By Deborah Byrd
EarthSky Communications

Friday, Sept. 25, 2009

Every year around this time, we get questions from people who see a bright star twinkling with red and green flashes, low in the northeastern sky. Capella is a golden star when seen higher up in the sky. If you could travel to it in space, you’d find that it’s actually two golden stars, both with roughly the same surface temperature as our local star, the sun . . . but both larger and brighter than the sun.

So here is a golden star that flashes red and green when it’s low in the sky. Why does it do that? The reality is that every star in the sky undergoes the same process as Capella, to produce its colorful twinkling. That is, every star’s light must shine through Earth’s atmosphere before reaching our eyes. The key is that, when you look at an object low in the sky, you are looking through more atmosphere than when the same object is overhead. The atmosphere splits or “refracts” the star’s light, just as a prism splits sunlight.

That’s where Capella’s red and green flashes are coming from … not from the star itself … but from the refraction of its light by our atmosphere. When you see Capella higher in the sky, these glints of red and green will disappear.

By the way, why are these flashes of color so noticeable with Capella? The reason is simply that it’s a bright star. It’s the sixth brightest star in Earth’s sky, not including our sun. Capella is in the constellation Auriga the Charioteer, but since antiquity it has carried the name “Goat Star.”

More at www.EarthSky.org.