WD J1330+6435: des chercheurs ont trouvé une naine verte dans l'espace

Astronomers have discovered a green star for the first time, identifying a dense stellar remnant known as a white dwarf that exhibits a rare chartreuse hue after absorbing rocky debris from planets that once orbited it. The star, designated WD J1330+6435, lies approximately 280 light-years away in the constellation Draco within the Milky Way galaxy. A light-year corresponds to the distance light travels in one year, roughly 9.5 trillion kilometers. The color of a star is generally determined by its temperature, with cooler stars appearing orange, stars like the Sun appearing white, and the hottest stars displaying a pale blue hue. However, the coloration mechanism differs for white dwarfs, which are the remnants of low- to medium-mass stars that have exhausted their nuclear fuel and shed their outer layers. While most white dwarfs appear white, changes in their chemical composition over time can result in unusual colors. WD J1330+6435 stands out due to its unique greenish appearance, a phenomenon attributed to the interaction between sodium atoms and surrounding helium atoms in its extremely dense atmosphere. This interaction alters the energy levels of sodium atoms, causing them to absorb a broader range of wavelengths, specifically yellow, orange, and parts of the red spectrum, resulting in the star's distinctive chartreuse glow. Such a feature is not observed in sun-like stars. The discovery was part of a research initiative aimed at determining how white dwarfs would appear to the human eye based on their spectral characteristics. The team examined 151 white dwarfs within a radius of about 65 light-years from Earth. They were particularly intrigued by WD J1330+6435 due to the presence of planetary material on its surface, enriched with elements such as calcium, lithium, and notably sodium. According to astrophysicist Mark Hollands from the University of Warwick in England, who led the study published in the journal RAS Techniques and Instruments, the high density of WD J1330+6435's atmosphere plays a crucial role in this phenomenon. The extreme density, reaching up to one gram per cubic centimeter, allows interactions between trace elements like carbon, calcium, and sodium with the helium plasma, leading to the absorption of light across a wider spectrum compared to less dense environments. This finding highlights the diversity of colors among white dwarfs, with previous studies indicating that about one in thirty white dwarfs exhibit unusual hues. These colors arise not primarily from temperature but from the chemical composition of the stars. Other notable examples include the reddest star found, WD J1011+2845, whose color is attributed to carbon. White dwarfs represent the typical end stage of most stars, making them relatively common in the universe. The Sun itself will eventually become a white dwarf following its expansion into a red giant. Their sizes vary inversely with their masses due to the absence of nuclear fusion, which balances gravity in conventional stars. Instead, their dimensions are dictated by quantum mechanical degeneracy pressure exerted by electrons. While the discovery of a green star might seem fantastical, it underscores the complexity and unpredictability of celestial phenomena. As noted by physicist Boris Gänsicke, even though only a fraction of white dwarfs display non-white colors, this still indicates a substantial number of stars with peculiar appearances scattered throughout the cosmos.