Astronomers identify a green star in the Universe for the first time

"People have stars that are not the same. For some, who travel, the stars are guides. For others, they are no more than small lights." This quote from The Little Prince reminds us that human eyes attribute multiple meanings to the night sky. In astronomy, however, the light emitted by a celestial body follows well-defined physical laws, among them Wien's Law, the fundamental rule that associates a star's temperature with the color it displays.

The recent discovery of WD J1330+6435, a white dwarf located approximately 280 light-years from Earth in the constellation Draco, has surprised the scientific community with its green color. This is the first recorded star of this color in the Universe.

White dwarfs are the dense core remnants of low to medium-mass stars that have exhausted their nuclear fuel and expelled their outer layers. Concentrating roughly half the mass of the Sun into a volume similar to that of the Earth, these bodies often shine with a conventional pale white glow. In the case of WD J1330+6435, however, the chemical composition of the surface layers altered the final outcome.

A planetary feast that altered the light

The change in coloration occurred because the extreme gravity of the white dwarf attracted and destroyed rocky debris from ancient planets orbiting around it. The assimilation of this material enriched the star's atmosphere with elements such as calcium, lithium, and especially sodium.

Under normal conditions, sodium atoms absorb a very specific range of orange light, which is the same hue seen in certain public lighting lamps. However, the environment of WD J1330+6435 altered this behavior.

In this particular white dwarf, the density is so high that sodium atoms are constantly bombarded by helium atoms around them, altering the atomic energy levels of sodium, explained astrophysicist Mark Hollands from the University of Warwick, England, and lead author of theA study published in the journal RAS Techniques and Instruments .

As the star packs about 165,000 times the mass of the Earth into a compact volume, the presence of sodium began to block not only the orange light but also portions of the yellow and red spectra.

The final result is that sodium atoms no longer absorb a very narrow wavelength of orange: they absorb yellow, orange, and some red wavelengths. And what remains is a star that has a beautiful yellowish-green glow, Hollands noted.

Astrophysicist Boris Gänsicke, a co-author of the research, compares the phenomenon to the work of a palette of paints: by removing certain colors from the original mixture emitted by the star, the light that manages to escape takes on a greenish tone.

Simulated images with precise color accuracy of white dwarfs. Top left images: Sirius B is the nearest white dwarf to the Solar System. Its hot hydrogen atmosphere gives it a pale blue color. In contrast, WD J1147–7449 is among the coldest white dwarfs in the neighborhood, presenting a featureless spectrum and an orange appearance. Top right images: In the convective layers of these cold white dwarfs, carbon is dragged from the core to their dense helium-dominated atmospheres. This forms diatomic carbon molecules, resulting in different shades of pink. Bottom images: The attraction of rocky debris from planetary systems around white dwarfs enriches their cold helium atmospheres with elements such as calcium, magnesium, and sodium. Variations in the amount and composition of the absorbed material, combined with the different temperatures of the white dwarfs, produce a wide range of colors, including green. In reality, all stars would be millions of times brighter than what can be displayed on a screen. Source: Mark A. Garlick/University of Warwick.

What is Wien's Law and why doesn't it produce "green stars"?

The rarity of the discovery can only be understood through Wien's Law, formulated at the end of the 19th century by German physicist Wilhelm Wien. This law states that the dominant color emitted by a heated body depends directly on its temperature:

• Cooler stars: they radiate primarily lower-energy light, appearing reddish or orange;

• Extremely hot stars: they emit high-energy light, glowing in bluish tones;

• Intermediate temperature stars: like our Sun, their maximum emission point is precisely in the green light range.

Why doesn't the Sun appear green to us?

Although the Wien Law shows that most of the energy emitted by the Sun is in the green band, it also emits large amounts of blue and red light simultaneously. When all these colors reach the human eyes together, the mixture stimulates our visual receptors in a balanced way. As a result, we perceive sunlight as white (or yellowish due to the effects of the Earth's atmosphere).

It is for this strictly physical reason that there are no common green stars in the sky. WD J1330+6435 was only able to display this unique hue because it did not depend solely on its natural thermal brightness: its atmosphere acted as a chemical filter that subtracted the blue, red, and yellow tones, allowing only the green light to pass.