Astronomers Find a 'Ghostly Green' White Dwarf, a Likely First

Why a white dwarf can look green

Most stars behave like perfect radiators, so their color is set almost entirely by temperature. White dwarfs break that rule because their gravity is so extreme.

White dwarfs are among the densest objects in the known universe, and their atmospheres are dense too. In cool, helium-dominated white dwarfs, the helium becomes more transparent as the star ages, letting light reach deeper, denser layers. Trace elements there — sodium or calcium pulled in from a surrounding planetary system, or carbon dredged up from the core — absorb light across a wider range of wavelengths than you would expect.

The result is a star that can appear in colors no temperature scale would predict. Mark Hollands of Warwick University's astronomy and astrophysics group, the study's lead author, told CNET that the green star, WD J1330+6435, sits in a rocky planetary system whose debris feeds it the trace elements needed for that ghostly green tint.

The team took the same spectroscopic data for 151 white dwarfs and ran it through a simulation that converts it to human color vision. Of those, 146 averaged about 6,800K and would look bright white, perhaps with the faintest hint of blue. The other five came out cyan, yellow, purple/violet, pink and green.

The pink one is the odd one out

WDJ0902+2010 has a carbon-heavy atmosphere that absorbs light across the spectrum, giving it a salmon-pink appearance. Unlike the others, it should keep that color for billions of years.

Most of these colors are temporary. A white dwarf starts out at roughly 100,000K, which would look sky blue, and only reaches the interesting hues around 5,000K. Eventually, high gravity drags the trace elements back into the core and the star returns to a normal white or orange.

The pink star is different. Its carbon comes from inside the star itself, driven to the surface by natural convection, so the color should persist for billions of years rather than a few million.

Hollands said the green, pink and cyan colors are all likely firsts, since he could find no credible prior reports of stars in those hues. The findings were published this month in RAS Techniques and Instruments.

What it means for you

This is pure astronomy, so there is no product to buy or wait for. The practical value is in how the work was done.

No white dwarf has ever been seen with human eyes; none is close enough. Astronomers normally study them through spectroscopy, focusing on chemical makeup and brightness rather than color. Hollands said the question of what these stars would actually look like if you visited one is something the public asks all the time, and answering it turned out not to be too difficult.

That is a useful reminder of how much science sits in data that has already been collected. The same spectra that tell researchers what a star is made of can also tell them what it would look like. There is no new telescope or mission behind this result, just a different question asked of existing observations.

Availability and pricing

There is nothing to buy here. The study is published in RAS Techniques and Instruments, a peer-reviewed journal, and is accessible to researchers and the public through its publisher. Readers in the US, UK and EU can follow the work through the University of Warwick's astronomy group and the journal's open pages.

A green star is a good headline, but the real finding is that white dwarf color is a story about atmospheres, not just thermometers. The same data astronomers already use to weigh and dissect dead stars can also tell us what they would look like up close.