A dead star about 280 light-years away would glow chartreuse green up close, thanks to the remains of planets it swallowed. Here's how the color works.
Astronomers at the University of Warwick say they've found the first known green star. The white dwarf WD J1330+6435, about 280 light-years away, would glow chartreuse to human eyes because sodium, calcium and lithium from rocky debris it swallowed sit in its crushed, ultra-dense atmosphere and absorb the yellow, orange and some red light.
The team ran the spectra of 151 white dwarfs through a model of human vision. Most looked plain white, but five came out green, pink, cyan, yellow and purple or violet. They estimate roughly one in 30 white dwarfs has an unusual color.
The colors are clues to planets being destroyed as stars die. Most fade within a few million years as gravity pulls the elements inward, but carbon-pink stars can stay that way for billions of years.
No star had ever been credibly identified as green. Now one has, and the color comes from something grim: the shredded remains of the rocky worlds that once orbited it. Astronomers at the University of Warwick say the white dwarf WD J1330+6435, about 280 light-years from Earth, would glow a chartreuse green if you could get close enough to see it.
It sounds like trivia, but it answers a question people ask astronomers all the time: what do stars actually look like? It also turns color into a clue. The green white dwarf color is, in effect, a visible record of planets being destroyed, which tells scientists how often dying stars consume the worlds around them, including, one day, what might happen in our own solar system.
The green color comes from sodium. Over time, WD J1330+6435 has pulled in rocky debris from its old planetary system, loading its atmosphere with sodium, calcium and lithium. Normally sodium soaks up a very narrow slice of orange light. Inside this star, it absorbs yellows, oranges and some reds too, and what’s left over reaches the eye as green.
Lead author Mark Hollands, an astronomer at Warwick, called the result a “ghostly green glow” and said it feels like something from science fiction. The star is cool by stellar standards, roughly 4,000 degrees Celsius.
Why can a trace of sodium change a star’s whole color? Density. White dwarfs are the tiny, crushed cores left when stars like the sun run out of fuel. They pack about half the sun’s mass into something only a bit bigger than Earth. In the coolest ones, the atmosphere can reach about a gram per cubic centimeter, about as dense as water, while the sun’s outer layer is millions of times thinner. At that pressure, small amounts of other elements interact with the surrounding helium and absorb a much wider range of light than usual.
Nobody has seen a white dwarf’s color directly. None is close enough to view with the naked eye, and astronomers normally study starlight as a spectrum, light split into its separate wavelengths the way a prism makes a rainbow, to learn what a star is made of and how hot it is.
Hollands’ team took existing spectra for 151 white dwarfs and ran them through a model of human color vision, calculating what a person would actually see. Of those, 146 came out ordinary: bright white with perhaps the faintest touch of blue, averaging about 6,800K in color temperature. The other five were the surprises, coming out green, pink, cyan, yellow and purple or violet. The study appeared in the journal RAS Techniques and Instruments in mid-October.
The team found the odd colors fall into groups that follow each star’s chemistry:
Hollands said the pink and cyan colors also appear to be firsts, since astronomers could find no earlier reports of stars in those shades.
For anyone curious about our place in the universe, this is the striking part. The most vivid colors are, in effect, the visible aftermath of planetary destruction. When a star like the sun nears the end of its life, it swells into a giant, can engulf nearby planets, then sheds huge amounts of mass. That upsets the orbits of whatever is left. Some planets get flung into interstellar space, while asteroids and other rocky bodies can be dragged in and torn apart by the white dwarf’s gravity.
Hollands says around a quarter or more of white dwarfs contain traces of planetary debris in their atmospheres. Astronomers expect our own sun to end its life in a similar way.
Color gives astronomers a simple new way to spot these cases. By studying a representative sample of white dwarfs within about 65 light-years of Earth, the team estimates that roughly one in 30 shows an unusual hue. Outside researchers described the work less as new physics and more as a fresh way of looking, one that connects astronomy with how human eyes work.
Probably not for long, in cosmic terms. White dwarfs are born extremely hot, at about 100,000 degrees Kelvin, which would make them look sky blue. The unusual colors appear only after they cool to roughly 5,000K. Over time, the star’s intense gravity drags the trace elements down toward the core, and the color fades back to ordinary white or orange within a few million years.
Carbon-colored pink stars are the exception. Because their carbon comes from inside the star and is kept stirred up by convection, that pink can last for billions of years. So if you want a star that stays colorful, pink is the safer bet than green.
Yes, as far as astronomers know there is now one: the white dwarf WD J1330+6435, about 280 light-years away, which would look chartreuse green to human eyes.
Sodium from rocky planetary debris sits in its extremely dense atmosphere and absorbs yellow, orange and some red light, leaving a green glow.
Not as green. White dwarfs are small and faint, so astronomers calculated the color from the star’s spectrum using a model of human vision.
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