Astronomers discover rare greenish-yellow white dwarf in the Milky Way

Astronomers have identified a white dwarf in the Milky Way with a rare pale greenish-yellow hue, which researchers attribute to chemical elements accumulating on its surface after it swallowed rocky debris from planets that once orbited it, Reuters reported.

The object, designated WDJ1330+6435, lies about 280 light-years from Earth in the constellation Draco. White dwarfs are extremely dense stellar remnants that form when low- and intermediate-mass stars exhaust their nuclear fuel and shed their outer layers.

The findings were published in the journal RAS Techniques and Instruments by researchers at the University of Warwick in England. The team set out to investigate the colors of white dwarfs as they would appear to the human eye.

Astrophysicist Mark Hollands, who led the study, said the team became interested in the star after discovering evidence that planetary material was falling onto its surface. This process enriched its outer layers with chemical elements, including calcium, lithium and sodium, with sodium playing a key role in producing its unusual color.

Under ordinary conditions, sodium atoms absorb a specific wavelength of orange light, a property also responsible for the characteristic color of sodium streetlights. But the extreme density of a white dwarf causes sodium atoms to collide frequently with surrounding helium atoms, altering their atomic energy levels.

As a result, sodium absorbs a broader range of wavelengths, including yellow, orange and part of the red spectrum. The resulting light gives the star its unusual greenish-yellow appearance, a color that researchers say cannot be produced in the same way in stars similar to the Sun.

Stellar colors are generally determined by surface temperature. Cooler stars tend to appear orange, while the Sun and similar stars appear white, and hotter stars often have a pale blue hue. The chemical composition of a white dwarf’s atmosphere, however, can significantly alter its apparent color.

Astrophysicist Boris Gänsicke, a co-author of the study, said the extraordinary density of white dwarfs means even small amounts of sodium can block a substantial portion of their emitted orange light.

White dwarfs are roughly the size of Earth, although their mass can reach around 165,000 times that of the planet, according to the report. Gänsicke compared the color changes to mixing paints, in which removing one color from a mixture can produce a different hue.

The researchers also examined 151 white dwarfs located within about 65 light-years of Earth. White was the most common color in the sample, but the team identified rarer hues associated with differences in atmospheric chemical composition.

One object that attracted their attention was WDJ1011+2845, a white dwarf located about 48 light-years away in the constellation Leo Minor. It has a pink hue resembling the color of salmon flesh and is believed by researchers to be among the reddest known stars.

Unlike the greenish-yellow white dwarf, whose color is linked to sodium from planetary debris, the pink object’s appearance is associated with carbon rising from its interior layers toward the surface.

Gänsicke said the carbon originates inside the star rather than from external material falling onto it, highlighting the different processes responsible for the two objects’ colors.

The researchers used spectral data from the Sloan Digital Sky Survey, collected with a telescope in New Mexico, alongside spectral models they developed to investigate the physical and chemical properties of white dwarfs.

Their findings showed that the objects’ colors fall into groups associated with the chemical composition of their atmospheres. Carbon-rich white dwarfs tend to appear pink or purple, while those containing remnants of disrupted planetary systems can appear yellow. Cooler white dwarfs with hydrogen- and helium-dominated atmospheres tend to look cyan or pale blue.

The study suggests that chemical differences among white dwarfs can produce colors spanning nearly the entire visible spectrum, expanding the range of hues associated with these dense stellar remnants beyond the white appearance commonly attributed to them.