astrophysicist Mark Hollands said following the identification of the first documented green star. Astronomers have discovered a white dwarf, designated as WD J1330+6435, which exhibits a rare chartreuse or pale yellowish-green glow. Located approximately 280 light-years from Earth in the constellation Draco, the star is situated within the Milky Way galaxy, according to research published in the journal RAS Techniques and Instruments.
The Chemistry of a Green Star
The unusual coloration of WD J1330+6435 is the result of the star consuming rocky debris from planets that previously orbited it. This process enriched the white dwarf’s outer layers with elements such as lithium, calcium, and specifically sodium. While sodium typically absorbs a very specific wavelength of orange light in normal stars—the same effect seen in sodium street lamps—the environment of this white dwarf is fundamentally different.
According to Hollands, who led the study from the University of Warwick in England, the extreme density of the white dwarf causes sodium atoms to be constantly bombarded by surrounding helium atoms. This interaction alters the atomic energy levels of the sodium, causing it to absorb a broader range of wavelengths, including yellow, orange, and some red light. This shift in absorption results in the star’s distinct yellowish-green appearance, a phenomenon Hollands noted would be impossible for stars similar to the sun.
Boris Gänsicke, a University of Warwick astrophysicist and co-author of the study, explained that white dwarfs are super-dense, typically containing about half the mass of the sun compacted into a size roughly equal to Earth. He noted that because they contain approximately 165,000 times the mass of Earth, even small amounts of sodium can block significant quantities of orange light. Gänsicke compared the process to a painter’s palette, stating that removing certain colors from a mix changes the final output.
A Spectrum of Stellar Remnants
The discovery was part of a broader project to assess how white dwarfs appear to the human eye. The researchers analyzed 151 white dwarfs within 65 light-years of Earth, finding that while white remains the average color, chemical variations can produce rare hues across almost the entire spectrum.
The study identified another outlier, WD J1011+2845, located 48 light-years away in the constellation Leo Minor. This star exhibits a salmon pink hue, which researchers believe may be the reddest shade of any known star. Unlike the green star, this coloration is caused by carbon rising from the star’s internal depths rather than external planetary debris.
The research team utilized spectral light observations from the Sloan Digital Sky Survey, conducted via a telescope in New Mexico, alongside proprietary spectral models. Their findings categorize white dwarf colors based on atmospheric composition:
- Pink or Purple: Typical of white dwarfs rich in carbon.
- Yellow: Common in those containing debris from shattered planetary systems.
- Cyan or Pale Blue: Found in lower-temperature white dwarfs with atmospheres composed of hydrogen and helium.
While white dwarfs are the leftover cores of low- to medium-mass stars that have exhausted their nuclear fuel and expelled their outer layers, this study demonstrates that their chemical evolution can lead to a diverse array of visual appearances. The researchers concluded that these colors can help scientists track the remnants of destroyed planetary systems and understand the fate of stars after their primary life cycles end.
