Astronomers find the first ever green star

 Astronomers have found the first ever green star, in a study that shows white dwarves can actually come in a wide array of different colours.

The colour of most stars is decided by their colour: when they are cooler, they are orange, whereas our own warmer Sun is white, and the very hottest are a pale blue. Previously, researchers had thought that white dwarves – the remains that are left behind after stars like our Sun die, would work in the same way.

But new research suggests that, despite their name, white dwarves can actually come in a wide array of colours. If we were able to get up close with them, they would come in pink, cyan, yellow, purple – and even green, according to the authors of the study, who found the first such example ever seen.

“Most stars, including most white dwarfs, behave like ‘perfect radiators,’ so their colour is set almost entirely by temperature. But nobody had really looked into what else might be possible for white dwarfs,” said Mark Hollands, from the University of Warwick, who led the work. “It turns out these incredibly dense stars can produce colours nobody expected, and some of them genuinely surprised me."

White dwarves are unusual because they are so dense, the researchers say: they are millions of times denser than the outer shell of the Sun, roughly the same as water. That density means that any elements that might be around are able to interact with the plasma in the white dwarf, absorbing light across many more colours.

As such, those white dwarves are not those perfect radiators. Instead, they will reflect the chemical makeup of the material that are around them.

There is not a white dwarf near enough for us to see its colour. Instead, researchers int he new study calculated it, using data about its spectrum, the “fingerprint” that can be found by splitting starlight apart into its different wavelengths.

Scientists were then able to run that data through a model that simulates how the light would look to a human who was looking at them. As such, they were able to calculate the stars’ true colour.

It showed that the colour is decided by the material in the star’s atmosphere, putting them in different groups. Those with lots of carbon will appear pink or purple, others that are surrounded by pieces of old planetary systems will look yellow, and cooler ones with hydrogen and helium atmospheres will be blue.

But researchers were most excited by WD J1330+6435, a white dwarf that is about 4,000 degrees Celsius and has taken on rocky material from the planetary system for which it was once the Sun. That material has added calcium, lithium, and sodium to the atmosphere, making it glow with a chartreuse green.

It is the first star of that colour ever found, the researchers say.

“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 Hollands.

Imagine standing on a planet and looking up at a green star. It feels like something out of science fiction."

They also found what they believe to be the reddest star ever seen. WD J1011+2845 is relatively nearby – just 48 light years away – and carbon is being lifted up to its surface which then distorts the light so that almost all of the light is squeezed into particular wavelengths that make it glow a deep salmon-pink.