Astronomers Just Found the First Green Star Ever Discovered—Right After It Swallowed Its Planets

When we look up at the Sun, we see a star glowing in shades of yellow, orange, and white. Across the universe, stars display a range of colors, from fiery reds to brilliant blues, with their hues shaped primarily by their surface temperatures. But what if the stellar palette extends beyond the colors we’re accustomed to seeing? New findings are bringing some unexpected possibilities into view, suggesting that white dwarfs, the dense remnants of dead stars, could glow in shades of pink, purple, and even green.

For the first time, astronomers have spotted a star that appears green to the human eye. The star had reached the end of its lifespan and swallowed up rocky material from its planetary system, giving it a chartreuse green appearance. The findings, published in RAS Technique and Instruments, suggest that planetary debris can cause white dwarf stars to shine in a whole array of colors if viewed up close.

Zombie star

When a star like the Sun burns through its nuclear fuel, it sheds its outer layers to form a planetary nebula, leaving behind its dense, dead core in the center. These cosmic zombies, known as white dwarfs, are extremely hot and have been known to shine in a white/pale blue. (The hotter a star is, the more of its light is emitted through shorter, bluer wavelengths.)

“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,” Mark Hollands, an astronomer at the University of Warwick in England who led the work, said in a statement. “It turns out these incredibly dense stars can produce colors nobody expected, and some of them genuinely surprised me.”

The key to this surprising display of color is the star’s density. The atmospheres surrounding the coolest white dwarfs can reach the same density as water, millions of times more dense than the Sun’s photosphere, the lowest layer of its atmosphere. Under these conditions, trace elements such as carbon, calcium and sodium, interact with the surrounding dense helium plasma of the star.

That makes the elements absorb light across a wider range of colors than they would at lower densities. As a result, the star’s color changes from the more typical orange, white, and blue, according to the study.

Pick a color

The team of astronomers behind the study set out to see the true colors of white dwarfs. Unfortunately, there are no white dwarfs close enough to observe directly. Instead, the astronomers relied on the star’s spectrum.

The spectrum is a distinct fingerprint produced when light from a star is split into its constituent colors, providing clues to its chemistry. By running the spectrum through a model of human color vision, the astronomers found that the colors of white dwarfs fall into distinct groups based on the composition of their atmosphere.

Carbon-rich white dwarfs are typically pink or purple, while those that have fed off debris from shattered asteroids and comets, or the remains of old planetary systems, leaned more towards yellow. Cooler white dwarfs with mixed hydrogen and helium atmospheres often take on a cyan or pale blue tinge, caused by hydrogen molecules colliding with helium atoms in their ultra-dense atmospheres.

One particular star, designated as WD J1330+6435, stood out. The star is roughly 7,232 degrees Fahrenheit (4,000 degrees Celsius) and around 284 light-years away from Earth. It had swallowed the rocky material from its orbiting planets which were rich with calcium, lithium, and sodium. Once these elements hit the white dwarf’s athmosphere, the interaction results in a chartreuse green light, according to the team’s models.

“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.”

The team also found what may be the reddest star ever recorded: WD J1011+2845, a nearby white dwarf just 48 light years from Earth. The star has a deep convection zone that has brought carbon from its core to its surface, distorting its light so heavily to produce a deep salmon-pink color.

These white dwarfs, however, may be one of a few colorful outliers. The team also studied 151 white dwarfs located within about 65 light-years from Earth and only five of these showed unusual colors (two yellow and three pink).

This suggests that roughly one in thirty white dwarfs spread across the cosmos may shine in unusual colors. “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, physics professor at University of Warwick and co-author of the study, said in a statement. “Yet if we take the median across a representative population of white dwarfs, we found that the average white dwarf really is white.”