The realm of stars resembles a fairy tale. In it, we find giants, supergiants, and also many dwarfs. Rather like the flames of a fire, most stars’ colors are based on temperature. Cooler stars look orange, sun-like stars appear white, and the hottest ones shine light blue. However, there are exceptions, and astronomers have recently discovered some among the class of stars known as white dwarfs.
At the end of their lives, stars with a mass similar to the sun expand and turn into red giants. Subsequently, they shed their gaseous envelope, creating a planetary nebula that we can admire through telescopes. In place of the star, a white dwarf remains—a highly condensed object the approximate volume of the Earth but the mass of the sun.
Initially, white dwarfs have extremely high temperatures, and so the first ones discovered shone from white to light blue. That’s how they got their name. But according to a new study published in the journal RAS Techniques and Instruments, the world of white dwarfs is actually far more colorful—with some stars turning pink, green, and even purple.
The researchers posit that density is the deciding factor. The atmospheres of the coolest white dwarfs can reach a density of approximately 1 gram per cubic centimeter. That’s roughly the same density as water, which may not sound like much, but it’s packed for a star, this being about a million times denser than the sun’s atmosphere. Under these conditions, any trace elements like carbon, calcium, and sodium interact with the surrounding dense helium plasma and absorb light across a wider range of colors.
Material from ancient planetary systems also plays a role. The closest planets can perish in the fiery embrace of the red giant phase. Other planets, and especially asteroids, begin to collide and migrate toward the dead star. Fragments of these bodies then integrate into the white dwarf’s atmosphere.
Through a hobbyist’s telescope, a white dwarf looks just like any other faint star. However, if observed up close, their colors would be much more varied. Carbon-rich white dwarfs tend to be pink or purple. According to calculations, white dwarfs whose atmospheres have been enriched with material from shattered asteroids or other bodies will exhibit yellow and, in some cases, green shades. The coolest white dwarfs are often a baby blue, which is caused by hydrogen and helium atoms colliding in their extremely dense atmospheres.
One example is the star WD J1330+6435. With a temperature of around 4,000 degrees Celsius, it has accumulated rocky material from its planetary system, enriching its atmosphere with calcium, lithium, and sodium. These elements give it a light chartreuse color, making it the first known green star in our galaxy.