When we look at the Sun, we see a star that shines in shades of yellow, orange and white. Throughout the universe, stars display a range of colors, from deep reds to bright blues, and their hues are determined primarily by their surface temperatures. But what if the stellar palette extends beyond the colors we’re used to seeing? New findings are bringing to light some unexpected possibilities, 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 detected a star that appears green to the human eye. The star had reached the end of its lifespan and swallowed 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 make white dwarf stars glow in a wide range of colors if viewed up close. Zombie star When a star like the Sun burns up 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 are known to glow a pale white/blue color. (The hotter a star is, the more light is emitted through shorter, bluer wavelengths.) “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,” Mark Hollands, an astronomer at the University of Warwick in England, who led the work, said in a statement. “It turns out that these incredibly dense stars can produce colors that no one expected, and some of them really surprised me.” The key to this amazing display of color is the density of the star. The atmospheres surrounding the coldest white dwarfs can reach the same density as water, millions of times denser 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 star’s surrounding dense helium plasma. This causes elements to absorb light in a wider range of colors than at lower densities. As a result, the star’s color changes from the more typical orange, white and blue, according to the study. Choose 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, astronomers relied on the star’s spectrum. The spectrum is a distinctive signature produced when a star’s light is split into its constituent colors, providing clues to its chemistry. By analyzing the spectrum through a model of human color vision, astronomers discovered that the colors of white dwarfs are divided into different groups depending on the composition of their atmosphere. The team ran the spectra through a model of human color vision to reveal what color the light from each star would appear to the eye. Credit: Mark Hollands/University of Warwick Carbon-rich white dwarfs tend to be pink or purple, while those that have fed on the remains of 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 tint, caused by the collision of hydrogen molecules with helium atoms in their ultradense atmospheres. One star in particular stood out, designated WD J1330+6435. The star is approximately 7,232 degrees Fahrenheit (4,000 degrees Celsius) and about 284 light years from Earth. It had swallowed the rocky material of orbiting planets, which were rich in calcium, lithium and sodium. Once these elements hit the white dwarf’s atmosphere, the interaction results in chartreuse green light, according to the team’s models. “Traces of sodium in this white dwarf’s atmosphere absorb the orange part of the spectrum, leaving behind a ghostly green glow,” Hollands said. “Imagine being on a planet and looking at a green star. It seems 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 zone of deep convection that has brought carbon from its core to its surface, distorting its light so much that it produces a deep salmon pink color. These white dwarfs, however, may be one of the few color outliers. The team also studied 151 white dwarfs located about 65 light years from Earth and only five of them showed unusual colors (two yellow and three pink). This suggests that about one in every thirty white dwarfs spread across the cosmos may shine in unusual colors. “One in thirty may not seem like a huge fraction, but in the entire population of white dwarfs in the Universe, there are still many strangely colored stars,” Boris Gänsicke, professor of physics at the University of Warwick and co-author of the study, said in a statement. “However, if we take the median of a representative population of white dwarfs, we find that the average white dwarf is actually white.”
