoms constantly bombard the sodium atoms, altering their energy levels. As a result, the sodium no longer absorbs one narrow band of orange; it absorbs yellows, oranges and some red wavelengths. What remains is a chartreuse green glow that would be impossible for a sun-like star.
«Because white dwarfs are super-dense stars — they are roughly the size of the Earth, but contain about 165,000 times its mass — even a small amount of sodium blocks out a large amount of the orange light coming out of the star,» said University of Warwick astrophysicist Boris Gänsicke, a co-author of the study published in the journal RAS Techniques and Instruments. He compared the effect to a painter's palette: start with a few basic colors, mix them, and remove one ingredient to change the result.
The researchers studied 151 white dwarfs within about 65 light-years of Earth. White was the average color, but the survey revealed distinct groups based on atmospheric composition. Carbon-rich white dwarfs usually appeared pink or purple. Those enriched with debris from shattered planetary systems leaned toward yellow. Cooler white dwarfs with mixed hydrogen and helium atmospheres tended toward cyan or pale blue. Together they spanned almost every color of the spectrum.
Among them was WD J1011+2845, a white dwarf 48 light-years away in the constellation Leo Minor that may be the reddest known star, with a salmon-pink hue. In that case carbon, rather than sodium, reshapes the palette — and it comes from the star's deep interior rather than from an impacting body. «Rather than something hitting the star, in this case the carbon comes out from its deep interior,» Gänsicke said.
The analysis drew on light spectrum observations from the Sloan Digital Sky Survey using a telescope in New Mexico, along with white dwarf spectrum models developed by the team. Mark Hollands of the University of Warwick, the study's lead author, said the work began because planetary material had fallen onto the white dwarf's surface. While WD J1330+6435 is the first documented green star in the real world, green stars have long appeared in science fiction, including in the recent comic book movie «Supergirl.» «It does show that the universe is sometimes just as weird as anything science fiction can conjure up,» Hollands said.
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