How awesome 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 may indeed go green, but only billions of years from now 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 remnants sun-sized stars leave behind when they die, can actually take on a range of colors including green if they snack on their rocky planets like cosmic cannibals.
For stellar scientists, the colors of stars have always been indicative of their temperatures. Cool stars look orange, while stars like the sun look white, and the hottest stars glow pale blue. This is akin to a blacksmith’s iron glowing red, then orange, then white-hot 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 range of colors including green, pink, cyan, yellow, and purple. That means the universe is far more colorful than we suspected.
“Most stars, including most white dwarfs, behave like ‘perfect radiators,’ so their color is set almost entirely by temperature. But nobody had really looked into what else might be possible for white dwarfs,” team leader Mark Hollands of the University of Warwick said in a statement sent to Space.com.
“It turns out these incredibly dense stars caaaaan produce colors nobody expected, and some of them genuinely surprised me.”
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 dwarf is close enough for Earthbound astronomers to see their true colors.
The key to the rainbow-like array of colors this team suggests white dwarfs could possess is their density. White dwarfs form when stars with a mass roughly 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 is around the density of water, millions of times denser than the sun’s surface, the photosphere.
In these conditions, elements like carbon, calcium, and sodium can interact with the surrounding dense helium plasma and absorb light across a wider range of colors than they would at lower densities. That shifts the colors of white dwarfs away from the white/light blue seen for a perfect radiator of heat.
The researchers found that the colors of white dwarfs would fit into distinct groups tied to the individual chemical makeup of the stellar remnant’s atmosphere.
White dwarfs abundant in carbon should be pink or purple, while those that have been enriched with debris from consumed asteroids and comets, and the remains of old planetary systems, would take a more yellow hue. Cooler white dwarfs with mixed hydrogen and helium atmospheres often would glow cyan or pale blue, caused as hydrogen molecules collide with helium atoms in their dense atmospheres.
One of the most striking examples of white dwarf coloration, however, would arise when one of these living dead stars starts feeding on the planets around it like a horror movie zombie.
Zombie white dwarfs go green
The researchers found that some dead star white dwarfs can go a little green around the gills after snacking on orbiting planets.
An example of this is WD J1330+6435, a cool white dwarf located around 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 model suggests this would have led WD J1330+6435 to adopt 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.
The researchers think that no other star, living or dead, has ever been credibly reported as green. That makes WD J1330+6435 the first known genuinely green star.
“The traces of sodium in the atmosphere of this white dwarf absorb the orange part of the spectrum, leaving behind a ghostly green glow,” Hollands said. “Imagine standing on a planet and looking up at a green star. It feels like something out of science fiction.”
Scientists see red
The team didn’t just discover that a known white dwarf is the first-ever green star; they also discovered what could be the reddest star ever in the form of the white dwarf WD J1011+2845.
Located just 48 light-years from Earth, this white dwarf has a deep convection zone that carries carbon from its core to its surface. These carbon molecules distort light from the white dwarf, causing a deep salmon-pink glow unlike anything seen in any other star.
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 within around 65 light-years of Earth have extraordinary coloring. Extrapolating this to the total population of white dwarfs, the team estimates that only one in 30 of these dead stars has an unusual color.
“One in thirty might not sound like a huge fraction, but across the entire population of white dwarfs in the universe, that is still a lot of oddly colored stars out there,” Boris Gänsicke, Department of Physics, University of Warwick, said. “Yet if we take the median across a representative population of white dwarfs, we found that the average white dwarf really is white.”
The team’s research was published on Thursday (Oct. 14) in the journal RAS Techniques and Instruments.
