THE FIRST GREEN STAR EVER FOUND: Why a Dead Star 280 Light-Years Away Glows Chartreuse

Quick answer: Astronomers at the University of Warwick have identified what they describe as the first genuinely green star known in our galaxy. It is a cool white dwarf called WD J1330+6435, about 280 light-years away in the constellation Draco. Rocky debris from its old planetary system has polluted its ultra-dense atmosphere with sodium, which soaks up yellow, orange and some red light. What is left over would look chartreuse green to a human eye seen up close. The finding comes from a study in RAS Techniques and Instruments that worked out the true colours of white dwarfs.

Ask anyone what colour stars come in and you will probably hear red, orange, yellow, white and blue. Green is missing from that list for a good physical reason: ordinary stars simply cannot produce it. That is why the news of a “green star” spread so quickly online, with people trading jokes and science-fiction comparisons. The real explanation is stranger and more interesting than the jokes, and it involves dead stars, shattered planets and an atmosphere as dense as water.

Why ordinary stars are never green

For most stars, colour is a thermometer. The University of Warwick explains that cooler stars look orange, stars like the Sun look white and the hottest ones glow pale blue, much like a blacksmith’s iron changes colour as it heats up.

The reason is that stars behave almost like what physicists call “perfect radiators” (blackbodies). They pour out light across a broad range of wavelengths at once. The peak shifts with temperature, but the light is always spread so widely that our eyes blend it into orange, white or blue. As the study’s abstract puts it, the atmospheres of normal stars cause “only minor deviations” from blackbody colours once the light is filtered through the broad colour sensitivity of human vision. A star would need to lose a big chunk of the rest of the spectrum to look green, and normal stars don’t.

What a white dwarf is

A white dwarf is the leftover core of a low- or medium-mass star that has used up its nuclear fuel and shed its outer layers. NASA notes that our own Sun will become one in about 10 billion years, after its red giant phase, and that white dwarfs can be surrounded by debris and even planets left over from the original star.

These objects are extreme. A typical white dwarf packs roughly half the Sun’s mass into a body about the size of Earth. Warwick astrophysicist and co-author Boris Gänsicke told Reuters that they “are roughly the size of the Earth, but contain about 165,000 times its mass.”

That density is the key to the whole story. According to the researchers, the atmospheres of the coolest white dwarfs can reach about 1 gram per cubic centimetre, roughly the density of water and millions of times denser than the visible surface of the Sun. In conditions like these, trace elements such as carbon, calcium and sodium behave very differently from the way they do in an ordinary star.

Not every dead star is quiet

White dwarfs turn up in some of astronomy’s most dramatic stories. The glowing remnant of a supernova that medieval sky-watchers saw in 1181 is thought to come from a rare kind of white dwarf explosion. You can read about it in our report on the most detailed view yet of the 1181 supernova remnant.

How sodium turns a star green

WD J1330+6435 has been swallowing pieces of its own past. Lead author Mark Hollands, a Warwick Prize Fellow, told Reuters that the team first became interested in the star “because bits of planetary material have fallen onto its surface, enriching its outer layers in elements including calcium, lithium and, in particular, sodium.”

In a normal star, sodium absorbs a very narrow band of orange light. It is the same orange you see in old sodium street lamps. In this white dwarf the atmosphere is so crowded that sodium atoms are constantly bombarded by surrounding helium atoms, and that changes how they absorb light.

“The end result is that the sodium atoms no longer absorb one very narrow wavelength of orange — they absorb the yellows, oranges and some of the red wavelengths. And what you’re left with is a star that has a beautiful chartreuse green glow, something that wouldn’t be possible for sun-like stars.” — Mark Hollands, via Reuters

Calcium plays a supporting role at the other end of the rainbow. The paper reports that calcium absorbs violet wavelengths while the broad sodium feature removes the orange, so the star’s light ends up peaking around 500 nanometres, in the green part of the spectrum. The team’s models put the star’s temperature at about 4,240 kelvin, or roughly 4,000 °C, which is cool for a star.

Gänsicke compared the process to a painter’s palette: start with a few basic colours, take some of them out of the mix, and the colour that comes out changes. As far as the researchers know, no other star, “living or dead”, has ever been credibly reported as green.

How you measure the colour of a star you can’t see

There is an important catch. Nobody has looked at WD J1330+6435 and seen it glow green. White dwarfs are too faint to spot with the naked eye, and none is close enough for its colour to be seen directly.

Instead, the team calculated it. They started from each star’s spectrum, the “fingerprint” made when starlight is split by wavelength, using observations from the Sloan Digital Sky Survey and their own white dwarf atmosphere models. Then they ran those spectra through a standard model of human colour vision to work out how each star would look to a person close enough to see its disc. The full method is described in the paper, “Any colour you like: The diverse chromaticities of white dwarfs” (DOI: 10.1093/rasti/rzag069).

The authors are open about the uncertainty. They note that the star’s spectrum is “somewhat noisy,” so its exact shade could differ slightly in reality. But an older model of the same star also came out green, which they say gives them confidence in the result.

A whole palette of strange stars

The green star was the headline, but the study found more surprises. White dwarf colours fall into groups linked to what is in their atmospheres:

• Pink and purple: carbon-rich white dwarfs, where carbon dredged up from the core forms molecules that absorb much of the visible spectrum.
• Yellow: stars 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 change the light.

At the opposite extreme from the green star, the team identified what may be the reddest star ever recorded. WD J1011+2845, just 48 light-years away in the constellation Leo Minor, has carbon molecules that squeeze almost all of its light into wavelengths beyond 600 nanometres, producing a deep salmon-pink glow. “Rather than something hitting the star, in this case the carbon comes out from its deep interior,” Gänsicke said.

Is the average white dwarf actually white?

Yes. To get a fair picture, the researchers examined all 151 known white dwarfs within about 65 light-years of Earth. Only five had unusual colours: two yellow and three pink. That suggests roughly one in thirty white dwarfs shines with an unusual colour.

For the other 146, the median colour temperature came out at about 6,800 kelvin, which we perceive as white. The team says this corrects a common misconception that white dwarfs are typically blue. Hotter, bluer white dwarfs are brighter and can be seen from farther away, so they are over-represented in surveys that rely on brightness. The featured image on this page is a good example of the classic picture: NASA’s artwork shows the bright star Sirius A with its small, hot companion Sirius B, a white dwarf drawn as a vivid blue point.

Why this matters

The study is a reminder of how much information hides in a beam of starlight. A spectrum can reveal a star’s temperature and chemistry, and, as this work shows, its true colour. For white dwarfs, colour also carries a record of history: sodium-rich debris on WD J1330+6435 is evidence that rocky material from its planetary system ended up falling onto the star.

If you enjoy stories about distant worlds, see how the James Webb Space Telescope photographed the coldest alien planet ever imaged. And if you’d rather look up yourself, our guides to northern lights alerts and lunar occultations explain what you can see with your own eyes. More space stories are in our Science section.

Frequently asked questions

Is there really a green star?

According to University of Warwick researchers, yes. The white dwarf WD J1330+6435 would appear chartreuse green to a human eye seen up close. They say no other star has been credibly reported as green.

Where is the green star?

WD J1330+6435 lies about 280 light-years from Earth in the northern constellation Draco, inside our Milky Way galaxy.

Why is WD J1330+6435 green?

Planetary debris added sodium to its extremely dense helium atmosphere. Collisions with helium atoms make the sodium absorb a wide range of yellow, orange and red light, while calcium absorbs violet. The remaining light peaks around 500 nanometres, which looks green.

Can I see the green star with a telescope?

Not as a green disc. White dwarfs are far too faint to see with the naked eye, and the colour was calculated from the star’s spectrum using a model of human colour vision. It describes how the star would look from close enough to see its disc.

What colour are most white dwarfs?

White. In a sample of 151 nearby white dwarfs, the median colour temperature was about 6,800 kelvin, and only about one in thirty showed an unusual colour such as yellow or pink.

Sources: University of Warwick; Hollands, Gänsicke, Tremblay & Koester, RAS Techniques and Instruments (preprint on arXiv); Reuters via CNA; NASA Science.

Featured image: Artist’s impression of Sirius A and its white dwarf companion Sirius B, NASA, ESA and G. Bacon (STScI), public domain, via Wikimedia Commons.