A 'rare color' in space: The sighting of a 'green star' astonishes astronomers

Astronomers have spotted a green star for the first time, documenting a dense stellar remnant known as a white dwarf that took on a pale yellowish-green hue after engulfing rocky debris from planets that once orbited it.

Most white dwarfs resemble other stars, with white being their dominant color. However, chemical changes over time can transform some of them into rare colors.

The newly observed green star, named WD J1330+6435, is a white dwarf in the Milky Way galaxy, located about 280 light-years from Earth in the constellation Draco. A light-year is the distance light travels in one year, approximately 9.5 trillion kilometers.

Mark Holland, an astrophysicist at the University of Warwick in England, who led the study published in the journal *Research Notes of the AAS*, issued by the Royal Astronomical Society, said, “We initially became interested in this white dwarf because chunks of planetary material fell onto its surface, enriching its outer layers with elements including calcium, lithium, and, in particular, sodium.”

In normal stars, sodium absorbs a very specific wavelength of orange light—the same orange that gives sodium-containing streetlights their distinctive color.

Holland said, “In this specific white dwarf, the density is so high that sodium atoms are constantly bombarded by surrounding helium atoms, altering the sodium’s atomic energy levels.”

He added, “The end result is that sodium atoms no longer absorb a very narrow orange wavelength, but instead absorb yellow, orange, and some red wavelengths. The result is a beautiful yellowish-green glowing star, which is not possible for Sun-like stars.”

This discovery was part of a project to assess the colors of white dwarfs as seen by the human eye. A white dwarf typically has half the mass of the Sun compressed into the size of Earth.

Boris Gänzi, an astrophysicist at the University of Warwick and co-author of the study, said, “Because white dwarfs are extremely dense stars, roughly the size of Earth but containing about 165,000 times their mass, they block even a small amount of sodium from emitting a large amount of the orange light produced by the star.”

He added, “Ultimately, the whole process is very much like mixing colors on a painter’s palette. A painter starts with a few basic colors, then mixes them to create any color needed for the painting. If you remove some colors from the mix, the result changes.”

The researchers studied 151 white dwarfs located about 65 light-years from Earth, with white being the dominant color on average.

One of them displayed what may be the most vivid red shade among all known stars—a pinkish hue resembling salmon flesh. Located 48 light-years from Earth in the constellation Leo Minor and named WD J1011+2845, its color change was not caused by sodium, but by carbon.

Gänzi said, “Instead of something colliding with the star, in this case, carbon is emerging from its deep interior.”

The data used by the researchers included spectral light observations from the Sloan Digital Sky Survey using a telescope in New Mexico, as well as spectral models of white dwarfs that they developed.

Carbon-rich white dwarfs are typically pink or purple, while those rich in debris from shattered planetary systems tend to appear yellow. Low-temperature white dwarfs, whose atmospheres contain a mixture of hydrogen and helium, tend to exhibit a sky-blue or pale blue hue.