Colors in the sky

As the final fall colors fade, we can get a “refill” by looking up. And when it comes to the sky, we learn a lot from its colors.

Start with a cloudless day. We all know that deep blue means clean dry air while a milky sky is the sure sign of moisture. This is true because tiny water droplets reflect all the sun’s colors equally. When red, green, and blue light strike us simultaneously the mix looks milky white. Since October is one of our drier months, the sky tends to be deep blue far more often than in July.

Now check out the stars, say at 10 p.m. The brightest are Vega, halfway up the western sky, Capella, low in the northeast, and Aldebaran, rising in the east. Their pastel tints are obvious at a glance: blue, white, and orange.

Each has meaning. Capella’s whiteness tells us that it emits all colors equally. It precisely matches the color of our own sun. A blue star like Vega is always super-hot; a ruddy star like Aldebaran is cool. The temperature indicates the rate at which the star’s nuclear furnace consumes fuel, which is a function of mass. So, heavy stars are blue. Their rapid consumption of hydrogen makes the star “burn itself out” quickly, and this gives yet another morsel of information: Blue stars are always young stars, because they die in their adolescence. All this from a quick glance at a star’s color.

People sometimes wonder why a reddish star should be cool, when things glow red when they’re hot. But think: When you heat a piece of metal it first starts glowing a dull red. As it gets hotter it becomes orange, then yellow, and then white. It would turn blue when it got super-hot, except that it melts first, before this can happen.

Meteor colors are yet another story. Here we have a “flame test” for the meteor’s composition. A green meteor might indicate an unusually high copper content. Indeed, green is a rare sky color except for auroras. The Northern Lights are mostly green because that’s the dominant color oxygen emits when it’s stimulated.

Auroras appear on other planets, too, especially Jupiter and Saturn. But on no other do they appear green. A green aurora is a tell-tale sign of abundant oxygen. And since copious free oxygen only occurs on a planet that has plants to produce it, seeing a green glow on a far-away world would be a wonderful clue that it may harbor life.

Other colors materialize in the sky too, and each is a lesson in physics. Sky colors at dawn and dusk come from Rayleigh scattering, which means that shorter bluer waves bounce around much more easily than redder ones. When the sun is low or just below the horizon, each color or wavelength gets scattered differently during its long path through our air, so that colors separate rather than combine, letting us see the sun’s emissions individually rather than scrambled together.

Colors on the fringes of white clouds that are near the sun come from a different process — diffraction. Here, light’s waves interfere with other waves, to cancel out some colors entirely and produce new ones. The result is a dramatic modern art display of pink, aqua, maroon, and brown fringes. These colors are not created within the sun. These are new colors, concocted colors, colors created by blending and destruction. It’s a very special process.

Colorful rings and arcs in the sky come from yet another process, the bending of light rays, called refraction. Around the sun or moon (i.e. a halo), red is always on the inside of the ring. But a rainbow, whose arc is centered on the spot opposite the sun in the sky - has violet or blue on the inside of the arc. Such haloes and rainbows always display spectral colors, the same ones that prisms project onto walls. They’re the actual colors emitted by the sun, not newly concocted ones.

Colors in the sky: Lessons in how the universe works.

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