Warwick astronomers reveal white dwarf stars can glow green, pink and purple
Researchers say they have found the first known green star
Astronomers at the University of Warwick say white dwarf stars can shine in a range of colours, including what they believe is the first known green star.
The new research is published in the journal RAS Techniques and Instruments.
It says that if white dwarfs could be viewed up close, some would appear green, pink, cyan, yellow or purple.
White dwarfs are the dense remnants left behind when Sun-like stars reach the end of their lives.
They got their name because the earliest examples found were hot and glowed white or pale blue.
For most stars, colour is a guide to temperature. Cooler stars look orange, stars like the Sun look white, and the hottest glow pale blue.
White dwarfs were thought to follow the same rule.
Dr Mark Hollands, Warwick Prize Fellow in the Astronomy and Astrophysics Group at the University of Warwick, who led the work, said: "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. It turns out these incredibly dense stars can produce colours nobody expected, and some of them genuinely surprised me."
Why white dwarfs break the rule
The researchers say the key is density.
The coolest white dwarf atmospheres can reach around one gram per cubic centimetre, roughly the density of water and millions of times denser than the Sun's photosphere.
In those conditions, trace elements such as carbon, calcium and sodium interact with the surrounding dense helium plasma.
They then absorb light across a wider range of colours than they would at lower densities.
That shifts a star's colour away from the orange, white and blue of a perfect radiator.
No white dwarf is close enough for its colour to be seen directly.
The team instead worked it out from a star's spectrum, which is produced when starlight is split apart by wavelength, much like a prism splits sunlight into a rainbow.
They ran these spectra through a model of human colour vision to show what colour each star would appear to the eye.
A range of colours
The team applied the method to white dwarfs across the sky and found their colours fall into distinct groups, each linked to what is in the star's atmosphere.
Carbon-rich white dwarfs are typically pink or purple.
Those enriched with debris from shattered asteroids and comets, the remains of old planetary systems, tend towards yellow.
Cooler white dwarfs with mixed hydrogen and helium atmospheres often take on a cyan or pale blue tinge.
The researchers say that is caused by hydrogen molecules colliding with helium atoms in their ultra-dense atmospheres.
The most striking example is a white dwarf called WD J1330+6435, which is roughly 4,000 degrees Celsius.
It has taken in rocky material from its planetary system, enriching its atmosphere with calcium, lithium and sodium.
That gives it a chartreuse green appearance in the team's models.
As far as the researchers are aware, no other star, living or dead, has ever been credibly reported as green.
They say that makes it the first known genuinely green star in the Galaxy.
Dr Hollands said: "The traces of sodium in the atmosphere of this white dwarf absorb the orange part of the spectrum, leaving behind a ghostly green glow. Imagine standing on a planet and looking up at a green star. It feels like something out of science fiction."
At the opposite extreme, the team identified what may be the reddest star ever recorded.
The white dwarf, called WD J1011+2845, is just 48 light years from Earth.
It has a deep convection zone that has lifted carbon from its core to its surface.
The researchers say the resulting carbon molecules distort the star's light so heavily that almost all of it is squeezed into wavelengths beyond 600 nanometres.
That produces a deep salmon-pink glow.
The team also studied a representative sample of the 151 white dwarfs within 20 parsecs, or about 65 light years, of Earth.
Five showed unusual colouring, two yellow and three pink.
The researchers say that suggests roughly one in 30 white dwarfs in the Universe will shine with an unusually coloured light.
For the other 146, the team measured the colour temperature, which compares the colour of an object with that of a perfect radiator.
The median colour temperature of the sample was approximately 6,800 Kelvin, which is perceived as white.
The researchers say this corrects a common misconception in astronomy that white dwarfs are typically blue.
They say hotter, bluer white dwarfs are brighter and can be seen from further away, so they are over-represented in surveys that rely on brightness alone.
Professor Boris Gänsicke, from the Department of Physics at the University of Warwick, said: "One in thirty might not sound like a huge fraction, but across the entire population of white dwarfs in the Universe, that is still a lot of oddly coloured stars out there. Yet if we take the median across a representative population of white dwarfs, we found that the average white dwarf really is white."
The paper, called 'Any colour you like: The diverse chromaticities of white dwarfs', is published in RAS Techniques and Instruments.