A New Color of Star - How Can a Star Be Green?

An artist's depiction of white dwarf with a green coloration. Such an object would need to be eating rocky material that orbits it.

From red to white to blue, there are lots of different colors of stars, but traditionally it has been said that it is impossible for a star to be green. But on October 14, 2026, it was announced that astronomers had discovered the impossible and found a green star. But how can a star be green if we’ve always said they can’t exist?

How Stars Get Their Colors

Stars behave roughly like black bodies. A black body is an object that absorbs all incoming light, such that no light reflects off it. This means that the only way you’ll ever see a black body is with the light that it emits on its own. It is also known as a perfect radiator. No object in the real world is a true black body, but the light radiated by stars is so much brighter than any light bouncing off them that they can be approximated as a black body.

One useful property of a black body is that you can predict what color they are based on their temperature alone. This is something we see in stars. Cooler stars are in the red and orange range, average stars shine white, and the hottest stars are blue. It works in reverse as well. If you know the specific color of a star, you can accurately predict how hot it is. This is how astronomers can confidently predict the temperatures of distant stars without ever sending a thermometer anywhere near it. The reason for this coloration is that hotter stars release more energetic waves, which have shorter wavelengths. On the spectrum of visible light, red light has the longest wavelengths, and therefore the lowest energy, while blue is the opposite.

The black body curves of several stars. The graph shows each star's total output of light and where it relates to the visible spectrum. The color under the curve shows what color the star looks to the human eye. Image Credit: Daniel William "Danny" Wilson (CC-BY 4.0)

Why A Green Star Should Be Impossible

As it turns out, it is impossible for black body radiation to produce a color that our eyes would perceive as green. No star emits a single color of light. Instead, they release a wide spectrum, which is centered around one peak wavelength. If we see a star as red, that means that the red light is at the center of that spectrum and dominates the other colors in the spectrum. But imagine a rainbow. Red and blue are at either end of the rainbow, so a spectrum peaking at one of those wavelengths will have mostly red or blue light. But a spectrum that peaks at green, which is in the center of the rainbow, would have light spread out across the entire rainbow. When all the colors of the rainbow are mixed together, you get white light. Therefore, any star peaking at green wavelengths would instead appear white to the human eye. In fact, the Sun—which is technically white, not yellow—has peak light output in green wavelengths.

A Green Dwarf?

And if the Sun’s not green with a peak output of green light, how can any star be green? Well, the green star discovered, WD J1330+6435, isn’t a usual star. It’s a white dwarf. A white dwarf isn’t a living star. Instead, it’s what we call a stellar remnant—the dead remains of a once-living, Sun-sized star that no longer produces new energy via nuclear fusion. In theory, white dwarfs are also near black body emitters, at least that’s what we thought. Even the name white dwarf implies as much.

White dwarfs are incredibly dense. They form from the cores stars the size of our Sun collapsing down to about the size of the Earth. This resulting ultra-dense atmosphere of the stellar remnant can significantly alter its light by absorbing specific wavelengths, resulting in it no longer resembling a black body. Usually, a white dwarf wouldn’t be expected to have the types of materials in its atmosphere needed to alter its spectrum, but if the white dwarf got materials from an outside source, then that could change. WD J1330+6435 has an atmosphere rich with calcium, lithium, and sodium, elements it could have acquired by eating rocky planets that once orbited it. Different elements will absorb different wavelengths of light. Usually, only a specific narrow band of light, but the density of a white dwarf will actually broaden the absorption via the constant bombardment from helium atoms. In this case, the sodium will absorb much of the white dwarf’s emitted red-orange light, while the calcium will absorb much of the blue-violet light. With the red and blue light emitted by the white dwarf significantly decreased by this absorption, the visible light emitted by the stellar remnant would actually look green.

The graph on the left shows the spectrum of WD J1330+6435. 500 nm, where the output peaks is green light, while the dips on either side is where the blue and red light are absorbed before leaving the star. The right image shows what color of the white dwarf would look to the human eye. Image Credit: Mark A Hollands, Boris T Gänsicke, Pier-Emmanuel Tremblay, Detlev Koester (CC-BY 4.0)

We also know of white dwarfs with other colors. Different elements taken into their atmospheres can lead to a rainbow of different colors. We’ve also found white dwarfs that are yellow, blue, or even pink. Of the 151 white dwarfs within 65 light-years of Earth, five of them have unusual colors. Leading to the possibility that as many as 1 in 30 white dwarfs across the universe may have these odd colors. Given how many white dwarfs are out there, that’s a lot of unexpected color to find.