This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: University of Warwick astronomers have shown that white dwarf stars can glow in a range of colors, including the first star ever found that would appear green to the human eye. For most stars, color is a guide to temperature.
Cooler stars look orange, stars like our sun look white, and the hottest glow pale blue, much like a blacksmith's iron glowing red, then orange, then white-hot as it heats up. White dwarf stars, the dense remnants left behind when sun-like stars reach the end of their lives, were thought to follow the same rule. They earned their name because the earliest examples found were surprisingly hot and glowed white or pale blue.
But are orange, white and blue the only colors a star in the night sky can be? The answer, according to new research from the University of Warwick, is "no." Published in the journal RAS Techniques and Instruments, the new study shows that if we could view them up close, certain white dwarf stars would shine in colors such as green, pink, cyan, yellow and purple. "Most stars, including most white dwarfs, behave like 'perfect radiators,' so their color is set almost entirely by temperature.
But nobody had really looked into what else might be possible for white dwarfs," said Dr. Mark Hollands, Warwick Prize Fellow in the Astronomy and Astrophysics Group, University of Warwick, who led the work. "It turns out these incredibly dense stars can produce colors nobody expected, and some of them genuinely surprised me." The key is density.
The coolest white dwarf atmospheres can reach around 1 gram per cubic centimeter, roughly the density of water and millions of times denser than the sun's photosphere. In these conditions, any trace elements that may be present, such as carbon, calcium and sodium, interact with the surrounding dense helium plasma and absorb light across a wider range of colors than they would at lower densities. This shifts a star's color away from the orange, white and blue of a perfect radiator.
No white dwarf star is close enough for its color to be seen directly, so the team calculated it instead from data astronomers already collect routinely: a star's spectrum, the "fingerprint" produced when starlight is split apart by wavelength, much like a prism splits sunlight into a rainbow. Running these spectra through a model of human color vision revealed what color each star's light would appear as to the eye. It is a reminder of just how much a spectrum can reveal: not just a star's temperature or its chemical makeup, but its true color as well.
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