When gazing at the night sky, stars often appear as tiny, uniformly coloured pinpricks of light. However, these colossal cosmic furnaces inherently radiate a diverse spectrum of colours, contingent upon their surface temperature. Typically, incredibly hot, newly formed stars emit a blue light, moderately warm stars shine yellow, whilst older, cooler stars tend to glow in shades of orange and red. Traditionally, astrophysicists have maintained that all other colours, including green, are scientifically impossible for stars.
This long-held understanding has now been remarkably overturned by Mark Hollands, an astrophysicist at the University of Warwick, and his dedicated research team, who have made an extraordinary discovery: a genuinely green star.
The Discovery of WDJ1330+6435: A Celestial Anomaly
For decades, astronomers believed that white dwarfs—the remnants left behind after a star exhausts its nuclear fuel and dies—adhered to the same temperature-colour rule, typically appearing as a bluish-white due to their intense heat, hence their nomenclature. However, as documented in their groundbreaking paper published in the journal RAS Techniques and Instruments, Hollands and his co-authors revealed that these stellar husks can display colours previously unobserved in other types of stars.
The research team undertook a comprehensive study, calculating the colours that 151 white dwarfs would appear to the human eye. While 146 of these celestial bodies conformed to the expected bluish-white description, one particular dead star, WDJ1330+6435, situated approximately 280 light-years from Earth—an astounding 165 trillion miles away—in the constellation Draco, presented an astonishing ghostly green glow.
Unravelling the Mystery Behind the Green Hue
The unique green colour of WDJ1330+6435 is attributed to its atmospheric composition. This white dwarf has been absorbing a rich "slush" of iron, magnesium, and sodium from the rocky planets that once orbited it. As Hollands explained, "Think of something like asteroids or minor planets that have ventured too close to the white dwarf, have been shredded into dust by tidal forces and then rained down onto the stellar surface where they get mixed in."
The phenomenon hinges on the properties of chemical elements within the star's dense atmosphere. Elements, as listed in the periodic table, absorb specific wavelengths of light while reflecting others. Sodium, for instance, preferentially absorbs orange light, which is why sodium street lamps emit a dull yellow glow. In WDJ1330+6435, the star's incredibly dense helium plasma interacts with the absorbed sodium, causing it to soak up orange, yellow, and red light. "This absorbed radiation is then re-radiated at other wavelengths, with the emergent spectrum having a beautiful green colour," Hollands elaborated.
Potential Implications for a Green Star's Planetary System
Contemplating a hypothetical scenario where our own Sun—which Hollands clarifies is white, not yellow—was replaced by WDJ1330+6435, the impact on Earth would be profound. Hollands notes that the star would appear minuscule in the sky, being a hundred times smaller than our Sun. Furthermore, Earth's orbit would expand due to the white dwarf's weaker gravitational pull. Even if Earth could be repositioned closer to appear larger, the implications for life would be dire. "Plant life on Earth absorbs mostly violet and red light, reflecting the green, to grow," Hollands stated. "With some of these wavelengths missing from WDJ1330+6435, that certainly wouldn't be good news for us."
A Spectrum of Unexpected Stellar Colours
WDJ1330+6435 is not merely an isolated case; it is one of five white dwarf stars where astronomers have identified unusual colourations caused by the materials present in their atmospheres. These discoveries further underscore the unexpected diversity of stellar hues:
- White dwarfs with atmospheres rich in hydrogen and helium were observed to glow in shades of cyan or a pale blue.
- Those that had gravitationally captured and absorbed material from asteroids or rocky planets displayed yellowish tones, or, in the singular case of WDJ1330+6435, a distinctive chartreuse green.
- Carbon-rich white dwarfs exhibited pink or purple hues. For example, WD J1011+2845, located 48 light-years away in the constellation Leo Minor, glows with a salmon-pink colour. This is due to its deep convection zone, which allows carbon from its interior to leak into its atmosphere.
- Hollands added, "We also found some of these carbon-rich white dwarfs to be pale purple as well, if the stellar temperature is a bit higher and the abundance of carbon a bit lower."
Advancing Astronomical Understanding
Since white dwarf stars are too faint to be directly observed from Earth with the naked eye, the astronomers deduced their true colours using sophisticated star spectrum data. This "fingerprint" is created when starlight is split by wavelength, revealing the unique elemental composition of the star. Hollands emphasised that while these findings do not alter our visual perception of white dwarf stars, they profoundly illustrate the vibrant and colourful nature of the universe.
The research also sheds light on other celestial bodies, such as brown dwarf stars—failed stars too small to sustain hydrogen fusion—which can also exhibit dazzling colours. Hollands explained, "Most of their light is radiated in the infrared, but a small amount is still emitted at visible wavelengths, with absorption from sodium, potassium, and titanium oxide affecting their perceived colours." Previous studies have even determined that brown dwarfs could appear violet or purple based on their spectra, reinforcing the notion that the cosmos holds an abundance of colourful surprises. This scientific breakthrough deepens our understanding of stellar evolution and the diverse characteristics of stars across the Milky Way.