White Dwarfs Aren’t Just White. Scientists Discover Dying Stars Turning Pink and Green

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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.

Another white dwarf, WD J1011+2845, has a massive amount of carbon in its atmosphere. Almost all of its radiation is concentrated at wavelengths exceeding 600 nanometers, giving it a salmon-pink color. Meanwhile, the white dwarf WD J902-2010 leans toward the more bubblegum end of the spectrum.

However, there’s no reason to expect white dwarfs to start getting renamed, because most of these stars are, in fact, white. For this study, the authors examined 152 white dwarfs located within 65 light-years of Earth. Only five of them showed unusual coloration, suggesting that approximately one in thirty white dwarfs in the universe shines with unusually colored light.

This might seem like an expected result, but scientists previously assumed that white dwarfs were predominantly blue. That assumption may reflect bias in sampling the night sky. The hottest white dwarfs are also typically the brightest and can, therefore, be seen from greater distances. Because they’re easier to observe, many studies end up over-representing blue stars.

While you won’t be able to see the nearest white dwarf with the naked eye either, you can at least look in its direction. Sirius B is the companion to the brightest star in Earth’s night sky. During winter, Sirius is easy to spot shining brightly above the eastern horizon. Sirius B belongs to the group of “more boring” white dwarfs—its temperature hovers around 25,000 Kelvin, giving it a light blue color.

This story originally appeared on WIRED Czech Republic and Slovakia and has been translated from Czech.