Colors in the sky

Posted July 24, 2006 – When it comes to the sky, you can learn a lot just by noticing colors. Take the sky itself. We all know that deep blue means clean dry air while a cloudless milky sky is a 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 is perceived as white.

Now check out the stars. On summer nights the two brightest are Arcturus and Vega, both high up. Their pastel tints are obvious at a glance: orange and blue. Each has meaning. If a planet like Venus or Jupiter is out, its whiteness tells us that it reflects the sun’s colors equally. This in turn reveals that we’re not looking at liquid water or methane gas, which absorb red and reflect blue, and explain why Earth, Uranus, and Neptune appear bluish from space.

Star colors tell a different tale because they emit their own light. A blue star like Vega is always super-hot; a ruddy star like Arcturus is cool. The temperature indicates the rate at which the star’s nuclear furnace consumes fuel, which is a function of pressure and 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, around here, things glow red when they’re hot. But think: When you heat a piece of metal it first starts to glow 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.

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. Actually, 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 by electricity. Auroras are seen on other planets, too. 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 that can produce it — seeing green on a far-away world would be a wonderful clue that it may be inhabited by life!

Colors in the sky: They’re lessons in how the universe works.