Astronomers discover a white dwarf that should appear green — ScienceDaily

Astronomers at the University of Warwick have discovered the white dwarf WD J1330+6435, which, according to their calculations, should appear green to the human eye. The team believes this is the first known example of a star with such a color in the Milky Way, ScienceDaily reports.

White dwarfs are ultra-dense remnants of stars similar to the Sun. Usually, the color of stars is associated primarily with temperature: cooler ones appear orange, while the hottest ones are blue. However, researchers found that white dwarf atmospheres can also produce pink, purple, yellow and blue-green shades.

What changes the color of white dwarfs

In the coolest white dwarfs, the density of matter in the atmosphere can reach about 1 gram per cubic centimeter — comparable to the density of water and millions of times greater than in the visible layer of the Sun. Under such conditions, traces of carbon, calcium and sodium interact with helium plasma and absorb a broader range of light wavelengths.

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WD J1330+6435 has a temperature of about 4,000 degrees Celsius. Calcium, lithium and sodium from rocky debris left by a former planetary system entered its atmosphere. According to scientists’ estimates, sodium absorbs the orange part of the spectrum, so the star should have acquired a yellowish-green hue.

Spectra and unusual shades

Since no white dwarf is close enough for its color to be directly distinguished by the eye, the team analyzed stellar-light spectra and applied a model of human color vision. White dwarfs with large amounts of carbon often appear pink or purple, while objects that have accumulated matter from destroyed asteroids and comets may be yellow. In cool atmospheres containing hydrogen and helium, a blue-green hue is possible.

The researchers also studied a sample of 151 white dwarfs within 20 parsecs, or about 65 light-years, of Earth. Five of them had unusual colors: two were yellow and three were pink. Based on this sample, the authors estimated that about one in 30 white dwarfs may have an atypical color. The median color temperature of the remaining objects was about 6,800 kelvins, meaning that a typical white dwarf still appears white.