White Dwarf Stars Are More Colorful Than You Think

Astronomers at the University of Warwick have made a fascinating discovery – white dwarfs can exhibit a stunning array of colors, from green and pink to purple and cyan. In fact, they have even found the first known star that appears pure green to the human eye.

Typically, a star’s color corresponds to its temperature. Cooler stars appear orange, middle-range stars like our Sun appear white, and the hottest stars emit a pale blue light. This color-temperature relationship is similar to heating iron, which transitions from red to white as the temperature rises.

White dwarfs, remnants of dense stars like our Sun, were initially believed to follow this same pattern. The first white dwarfs discovered appeared white or pale blue due to their high temperatures. However, recent findings published in RAS Techniques and Instruments (RASTI) have revealed a broader spectrum of colors exhibited by these stellar remnants.

Dr. Mark Holland, the Warwick Prize Research Fellow in the University of Warwick’s Astronomy and Astrophysics Group, led the study and shared, “Most stars, including white dwarfs, act as ‘perfect radiators,’ so their color is primarily determined by their temperature. However, white dwarfs have surprised us with the variety of colors they can produce.”

Why Do Some Dead Stars Glow in Unexpected Colors?

The unique colors observed in white dwarf stars can be attributed to the density of their atmospheres. In the coldest white dwarfs, the atmospheric material reaches a density comparable to water, millions of times denser than our Sun’s visible surface.

Under such extreme conditions, elements like carbon, calcium, and sodium interact with surrounding helium plasma. These interactions enable these elements to absorb a wider range of wavelengths, resulting in unexpected colors emanating from white dwarfs.

To perceive these colors, astronomers analyzed the starlight using a spectrum, which splits light into different wavelengths similar to a prism separating sunlight into a rainbow of colors. Each spectrum provides a unique fingerprint revealing details about the star’s temperature and composition.

The Discovery of the First Green Star

Among their discoveries, researchers identified WD J1330+6435, a white dwarf star with a temperature of 4,000 degrees Celsius. This star, which accumulated rocky debris from previous planetary systems, showcases a distinctive chartreuse-green color attributed to elements like calcium, lithium, and sodium in its atmosphere.

Unlike any other known star, WD J1330+6435’s green appearance is caused by traces of sodium, which absorb the orange part of the spectrum, leaving behind the unique green glow.

Exploring the Reddest White Dwarf Star

Astronomers also observed WD J1011+2845, a white dwarf star just 48 light-years away, possibly the reddest star on record. The deep convective zone of this star transports carbon to the surface, altering the light’s wavelength and resulting in a deep salmon pink color.

How Common Are These Unique Stars?

By examining 151 white dwarfs near Earth, researchers discovered that about 1 in 30 white dwarfs exhibit unusual colors, such as yellow or pink. Despite this, most of these stellar remnants appear white to the human eye, challenging the common perception that white dwarfs are typically blue.

Professor Boris Gensik remarked, “Despite the unique colors observed in a small percentage of white dwarfs, the median color temperature of these stars remains around 6,800 Kelvin, making them appear white to us.”