Stars come in a wide range of colors, generally determined by their temperatures: cooler stars look orange, Sun-like stars appear white, and the hottest stars glow light blue. But a recent study suggests that white dwarfs can be far more colorful than their name implies, with some appearing pink, green, purple, yellow, or light blue.
How white dwarfs form
Stars with masses similar to the Sun expand into red giants near the end of their lives. They eventually shed their outer gas, creating planetary nebulae that can be observed through telescopes. The remaining core is a white dwarf—a highly compact object with roughly the volume of Earth but approximately the mass of the Sun.
Newly formed white dwarfs are extremely hot, which is why the first examples discovered appeared white or light blue. However, according to a new study published in RAS technology and equipment, the colors of these stellar remnants can change as they cool and as material from former planetary systems enters their atmospheres.
Density and debris affect their colors
Researchers argue that atmospheric density is a key factor. The atmospheres of the coldest white dwarfs can reach a density of about 1 gram per cubic centimeter—roughly the density of water and about a million times greater than the Sun’s atmosphere. Under these conditions, trace elements such as carbon, calcium, and sodium interact with the surrounding dense helium plasma and absorb a broader range of wavelengths of light.
Material left over from an ancient planetary system can also alter a white dwarf’s appearance. A planet close to its star may be destroyed during the red-giant phase, while other planets and asteroids can collide or migrate toward the stellar remnant. Debris from these bodies may then become incorporated into the white dwarf’s atmosphere.
Through a hobbyist telescope, white dwarfs generally look like faint stars. Closer observation, however, may reveal a wider variety of colors. Carbon-rich white dwarfs tend to appear pink or purple. Calculations suggest that white dwarfs whose atmospheres have been enriched by shattered asteroids and other celestial bodies may take on yellow or even green hues. The coldest white dwarfs are often light blue, a color caused by collisions between hydrogen and helium atoms in their extremely dense atmospheres.
A possible green white dwarf
One example is WD J1330+6435. With a temperature of about 4,000 degrees Celsius, the star is accumulating rocky material from a planetary system, enriching its atmosphere with calcium, lithium, and sodium. According to the study, these elements give it a bright chartreuse color, making it the first known green star in our galaxy.
Source: www.wired.com
