Look for This Colorful, Twinkling Star in the Night Sky This Fall
On autumn evenings, Capella can put on a dazzling display of red, green, and blue. Here's why it happens and where to look.
Look low in the northeastern sky on an autumn evening, and you might spot something unusual: a bright star flashing red, green, and blue. That’s Capella, one of the brightest stars in the night sky. But despite appearances, the star isn’t actually changing colors. The dazzling display is an optical effect caused by starlight passing through Earth’s atmosphere.
And Capella isn’t the only star that puts on this kind of show.
“Stars appear to twinkle or shimmer in the night sky, but this is not an inherent trait of stars,” says Emily Watson, astrophysicist and planetarium supervisor at the U.S. Space & Rocket Center. “As the light from a star passes through different densities and temperatures within our atmosphere, the light bends in a process called refraction.”
The color flashes happen because Earth’s atmosphere bends different wavelengths of light by different degrees. White starlight contains many colors, and atmospheric refraction can make those colors appear to separate and shimmer.
(Everything you wanted to know about stars.)
So why don’t planets twinkle like stars? “Because stars are so far away, they appear as point sources,” says Watson. “A planet, in comparison, is not a point source of light because it’s far closer to Earth. The larger size of planets also makes any distortion less apparent, so they do not appear to twinkle the same way stars do.”
Some stars, however, put on a more noticeable display than others. Capella is the sixth-brightest star in the night sky, making its colorful flashes particularly easy to spot. Though it appears to be a single star to the naked eye, Capella is actually a system of multiple stars, including two giant stars that orbit each other every 104 days.
But its brightness isn’t the only reason Capella stands out. On autumn evenings in the Northern Hemisphere, the star sits low in the early evening sky. Its light must pass through more atmosphere than the light from stars overhead, giving turbulent air more opportunity to distort it.
As autumn gives way to winter, Capella climbs higher in the night sky. With its light traveling through less atmosphere, its colorful scintillation transforms into a yellow-white light, closer to the starlight’s true color. It can still twinkle from up high, though, just not as much as it does near the horizon.
(Here’s how to find the Big Dipper.)
For the best chance of catching Capella’s colorful display, look toward the northeastern sky after dark on an autumn evening. The star is often one of the first bright points of light to catch your eye. “The Big Dipper can be used to find Capella,” says Watson. “First, locate the star where the handle meets the bowl, and draw a line from that star through the top outer star of the bowl. Continue on this line until you reach the next bright star, which will be Capella.”
Capella is also the brightest star in Auriga, the constellation known as the Charioteer. In winter, it marks one corner of the Winter Hexagon asterism, a large pattern of bright stars that spans much of the sky.
