Stars are usually described by three colors: orange, white and blue. Today’s NASA Astronomy Picture of the Day adds a surprising fourth. It shows an artist’s illustration of WD J1330+6435, a white dwarf that researchers say would look green to the human eye.

What you are looking at

The APOD image is an illustration, not a photograph. It shows the white dwarf surrounded by a disk of gas and rocky debris orbiting the star. White dwarfs are the small, faint remnants left when a star like the Sun runs out of nuclear fuel. No white dwarf is close enough for us to see its color directly, and WD J1330+6435 is far too faint to spot in the night sky.

How do you find the color of a star you cannot see?

A team at the University of Warwick, led by Dr. Mark Hollands, calculated it. They took each star’s spectrum, the pattern produced when starlight is split by wavelength, and ran it through a model of human color vision. The result is the color the star’s light would appear to a human eye. The work was published in the journal RAS Techniques and Instruments in a paper titled “Any colour you like: The diverse chromaticities of white dwarfs.”

Why this white dwarf is green

For most stars, color tracks temperature. Cooler stars look orange, Sun-like stars look white and the hottest look pale blue. White dwarfs were assumed to follow the same rule, but their extreme density changes things. The atmospheres of the coolest white dwarfs can reach about 1 gram per cubic centimeter, roughly the density of water. Under those conditions, trace elements such as carbon, calcium and sodium interact with the dense helium around them and absorb light over a much wider range of colors than they normally would.

WD J1330+6435 has a temperature of roughly 4,000 degrees Celsius and has accreted rocky material from its own planetary system. That material enriched its atmosphere with calcium, lithium and sodium. According to the team, the sodium absorbs the orange part of the spectrum, leaving a chartreuse green appearance in their models.

Why “first”

The researchers say that, as far as they are aware, no other star has ever been credibly reported as green, which makes this the first known genuinely green star in the Galaxy. The star itself was already catalogued. What is new is the realization of what color it would actually appear.

Other colors, and why most white dwarfs are still white

  • Pink or purple: carbon-rich white dwarfs. The team also identified WD J1011+2845, about 48 light years away, as possibly the reddest star ever recorded, with a deep salmon-pink glow.
  • Yellow: white dwarfs enriched with debris from shattered asteroids and comets, the remains of old planetary systems.
  • Cyan or pale blue: cooler white dwarfs with mixed hydrogen and helium atmospheres, where hydrogen molecules colliding with helium atoms tint the light.

These unusual colors are the exception. In a sample of the 151 white dwarfs within about 65 light years of Earth, only five (two yellow, three pink) showed unusual colors, which suggests roughly one in thirty white dwarfs is oddly colored. For the other 146, the median color temperature was about 6,800 kelvin, which we perceive as white. The team’s conclusion: the average white dwarf really is white. The common idea that white dwarfs are typically blue comes from the fact that hotter, bluer ones are brighter and easier to find in surveys.

What to keep in mind

  • The colors are calculated from spectra with a color-vision model. Nobody has seen these stars glow green. They are also much too faint to see by eye.
  • “First green star” is the researchers’ claim, based on their knowledge of the literature.
  • This post is based on the APOD entry and the University of Warwick press release.

Sources

Featured image: illustration of WD J1330+6435 and its debris disk. Image Credit & Copyright: Mark A. Garlick / University of Warwick https://markgarlick.com/. Image via NASA APOD, Oct. 22, 2026.