WASHINGTON: Astronomers for the first time have identified a green star, documenting a dense stellar remnant called a white dwarf that became a ghostly shade of chartreuse after ingesting rocky debris from planets that once had orbited it.
A star's colour typically is governed by its temperature, with cooler stars appearing orange, the sun and stars similar to it appearing white, and the hottest ones appearing a pale blue.
But the formula is different for a white dwarf, which is the leftover core of a low- to medium-mass star that burned through its nuclear fuel and blew off its outer layers. Most white dwarfs have colours similar to other stars, and their average colour is white. But changes in their chemistry over time can turn some of them into rarer colours.
The newly identified green one, named WD J1330+6435, is a white dwarf in the Milky Way galaxy that is located about 280 light-years from Earth in the constellation Draco. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).
"We first became interested in this white dwarf because bits of planetary material have fallen onto its surface, enriching its outer layers in elements including calcium, lithium and, in particular, sodium," said astrophysicist Mark Hollands of the University of Warwick in England, lead author of the study published in the journal RAS Techniques and Instruments.
In normal stars, sodium absorbs a very specific wavelength of orange - the same orange that gives sodium street lamps their characteristic colour.
"In this particular white dwarf, however, the density is so high that the sodium atoms are constantly being bombarded by surrounding helium atoms, which alters the sodium atomic energy levels," Hollands said.
"The end result is that the sodium atoms no longer absorb one very narrow wavelength of orange - they absorb the yellows, oranges and some of the red wavelengths. And what you're left with is a star that has a beautiful chartreuse green glow, something that wouldn't be possible for sun-like stars," Hollands said.
The discovery was part of a project assessing the colour of white dwarfs as human eyes would see them. A white dwarf typically has half the mass of the sun compacted into the size of Earth.
"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," University of Warwick astrophysicist and study co-author Boris Gänsicke said.
"In the end, it's all very similar to the mixing of colours in the palette of a painter. They start with a few basic colours, then mix them together to form any colour they need for their painting. Take away some of the colour going into the mix, and the output will change," Gänsicke said.
A star that is salmon pink
The researchers studied 151 white dwarfs located within about 65 light-years of Earth, with white the average colour.
One showed perhaps the reddest hue of any known star, a salmon pink. Named WD J1011+2845, it is located 48 light-years from Earth in the constellation Leo Minor. This time, it was not sodium that was changing the palette, but carbon.
"Rather than something hitting the star, in this case the carbon comes out from its deep interior," Gänsicke said.
Data used by the researchers included light spectrum observations from the Sloan Digital Sky Survey using a telescope in New Mexico, as well as white dwarf spectrum models they developed.
They found that white dwarf colours divide into distinct groups based on atmospheric composition. Carbon-rich white dwarfs usually were pink or purple. Those enriched with debris from shattered remnants of planetary systems leaned toward yellow. Lower-temperature white dwarfs with mixed hydrogen and helium atmospheres tended toward a cyan or pale blue hue.
In all, they spanned almost every colour of the spectrum.
While WD J1330+6435 was the first documented example of a green star in the real world, green stars have existed 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.