Astronomers find the first star that would look green to human eyes

Astronomers at the University of Warwick have found the first star that would look genuinely green to human eyes. It is not a young, blazing star but the dead core of one like our Sun, and it glows green for a reason that has nothing to do with being hot.

Colour usually means temperature

For most stars, colour and heat go together. Cool stars look orange, a star like the Sun looks white, and the hottest ones shine pale blue. The researchers compare it to a piece of iron in a fire, which glows red, then orange, then white as it warms up. Because colour follows temperature so closely, no star had ever been credibly identified as genuinely green, whether it was still burning or long dead. This one is the first known example in the Milky Way.

Dead stars like this are called white dwarfs, a name they earned because the first ones found were unusually hot and looked white or pale blue. The coolest of them are stranger than that name suggests. Their atmospheres can be squeezed to a density of about one gram per cubic centimetre, roughly the density of water and millions of times denser than the Sun's visible surface. In that crush, small amounts of carbon, calcium and sodium mix with the helium around them, and they soak up a much wider range of light than they would in thinner air. The light that finally escapes can take on colours that temperature alone would never predict.

The star that comes out green

The star is known as WD J1330+6435, and its temperature is about 4,000 degrees Celsius. Over time it has pulled in rocky debris from the planetary system that once surrounded it, adding calcium, lithium and sodium to its atmosphere. When the team worked out what those ingredients would do to its light, the answer was a chartreuse green. "The traces of sodium in the atmosphere of this white dwarf absorb the orange part of the spectrum, leaving behind a ghostly green glow," said Dr Mark Hollands, the Warwick Prize Fellow who led the work.

He added that the project began with a simple gap in knowledge: most stars, including most white dwarfs, behave like "perfect radiators," meaning their colour is set almost entirely by temperature, and nobody had really looked into what else might be possible for them. "It turns out these incredibly dense stars can produce colors nobody expected, and some of them genuinely surprised me," he said.

A hidden rainbow worked out from light alone

No white dwarf is close enough for anyone to make out its colour with the human eye, so the team could not simply look. Instead they measured the starlight and separated it into its component wavelengths, much as a prism divides sunlight into a rainbow. Those measurements were then processed through a model of human colour vision, which calculated the colours people would actually see.

Doing this across the sky sorted the dead stars into colour families. White dwarfs holding large amounts of carbon often appear pink or purple. Ones that have gathered material from destroyed asteroids and comets tend to look yellow. The coolest, with both hydrogen and helium in their atmospheres, can turn cyan or pale blue, because hydrogen molecules collide with helium atoms under the immense pressures inside these stars.

The findings were published in the journal RAS Techniques and Instruments. The green star itself stays out of reach of the eye: only from close up would its colour be obvious, and nothing is near enough to try.