Discovery of the world's first green star

New research by scientists shows that some white dwarfs can take on unusual colors, including green, by absorbing the remnants of rocky planets; A phenomenon that changes scientists' understanding of the color of dead stars.

Ambassador Arjan-Social Group: How cool would it be if the sun could take on a ghostly green glow for Halloween? Scientists have discovered that our host star might actually turn green, but only billions of years after it dies, and only if it then starts eating planets like a cosmic zombie.

Scientists have discovered that white dwarfs, the stellar remnants left behind by Sun-sized stars when they die, can actually take on a wide range of colors, including green, if they feed on their rocky planets like cosmic planet-eaters.

For astronomers, the color of stars has always been an indication of their temperature. Cool stars appear orange, while stars like the Sun appear white, and the hottest stars glow pale blue. The story is similar to a blacksmith's iron that glows red, then glows orange, then as it heats up again, it takes on a brilliant, white color.

Researchers have always assumed that white dwarfs follow a rule. However, new research shows that these stellar bodies can glow in a range of colors, including green, pink, turquoise, yellow and purple. This means that the world is much more colorful than we thought.

The color of most stars, including most white dwarfs, is governed almost entirely by temperature. "No one really looked at what else might be possible for white dwarfs," team leader Mark Hollands of the University of Warwick said in a statement. These super dense stars seem to produce colors that no one expected and some of them really surprised me.

How do white dwarfs break the star color rules?

White dwarfs get their name because when they were discovered, they looked surprisingly hot and glowed white or pale blue. However, this new research suggests that this slightly bland appearance is because no white dwarf is close enough for astronomers to see its true color.

The key to the rainbow-like array of colors the team suggests white dwarfs can have is their density. White dwarfs form when stars roughly the mass 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 material that makes up white dwarfs is as dense as water and millions of times denser than the star'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, making white dwarfs far from bright white/blue.

The researchers found that the colors of the white dwarfs fall into distinct groups related to the chemical composition of the stellar remnant's atmosphere.

Carbon-rich white dwarfs should be pink or purple, while those enriched with the remains of spent asteroids and comets and remnants of old planetary systems take on a more yellow hue.

Cooler white dwarfs with mixed hydrogen and helium atmospheres often glow a turquoise or pale blue, caused by hydrogen molecules colliding with helium atoms in their dense atmospheres.

However, one of the most striking examples of white dwarf coloring occurs when one of these dead stars begins to feed on the planets around it like a zombie.

Zombie white dwarfs turn green

Researchers have found that some white dwarfs can turn slightly green after being swallowed up by orbiting planets.

An example of this is a cool white dwarf known as WD J1330+6435, located 326 light-years away and collecting rocky material from a planetary companion. This has enriched its atmosphere with calcium, lithium and sodium, and the team's model shows that this gives WD J1330+6435 its green color. Of course, there are still no hordes of these green-faced cosmic zombies.

Researchers believe that no other star, living or dead, has ever been reliably reported to be green. This makes WD J1330+6435 the first true green star.

Traces of sodium in the white dwarf's atmosphere absorb the orange part of the spectrum, leaving behind a ghostly green glow, says Hollands. Imagine you are standing on a planet and looking at a green star. It feels like something out of science fiction.

The team didn't just discover that a known white dwarf is our universe's first green star. They also discovered what could be the reddest star ever in the form of a white dwarf, WD J1011+2845.

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 deflect the white dwarf's light, giving it a deep, pale pink glow unlike any other star.

Despite this rainbow of white dwarfs, astronomers theorize that white is still the most common color for these stellar bodies. Only five of the 151 white dwarfs within 65 light-years of Earth have such extraordinary coloring. Extrapolating this to the entire population of white dwarfs, astronomers estimate that only one in 30 dead stars has an unusual color.

Boris Gänsicke from the University of Warwick's Department of Physics says: "One in 30 may not seem like a huge fraction, but there are still a lot of strangely colored stars in the entire population of white dwarfs in the universe." However, if we take the population average of white dwarfs, we find that the average color of white dwarfs is indeed white.

The team's research has been published in the journal RAS Techniques and Instruments.