Astronomers Found the First Star That Would Look Green to the Eye

October 21 — According to a report by CCTV News' Global Information Radio, an astrophysics team at the University of Warwick in the UK has analysed the spectral data of white dwarfs and for the first time found a star that, to the naked eye, would appear a faint yellow-green: WD J1330+6435. The work was published in the international journal RAS Techniques and Instruments (RASTI).

The green star lies in the constellation Draco in the Milky Way, about 280 light-years from Earth. It is not a young, healthy star but a white dwarf — the final stage of stellar evolution, the dense core left behind when a star reaches the end of its life and runs out of fuel; in other words, a stellar remnant.

The paper's first author, University of Warwick astrophysicist Mark Hollands, said that the colour of the vast majority of stars depends on temperature and is limited to orange, white or blue. “Most white dwarfs are the same,” he said, “but some of them show an extraordinarily rich variety of colours.”

Note: the light a star emits is not a single colour but a continuous spectrum covering all wavelengths — like a continuous rainbow with red, orange, yellow, green, blue and violet, varying only in intensity; green is special because it sits in the middle of the spectrum. So normal stars have no “pure green.” For a star to look pure green, green light would have to be very strong while orange and blue were very weak, which is impossible in a continuous spectrum.

WD J1330+6435 breaks the idea that temperature determines colour. In a news release on Warwick's website, the team wrote: “Colour is not only determined by temperature but also by density.” The white dwarf is about the size of Earth but 165,000 times its mass, and at such extreme density its atmosphere is very thick.

Co-author and Warwick astrophysicist Boris Gänsicke added that the green is not a colour captured directly by a telescope but one the team calculated by feeding spectral data into a human vision model. On that model, if we stood on a planet near this star and looked up, the sky would appear green.

The team also used the method to study another 151 white dwarfs within about 65 light-years of Earth: most were white, and only five showed different colours. Among them they found what may be the reddest star observed so far: WD J1011+2845, a nearby white dwarf only 48 light-years away in the constellation Leo Minor. The team estimates that roughly one in every 30 white dwarfs in the universe emits anomalously coloured light.