Astronomers find unlikely colors glowing among stars

Extreme pressure and elements in their atmospheres give some dead stars unusual hues.(CN) — A dead star that would appear green to the human eye is among a handful of unusually colorful white dwarfs identified by astronomers, who say others could look pink, purple or yellow.In a study published Wednesday in RAS Techniques and Instruments, researchers at the University of Warwick in England examined the colors of white dwarfs, the dense remains of stars like our sun.Among them is what researchers believe may be the first known star that would appear genuinely green.In most cases, cooler stars tend to look orange, while hotter ones appear white or blue. But white dwarfs can behave differently because of their unusually dense atmospheres.“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,” said Mark Hollands, an astronomer at the University of Warwick who led the research, in a news release. “It turns out these incredibly dense stars can produce colors nobody expected, and some of them genuinely surprised me."White dwarfs form when stars like the sun exhaust their fuel and shed their outer layers, leaving behind extremely dense cores. Their atmospheres can become so compressed that the surrounding gases change how elements absorb light.One particularly unusual example is WD J1330+6435, a white dwarf with a temperature of roughly 4,000 Celsius (7,200 F).Researchers found that calcium, lithium and sodium in its atmosphere give it a green appearance.“In this particular white dwarf, however, the density is so high that the sodium atoms are constantly being bombarded by the surrounding helium atoms, which due to quantum effects alters the sodium atomic energy levels,” Hollands said in an email.In ordinary stars, sodium absorbs a narrow band of orange light. In this white dwarf, collisions with surrounding helium atoms cause it to absorb a much wider range, including yellow, orange and some red light.Researchers also identified WD J1011+2845, a white dwarf about 48 light-years from Earth that could be among the reddest stars ever recorded. Carbon brought to its surface changes the light it emits, giving it a deep salmon-pink appearance.Other white dwarfs rich in carbon can appear pink or purple, while those containing debris from shattered asteroids and comets tend to look yellow. Some cooler white dwarfs with hydrogen and helium atmospheres appear cyan or pale blue.Since these stars are too distant for their colors to be seen directly, astronomers calculated how they would look using measurements of the light they emit.They then applied models of human color vision to determine how those wavelengths would appear to our eyes.Hollands said they’re confident in their results because the methods used to calculate the stars’ colors are already well established.“Although color is a very subjective experience, the understanding of how the spectra of light sources map to different perceived colors is not at all new,” he said. “All digital cameras and display technology depend on having an accurate model of color that is measurable and reproducible.”However, most white dwarfs still appear white. Of the 151 nearby stars researchers examined, only five had unusual colors, suggesting about one in 30 white dwarfs may look different.The discovery doesn’t change what astronomers already know about how white dwarfs work, Hollands said, but it makes some of the complex physics behind them easier to visualize.Studying the elements in these stars can also reveal traces of rocky debris from former planetary systems, offering clues about what those systems were made of.“But now being able to work out how that material changes the color of the white dwarf adds a really nice human connection to these objects that we study,” Hollands said.Subscribe to our free newslettersOur weekly newsletter Closing Arguments offers the latest about ongoing trials, major litigation and rulings in courthouses around the U.S. and the world, while the monthly Under the Lights dishes the legal dirt from Hollywood, sports, Big Tech and the arts.Additional Reads