Some White Dwarf Stars Glow Pink and Yellow, Astronomers Find

Not every white dwarf is white. A team led by the University of Warwick has found evidence that some of these dense, dying stars glow pink or yellow, according to Universe Today. The results were published in RAS Techniques and Instruments.

The researchers studied 151 white dwarfs located within 20 parsecs (65 light-years) of Earth. Five of them showed unusual colors: two yellow and three pink. The team says that works out to about 3.3 percent of white dwarfs, though most of the stars still look white.

What did the researchers actually find?

Using a series of computer models, the team examined how the thick atmospheres of white dwarfs shape the light that reaches telescopes. They wanted to know whether elements such as carbon, calcium or sodium in those atmospheres could change how the stars glow.

They could. Of the 151 stars in the sample, five had atmospheres whose makeup pushed their glow toward pink or yellow, Universe Today reported. Dr. Boris Gänsicke, a professor of physics at Warwick and a co-author, said one in thirty may not sound like much, but across the universe it adds up to a lot of oddly colored stars.

Why do stars normally have a fixed set of colors?

Astronomers have long tied a star's color to its surface temperature. Dr. Mark Hollands, a postdoctoral researcher at Warwick and the study's lead author, explained to Universe Today that a hot object first glows red, then orange, as it heats up. Around 6000K, an ideal glowing object looks white because its light is spread across the visible range. Hotter still, it turns pale blue as more of its light shifts into the ultraviolet.

For a perfect “blackbody,” Hollands said, that sequence of red, orange, white and blue is the only one possible. Stars are usually treated as close to blackbodies, which is why their glow depends almost entirely on temperature.

Why are white dwarfs the exception?

White dwarfs have far denser atmospheres than main-sequence stars such as our Sun. That density lets certain elements absorb and reshape the light, so the star no longer follows the simple temperature-to-color rule.

The team linked specific colors to specific ingredients, according to the report:

• Carbon in the atmosphere makes a white dwarf look pink or purple.
• Cooler white dwarfs with hydrogen and helium tend to look cyan or blue.
• White dwarfs with debris from their original planetary system in the atmosphere are typically green.

Gänsicke added a caveat. Across a representative population, he said, the average white dwarf really is white.

Could other kinds of stars break the color rule too?

Hollands told Universe Today that brown dwarfs can. These are objects too cool to fuse hydrogen in their cores, though they can fuse deuterium. Because they are cold compared with normal stars, most of their radiation falls in the infrared, but some still emerges at visible wavelengths.

Brown dwarfs can also have dense atmospheres, he said. Absorption from sodium, potassium and molecules such as titanium oxide can shift their colors away from those of a blackbody. Hollands noted that some astronomers who study brown dwarfs have concluded in the past that these objects can look violet.

Why does it matter?

For readers who simply like looking at the night sky, the finding is a reminder that a star's color is not always a simple thermometer reading. For astronomers, the researchers say the work could offer new insight into how stars form and evolve across the cosmos.

The study does not change how most white dwarfs look. It points to a small fraction of them whose atmospheres, rich in elements like carbon, give them a distinct tint. The sample covers only stars close to Earth, and the findings rest on computer models of those 151 stars.

Around Prague, based on reporting by Universe Today