Astronomers have identified a green star. Is called WD J1330+6435has a surface temperature of around 4 thousand degrees and a color that, according to traditional astronomical knowledge, it should not have had. A light green tending towards yellow, similar to that of an unripe apple. What makes it so particular would be the traces of sodium and calcium present in its atmosphere, probably coming from the remains of an ancient planetary system.
The discovery comes from a group of researchers from the University of Warwick, in the United Kingdom, who reconstructed the colors of some stars that have now reached the final stages of their evolution. The study, published in the scientific journal RAS Techniques and Instrumentsalso had other surprises in store: there are white dwarfs that would appear pink, purple, yellow and even cyan. A chromatic variety that until now had remained hidden in astronomical data.
Because until now we hadn’t found any green stars
Stars can appear orange, white or blue. The color depends above all on the surface temperature: the colder ones tend towards red and orange, the warmer ones towards white and blue. Following this progression, it would be natural to also imagine green stars, halfway between the other shades. Yet, normally, it doesn’t work like that.
Stars emit light at many wavelengths at once. Even when a large portion of the radiation is concentrated in the green region of the spectrum, there are still enough red and blue components present for our eyes to perceive an overall white light. The Sun, for example, emits a lot of light in the green region, but seen from space it appears essentially white.
WD J1330+6435 is an exception thanks to its atmosphere. It is a white dwarf, what remains of a star after it has exhausted the fuel necessary to support nuclear reactions and has lost its outer layers. Even our Sun, in billions of years, should conclude its evolution in this way.
White dwarfs have dimensions comparable to those of the Earth, despite containing an enormous amount of matter. In their atmospheres, especially when they cool, the density can reach values comparable to that of water. It is in these extreme conditions that some chemical elements profoundly modify the light emitted.
The sodium absorbs the orange light and the star turns green
WD J1330+6435 was already known by astronomers for the presence of lithium, calcium and sodium in its atmosphere. Elements that indicate the accumulation of rocky material, probably coming from bodies that orbited around the star before the transformations of its planetary system.
In such a dense atmosphere, sodium absorbs a large portion of the orange light, while calcium subtracts part of the violet component. The distribution of the light that manages to escape is thus altered, to the point of producing a hue that the researchers describe as chartreuse greena light green with a yellow component.
As explained by astrophysicist Mark Hollands in the University of Warwick press release, traces of sodium present in the atmosphere absorb the orange part of the spectrum, leaving a particular greenish brightness.
A phenomenon that surprised the researchers themselves. Usually the chemical composition of stars only modifies the color we perceive to a limited extent. In white dwarfs, however, the extreme density of the atmosphere can greatly amplify the absorption of certain wavelengths.
To determine what color we would see, astronomers analyzed stellar spectra and used models of human vision. The stars are too small and far away to observe their colored disk directly, but the light collected allows them to reconstruct their appearance. In the case of WD J1330+6435, the result is precisely that unusual green, with a shade that may vary slightly compared to the reconstruction. The models even made it possible to reproduce how brightness and color would change between the center and the edges of the star. In the center the green would be more intense, while towards the outside it would become progressively lighter.
Also pink, purple and blue stars: what the study says
The green white dwarf is just one of the particular objects examined. The team spotted two other white dwarfs with much paler greenish hues, as well as stars that would appear pink, purple, yellow or cyan. The shades depend on the composition of the atmospheres. Carbon, for example, can absorb certain components of light to produce a pink or purplish color. In other white dwarfs, interactions between hydrogen and helium contribute to blue hues.
One of the most particular examples is WD J1011+2845a white dwarf about 48 light-years away from Earth. The carbon molecules present in its atmosphere absorb much of the visible light, leaving a particularly intense salmon pink color. According to researchers, it could be one of the reddest-looking stars ever identified.
To understand how common these colors are, astronomers also examined a sample of 152 white dwarfs within about 65 light years of the Sun. Five had significantly different colors from those normally expected: two yellow and three pink. The most evident green star, WD J1330+6435, belongs to another selection of analyzed objects.
Unusual colors therefore seem rather rare, but the quantity of white dwarfs present in our galaxy suggests how many other cases could still be identified. The research also offers a different way of reading the history of the stars. Color can tell something about their chemical composition and, in some cases, the objects that once surrounded them.
In the case of WD J1330+6435, the green could be the legacy of a now disappeared planetary system. Rocky fragments that ended up in the atmosphere of a dead star, rich enough in sodium and calcium to even change their color. Only chemical traces would remain of those celestial bodies. And a green star.
