In brief
Astronomers report WD J1330+6435, about 280 light-years from Earth in Draco, as the first green star, published in RAS Techniques and Instruments.
Fragments of former planetary material enriched the star's outer layers with calcium, lithium and sodium, and helium collisions in its dense atmosphere leave a yellowish-green glow.
The Warwick team led by Mark Hollands examined 151 or 152 white dwarfs within about 65 light-years, depending on the report, and found most would look white while some would look green, pink, purple, blue or yellow.
About 48 light-years away, this white dwarf is reported as possibly the reddest star so far, with carbon at its surface giving it a salmon-pink color.
A green star in Draco
Astronomers have identified what reports describe as the first green star, a white dwarf catalogued as WD J1330+6435, about 280 light-years from Earth in the constellation Draco. The findings appeared in the journal RAS Techniques and Instruments. White dwarfs are the dense remnants left when stars of low or intermediate mass exhaust their nuclear fuel and shed their outer layers, and most of them look white. This one has a yellowish-green tint linked to material that fell onto its surface from a former planetary system. One report adds that it is the first green star found in the Milky Way. Mark Hollands, an astrophysicist at the University of Warwick in England, led the study, and Boris Gänsicke, a Warwick professor, co-authored it.
Why the star is green
A star's color usually follows its temperature, but this white dwarf, reported to be about 4,000 degrees Celsius, owes its tint to chemistry. Fragments of planetary material enriched its outer layers with calcium, lithium and especially sodium. In ordinary stars sodium absorbs one narrow band of orange light, but this white dwarf is so dense that helium atoms constantly collide with the sodium atoms and shift their energy levels. As a result the sodium absorbs yellow, orange and part of the red light, leaving a yellowish-green glow. Hollands said such a glow would not be possible in stars similar to the Sun. Gänsicke likened the effect to mixing paints, where removing one color changes the shade that remains.
A wider survey of white dwarfs
The team worked from archived light spectra of white dwarfs, using each spectrum as a fingerprint of how much light a star emits at each wavelength. They converted those spectra into the colors a human eye would perceive. Sources differ on the sample size: one reports 152 white dwarfs and another 151, all within roughly 65 light-years of Earth. According to the model, most of them would look white, but some would appear green, pink, purple, blue or yellow. LaSexta reports that certain white dwarfs would show green, pink, cyan, yellow and purple if observed up close. Astronomers had assumed white dwarfs follow the same temperature rule as other stars, and the study challenges that assumption.
The reddest white dwarf
The team also points to WD J1011+2845, a white dwarf about 48 light-years from Earth that may be the reddest star identified so far. VRT NWS reports that carbon at its surface gives it an intense salmon-pink color that differs from most other stars. Gänsicke noted that although the average white dwarf is white, the population still holds a notable number of oddly colored stars. Gijs Nelemans of Radboud University in Nijmegen said that colorful telescope images of distant stars are usually recolored after the fact and do not show their true shades. The two stars sit at very different distances, with the green one about 280 light-years away and the pink one about 48 light-years away.
- WD J1330+6435 (green)
- 280 light-years
- WD J1011+2845 (salmon pink)
- 48 light-years
Data from the article text