Green stars do exist—and others shine pink or purple, too

One of science’s greatest strengths is its capacity to learn from mistakes. Confronted with an error, a good scientist will admit to being wrong and use the new information to learn even more.

So, having said, that: mea culpa.

In my January 16 The Universe column, I wrote that there are no green stars. This is because a star’s temperature governs the mix of colors of light it emits, and the way our eyes perceive that mix tips the colors to either side of the green part of the spectrum so that stars appear mostly white, blue, orange and red. The sun actually gives off most of its light in blue and green, but a variety of factors influence our perception of its color, making it appear white or perhaps slightly yellowish.

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But can a star be just plain ol’ green? Nope. And I’ve been saying so for years.

WD J133001.13+643523.7 would like to have a word, however. And that word is “chartreuse.”

The white dwarf star—let’s just call it J1330 for short—really has a greenish tint, according to some interesting research published in the journal Royal Astronomical Society Techniques and Instruments on October 15. Astronomers used real observations, together with calculated models of white dwarf spectra, and, using sophisticated techniques, converted them to approximations of what our eyes would see. Surprisingly, these odd stars come in a range of delightful hues, including, yes, even green in the case of J1330.

The important change over my earlier claim is that different kinds of stars emit light in different ways. Normal stars like the sun—those still fusing hydrogen into helium in their cores—are gigantic balls of very hot ionized gas. Material in this state generally emits light across the entire visible spectrum, spread out smoothly across the rainbow of colors, from blue to red. When each color’s brightness is graphed against its wavelength, the plot (called a blackbody curve for historical reasons) resembles a lopsided bell curve that peaks at a given color, depending on the star’s temperature. Hot stars peak in the blue, cooler stars do so in the red. Some emit more green or yellow, or what have you.

Normal stars also have a thin atmosphere of ethereal gas surrounding them. Material in this gas can absorb colors at very specific wavelengths, creating dips in the blackbody spectrum. This can affect the overall color of the star—by making it slightly bluer, say, if the dilute gas preferentially absorbs red light—but the effect is minimal in essentially all cases. That is, a star’s color is completely dominated by its temperature rather than these second-order atmospheric effects.

White dwarfs, however, are different. These are compact objects that used to be the cores of stars like the sun; when a sunlike star dies, it blows off its outer layers, revealing its inner heart, which slowly cools in space over the eons like an ember from an extinguished fire. This ultradense sphere (a cubic centimeter weighs a metric ton!) is fiercely hot and holds about half the mass of the sun crammed into a volume only about that of Earth. This means white dwarfs have tremendous surface gravities, with a typical value being about 100,000 times Earth’s! Standing on the surface of a typical white dwarf, I’d weigh about 6,800 metric tons—equivalent to over 200 fully loaded semitrucks. (Did I say, “Standing”? I meant, “Squashed into a millimeter-thin smear of vapor.”)

That intense gravity means white dwarfs can have much thicker atmospheres than a normal star—thicker but squatter: most of a white dwarf’s atmosphere only reaches centimeters in altitude above the surface. But it can be very dense, on the order of a gram per cubic centimeter (about the density of liquid water). The white dwarf’s gravity is so strong, in fact, that atoms and molecules in its atmosphere can be squeezed so hard that a condition arises called “pressure broadening,” in which gas absorbs a wider range of colors than it otherwise would. All this means a white dwarf’s atmosphere can be much more efficient than a normal star’s at absorbing light emitted from below.

Combining all this information, the astronomers who did the new research examined the colors of quite a few real white dwarfs to see how they’d look to human eyes. The results were a shock to me! Some “white” dwarfs, it seems, are instead yellow, pink or purple.

Or, yes, as demonstrated by our friend J1330, they can even be green. It’s more of a chartreuse, but that counts. So there does appear to be an actual green star. And where there’s one, more are bound to exist, even if we haven’t seen them yet.

I’d love to have a good true-color image of J1330, to be honest. Unfortunately it’s a very faint star, and a decent-sized telescope would be required for the task. Even then, getting the color balance of an image right is tricky. Still, it would be fun to see it.

And I’m kicking myself; at some point I actually considered the idea that selected absorption in a stellar atmosphere might tip a star’s emissions enough to make it green but dismissed this because I was only thinking of normal stars! I’m glad these astronomers were more clever than I was.

There’s another, darker aspect to this as well. Although dense, the surface of a white dwarf is technically a fluid. The ridiculously strong gravity means that heavy elements tend to sink rapidly into the white dwarf’s interior, leaving lighter elements (such as hydrogen and helium) on the surface. Other elements, such as sodium, should readily disappear, yet that’s the exact element that makes J1330 look green! How can it exist in the atmosphere at all?

White dwarfs, it so happens, are astrophysical zombies that will happily destroy and consume their own planets.

Long after a sunlike’s star demise, some of its planets might drop down too close to the remaining white dwarf, where its ferocious tidal forces can tear them apart, shredding them into rubble. Earth has a relatively high abundance of sodium in its crust, for example, so it’s not a stretch to think rocky exoplanets should, too. Asteroids and even comets have sodium in them as well, so there’s quite a supply. If there’s a semiconstant stream of material falling onto the white dwarf, that could explain the supply of sodium that turns the tint up.

While this “polluting” of the atmosphere is a relatively common phenomenon, it takes just the right mix of planetary chunks and white dwarf temperature to produce a star that looks like J1330. So while I will happily admit to being wrong in my earlier declaration, at least one thing I said remains correct:

For stars, it’s (still) not easy being green.