Astronomers have identified what appears to be the first documented green star in the universe, uncovering an unusual white dwarf whose distinctive color is caused by the strange behavior of sodium atoms under extreme conditions. Located approximately 280 light-years from Earth, the stellar remnant challenges conventional expectations about the colors stars can display and reveals how the remains of destroyed planetary systems can influence the appearance of their parent stars.
The object, known as WD J1330+6435, is located in the constellation Draco, also called the Dragon, within the Milky Way. Unlike ordinary stars, whose colors are primarily associated with their surface temperatures, this white dwarf owes its unusual green appearance to the chemical composition of its atmosphere and the extraordinary density of its material.
The findings also highlight the lasting influence of planetary systems, showing how material left behind by destroyed worlds can alter the light emitted by a star long after its most active phase has ended.
The unusual color of WD J1330+6435 is linked to sodium that accumulated in its outer layers after fragments of planetary material fell onto the stellar remnant.
Astrophysicist Mark Hollands of the University of Warwick in England, the lead author of the study published in RAS Techniques and Instruments, explained that the researchers initially became interested in the object because its atmosphere contained elements associated with the remains of planetary bodies.
“We started to be interested in this white dwarf because fragments of planetary material had fallen onto its surface, enriching the outer layers with elements such as calcium, lithium and, particularly, sodium,” Hollands said.
In most familiar stellar environments, sodium absorbs light at a particular wavelength associated with the orange part of the spectrum. This behavior is also responsible for the characteristic yellow-orange glow associated with sodium-vapor streetlights.
However, the conditions surrounding WD J1330+6435 are far from ordinary.
How Sodium Creates an Unusual Green Glow
Inside the dense atmosphere of this white dwarf, sodium atoms interact continuously with surrounding helium atoms. These repeated collisions alter the energy levels of sodium, changing the way the atoms absorb incoming light.
Rather than absorbing only a narrow range of orange wavelengths, the sodium absorbs a broader selection of visible light, including yellow, orange and some shades of red.
With more of those colors removed from the light reaching an observer, the remaining light produces the unusual green appearance.
Hollands explained that this effect would not occur in the same way in stars with conditions comparable to those of the Sun. The extreme density of a white dwarf changes the behavior of its atoms, creating an environment in which familiar elements can produce unexpected optical effects.
The result is not evidence that the star has a green surface in the same way that an object on Earth might be painted green. Instead, its apparent color emerges from the interaction between its atmosphere and the light escaping into space.
This distinction is important because astronomers often determine the properties of distant stars by analyzing their spectra, which reveal how different elements absorb or emit specific wavelengths of light.
What Makes White Dwarfs So Extreme?
White dwarfs are the compact remnants left behind when stars with relatively low or intermediate masses exhaust their nuclear fuel and shed their outer layers.
Unlike massive stars that can eventually collapse or explode in spectacular supernova events, Sun-like stars generally follow a different evolutionary path. After their fuel supply declines, they expand into red giants and release much of their outer material into space. The remaining core becomes a white dwarf.
Although the original star has lost much of its outer structure, its remnant remains extraordinarily dense.
A typical white dwarf has a mass comparable to roughly half that of the Sun, compressed into a body approximately the size of Earth. That combination makes these objects fundamentally different from ordinary stars.
A Small Amount of Material Can Change the Light
Boris Gänsicke, a University of Warwick astrophysicist and co-author of the study, emphasized how much material can be packed into such a small object.
“Since white dwarfs are incredibly dense stars, they are approximately the size of Earth, but contain around 165,000 times more mass,” Gänsicke said. He explained that even a relatively small quantity of sodium can therefore block a substantial portion of the orange light emitted by the star.
The researchers compared the process to mixing colors on an artist’s palette. Removing one color from a mixture changes the final appearance, even though the other colors remain present.
The discovery illustrates why the appearance of a celestial object cannot always be explained by temperature alone. In white dwarfs, atmospheric chemistry and the physical conditions surrounding atoms can be just as important in determining the colors observed from Earth.
Astronomers Find a White Dwarf With a Salmon-Pink Color
The green star was not the only unusual object identified during the research. The scientists also examined 151 white dwarfs located within approximately 65 light-years of Earth, finding that their colors varied according to their atmospheric compositions.
White remained the most common appearance among the objects studied. However, some displayed distinctive shades that would be unusual among ordinary stars.
One example was WD J1011+2845, a white dwarf approximately 48 light-years away in the constellation Leo Minor, or the Little Lion. The object displayed a salmon-pink hue, which the researchers identified as one of the more reddish stellar appearances documented in their sample.
Unlike the green white dwarf, whose color is linked to sodium, the pink appearance of WD J1011+2845 was associated with carbon originating from the star’s interior.
The difference demonstrates how separate chemical processes can produce dramatically different visual results in objects that belong to the same general class of stellar remnants.
Carbon and Planetary Debris Produce Different Colors
The researchers found that white dwarfs could be grouped into several color categories depending on the composition of their atmospheres.
Objects with carbon-rich atmospheres tended to appear pink or purple. By contrast, white dwarfs enriched by material from disrupted planetary systems were more likely to display yellowish hues.
The chemical composition of a white dwarf can preserve clues about its earlier history, including whether it has accumulated material from asteroids, rocky planets or other bodies that once orbited it.
Although the original planetary system may have been destroyed or dramatically altered, traces of its former contents can remain visible in the atmosphere of the stellar remnant.
The findings suggest that the colors of white dwarfs are not simply cosmetic differences. They can provide information about the material surrounding these objects and the events that occurred during the final stages of their evolution.
How Scientists Studied the Stars’ Colors
To investigate the range of colors displayed by white dwarfs, the team combined astronomical observations with theoretical models designed to reproduce their spectra.
The observational data included measurements from the Sloan Digital Sky Survey, a major astronomical project that has collected extensive information about the light emitted by celestial objects. The relevant observations were obtained using a telescope in New Mexico, United States.
The scientists compared these measurements with spectral models developed by the research team to understand how different atmospheric compositions influence the light reaching observers.
Rather than relying solely on the conventional scientific classification of stars, the project sought to determine how white dwarfs would actually appear to the human eye.
This approach allowed the researchers to identify patterns linking visible color with chemical composition and the physical conditions within the stars’ atmospheres.
Their results indicate that white dwarfs can display colors spanning almost the entire visible spectrum. While most remain white, unusual atmospheric conditions can produce shades ranging from yellow and salmon pink to purple and green.
