White Dwarfs Are Actually…Yellow, Green, Pink, and Purple?

The universe hosts a myriad of star types, including supergiants, giants, main sequence, red dwarfs, brown dwarfs, white dwarfs, and neutron stars. What makes these stars stand out are just their sizes, but their colors, as each one hosts a distinct color based on their surface temperature, specifically orange, red, blue, and white based on cooler to hotter temperatures. While the correlation between star colors and surface temperatures has long been known throughout astronomy, what if star colors could change based on specific criteria, including their atmosphere? Now, a team of researchers led by the University of Warwick might have introduced a new way to literally look at stars, as they revealed new evidence for white dwarfs to exhibit colors other than what their name implies. These findings were recently published in RAS Techniques and Instruments and could provide new insights into the formation and evolution of stars throughout the cosmos. This is because stars are referred to as “perfect radiators”, also called blackbody objects, meaning they absorb all electromagnetic radiation and reflect nothing. In a nutshell, their glow is entirely dependent on temperature. “As an object gets hotter it will emit more light at optical wavelengths, changing from red to orange (think of a blacksmith working on a piece of hot metal),” Dr. Mark Hollands, who is a postdoctoral researcher at the University of Warwick and the study’s lead author, tells Universe Today. “Eventually these orange hues will become paler, and by 6000K a black body will appear will just appear white as most of its radiation will spread across optical wavelengths. For even hotter temperatures an object will start to glow pale blue as the majority of its light will become emitted in the ultraviolet. This set of red/orange/white/blue are the only colors that are possible for a true blackbody.” For the study, the researchers used a series of computer models to study 151 white dwarfs with 20 parsecs (65 light-years) from Earth and how white dwarfs might challenge the longstanding understanding of stars being blackbodies. The reason white dwarfs were chosen was due to their extreme characteristics compared to main-sequence stars like our Sun, most notably white dwarfs having much denser atmospheres. The researchers aspired to learn if the presence of certain elements like carbon, calcium, or sodium in their dense atmospheres could influence the glow of white dwarfs. In the end, the researchers found that a total of five white dwarfs of the 151 sampled showed that a combination of a white dwarf’s dense atmosphere with certain elements could cause these five white dwarfs to glow pink and yellow, specifically two yellow and three pink. This indicates about 3.3 percent of white dwarfs in the cosmos could exhibit different colors aside from white. A white dwarf whose atmosphere is comprise of carbon appearing pink. (Credit: Mark A. Garlick/University of Warwick) “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,” said Dr. Boris Gänsicke, who is a professor in the Department of Physics at the University of Warwick and a co-author on the study. “Yet if we take the median across a representative population of white dwarfs, we found that the average white dwarf really is white." Regarding the specific elements responsible for altering the white dwarf’s colors, the researchers found that carbon causes white dwarfs to appear pink or purple while cooler white dwarfs with hydrogen and helium tend to be cyan or blue. Additionally, the researchers found that white dwarfs whose atmospheres have debris from their original planetary system are typically green. As noted, star colors are typically determined by their surface temperatures, with hotter stars to cooler stars ranging in several colors, including blue, blue-white, white, yellowish white, yellow (our Sun), orange, and red. The larger the star, the hotter and bluer it is. But while this study demonstrates there could be exemptions to this longstanding notion, could other stars also exhibit unexpected colors? “The other class of object that can disobey the rule are brown dwarfs," Dr. Hollands tells Universe Today. "These are substellar objects that are too cool to fuse hydrogen in their cores, but can fuse deuterium instead. Because these are quite cold objects (compared to normal stars) most of their radiation is in the infrared, but some still comes out at optical wavelengths. And because brown dwarfs also can have dense atmospheres, absorption from sodium, potassium, and molecules like titanium oxide can also alter their colors away from blackbody colors. Some astronomers who study brown dwarfs have looked into their appearance in the past and concluded that they can appear violet in color!” What new insights into white dwarfs and other star colors will researchers make in the coming years and decades? Only time will tell, and this is why we science! As always, keep doing science & keep looking up!