Astronomers Identify WD J1330+6435 as First Green-Appearing Star

Sodium in a dead star’s atmosphere removes so much orange light that the remaining spectrum peaks near green. That filtering mechanism gives WD J1330+6435 a modeled chartreuse appearance and makes it the first known star that would look green to human vision, according to research published October 16 in RAS Techniques and Instruments.

The green color was calculated rather than seen directly. No white dwarf is close enough for a human observer to discern its color unaided, so the researchers started with measured spectra and processed the light through a model of human color perception. The result represents how the star’s mix of wavelengths would appear, with its display brightness scaled while preserving the modeled color.

WD J1330+6435 is a white dwarf, the dense remnant left after a star has exhausted its nuclear fuel. Its temperature is about 4,240 degrees Celsius, but temperature alone does not explain its unusual appearance. The star’s helium-dominated atmosphere contains calcium, lithium and sodium associated with rocky material accreted from the remains of its former planetary system.

How sodium turns a white dwarf green

Most stars approximate thermal radiators: their color follows temperature, progressing broadly from orange through white to pale blue as they become hotter. WD J1330+6435 breaks that familiar relationship because its atmospheric elements selectively remove parts of the visible spectrum.

Broad sodium absorption strips out much of the orange light, while calcium absorbs light toward the violet end. The surviving output peaks at around 500 nanometers, producing the chartreuse-green result in the researchers’ human-vision model. In this case, a relatively small amount of atmospheric material reshapes the star’s visible color more decisively than its temperature does.

The white dwarf’s extreme atmospheric conditions amplify that effect. Cool, helium-rich white-dwarf atmospheres can become unusually dense and transparent, allowing metal absorption features to broaden across large wavelength ranges. The atmospheric calculations account for interactions between sodium atoms and surrounding helium that alter the sodium absorption profile. A simpler treatment of those lines had previously produced a cooler modeled solution.

The measured spectrum is somewhat noisy, so the exact chromaticity could shift slightly with improved data. An earlier model of the object nevertheless produced a similar green hue, supporting the broader conclusion that its spectrum falls well outside the colors expected from temperature alone.

The color carries traces of a destroyed planetary system

The sodium is more than a color filter. Heavy elements generally sink rapidly through a white dwarf’s intense gravity, so their presence in its atmosphere points to continuing or relatively recent accretion. Rocky bodies such as asteroids can be disrupted around white dwarfs, depositing material that lets astronomers examine the chemistry of planetary systems after their host stars have died.

That makes WD J1330+6435’s color a visible expression of two linked processes: the destruction and accretion of planetary debris, followed by absorption inside a dense stellar atmosphere. Calcium, lithium and sodium reveal the incoming material chemically; sodium’s broad orange-light absorption makes part of that history apparent as color.

The wider study found that white-dwarf chemistry can generate more than green. Carbon-rich atmospheres can produce pink or purple hues, while debris-enriched objects can tend toward yellow. Cool white dwarfs with mixed hydrogen and helium atmospheres may appear cyan or pale blue because of interactions in their dense outer layers.

Another object, WD J1011+2845, lies 48 light-years away and may be the reddest known star. Carbon brought to its surface concentrates much of its visible output at longer wavelengths, creating a modeled deep salmon-pink appearance.

Unusual examples remain exceptions. In the paper’s volume-limited sample of 152 white dwarfs within 20 parsecs, most tracked close to the color expected from thermal radiation. But WD J1330+6435 demonstrates the limit of treating stellar color as a thermometer: enough atmospheric absorption can make a star called a white dwarf appear chartreuse, with the decisive filter supplied by the rocky remains of worlds it once illuminated.

By David Whitaker — Associate editor for AMI’s aerospace and drone systems desk, translating flight systems, aircraft programs, spaceflight, and UAV developments into accessible technical stories.