How wonderful would it be if the sun could take on a spooky green glow for Halloween? It turns out that scientists have discovered that our star can turn green, but only within billions of years after its death, and only if it then starts eating planets like a cosmic zombie. That’s possible because scientists have discovered that white dwarfs, the type of stellar remains that Sun-sized stars leave behind when they die, can actually turn a variety of colors, including green, if they eat their rocky planets like cosmic cannibals. For stellar scientists, the colors of stars have always been indicative of their temperatures. Cool stars appear orange, while stars like the sun appear white and hotter stars glow pale blue. This is similar to a blacksmith’s iron glowing red, then orange, then white-red as it heats up. Researchers have always assumed that white dwarfs follow the same rule. However, new research suggests that these stellar corpses could glow in a variety of colors including green, pink, cyan, yellow and purple. That means the universe is much more colorful than we suspected. “Most stars, including most white dwarfs, behave as ‘perfect radiators,’ so their color is set almost exclusively by temperature. But no one had really investigated what else might be possible for white dwarfs,” team leader Mark Hollands of the University of Warwick said in a statement to Space.com. “It turns out that these incredibly dense stars can produce colors that no one expected, and some of them really surprised me.” You may like How White Dwarfs Break Stellar Color Rules White dwarfs get their name because when they were discovered, they appeared surprisingly hot and glowed white or pale blue. However, this new research suggests that this slightly bland and uniform appearance is because no white dwarfs are close enough for Earth-based astronomers to see their true colors. The key to the rainbow-like array of colors that this team suggests white dwarfs could possess is their density. White dwarfs form when stars with a mass approximately that of the Sun run out of fuel and can no longer support themselves against their own gravity, causing their cores to collapse. As a result, the matter that makes up white dwarfs has a density close to that of water, millions of times denser than the sun’s surface, the photosphere. Under these conditions, elements such as carbon, calcium and sodium can interact with the surrounding dense helium plasma and absorb light in a wider range of colors than at lower densities. This distances the colors of white dwarfs from the white/light blue that is seen as a perfect heat radiator. Simulated images of white dwarfs with accurate colors. (Image credit: Mark Hollands/University of Warwick) The researchers found that the colors of the white dwarfs would fit into distinct groups linked to the individual chemical composition of the stellar remnant’s atmosphere. Carbon-rich white dwarfs should be pink or violet, while those that have been enriched by the remains of consumed asteroids and comets, and remnants of ancient planetary systems, would take on a more yellow hue. Cooler white dwarfs with mixed hydrogen and helium atmospheres would often glow cyan or pale blue, because hydrogen molecules collide with helium atoms in their dense atmospheres. What to read next However, one of the most striking examples of white dwarf coloration would arise when one of these undead stars begins feeding on the planets around it like a zombie from a horror movie. Zombie white dwarfs turn green Researchers have found that some white dwarfs from dead stars can turn a little green around the gills after eating orbiting planets. An example of this is WD J1330+6435, a cool white dwarf located about 326 light-years away, which has been accreting rocky material from a planetary companion. This has enriched its atmosphere with calcium, lithium and sodium, and the team’s modeling suggests this would have led WD J1330+6435 to take on a green hue. But unlike the green-tinged zombies of George Romero’s classic Dawn of the Dead, there are no hordes of these green-faced cosmic zombies… yet. Researchers think that no other star, living or dead, has been credibly reported as green. This makes WD J1330+6435 the first known genuinely green star. An illustration of a green star (Image credit: Robert Lea (created with Canva)) “Traces of sodium in this white dwarf’s atmosphere absorb the orange part of the spectrum, leaving a ghostly green glow,” Hollands said. “Imagine being on a planet and looking at a green star. It feels like something out of science fiction.” Scientists see red The team not only discovered that a well-known white dwarf is the first green star; They also discovered what could be the reddest star ever created: the white dwarf WD J1011+2845. Located just 48 light years from Earth, this white dwarf has a deep convection zone that transports carbon from its core to its surface. These carbon molecules distort the white dwarf’s light, causing a deep salmon-pink glow unlike anything seen in any other star. Enriched from below: The cool white dwarf WDJ0902+2010 has a deep convection zone within its helium-dominated outer envelope. (Image credit: Mark A. Garlick/University of Warwick) Despite this rainbow of white dwarfs, the team theorizes that white is still the most common color for these stellar corpses. Only five of the 151 white dwarfs located about 65 light years from Earth have extraordinary colors. Extrapolating this to the total white dwarf population, the team estimates that only one in 30 of these dead stars has an unusual color. “One in thirty may not seem like a huge fraction, but in the entire white dwarf population of the universe, there are still many strangely colored stars,” said Boris Gänsicke, Department of Physics, University of Warwick. “However, if we take the median of a representative population of white dwarfs, we find that the average white dwarf really is white.” The team’s research was published Thursday (Oct. 14) in the journal RAS Techniques and Instruments.
