Bright Star in the Northeast
What Is the Brilliant Flickering Light in the Northeastern Fall Sky?
Autumn evenings are highlighted by a shining, twinkling star low in the northeast. Sometimes mistaken for a plane or UFO, this star is the sixth brightest in the sky.
As night falls in the Northern Hemisphere, a radiant light that appears to flash red, white, and blue shines low in the northeastern sky. This spectacular star is Capella, the sixth brightest star in all the night sky.
The Star Capella
The star Capella is a magnitude 0.8 beacon in the constellation Auriga. Auriga is the constellation known as the Charioteer. It is the brightest star in Auriga and is also known as Alpha Aurgiae. Another name for Capella is The Goat Star. It lies 45 light-years away, relatively close compared to most stars in the sky. Capella is also a large star, about 11 times the size of our sun.
Location Makes the Difference
Capella is more noticeable in the fall for a couple reasons, and it has to do with its location. During the fall Capella is lower to the horizon, making it easier to spot for those who catch it between the trees or right over the roof of the neighbor's house. With a location close to earthly objects, it is easy to at first believe it is also an earthly object, such as an airplane. But the curious observer will wait and watch and then notice that the sparkling light is not moving. Then, because of the multitude of colors that it seems to display, thoughts often turn to flying saucers or other extraterrestrial origins. And indeed it is extraterrestrial, being a distant star.
Its location near the horizon also means that in order to see it we are looking through more atmosphere than when we look at stars that are overhead. In the winter, as Capella moves higher into the sky, strange sightings of this flickering light will be reduced. Other stars fall prey to this same twinkling effect when near the horizon, but Capella's brightness and time of year, being up as the sky grows dark early again for winter, makes it especially noticeable.
The Science of a Star's Twinkle
A technical word for "twinkling" is scintillation. Another ten dollar term for this effect is atmospheric prismatic dispersion. This simply means that the atmosphere is acting like a giant prism, separating it into the colors that you see in the twinkling. This effect is not very strong when looking straight up through 100 kilometers of atmosphere. But the amount of atmosphere you are looking through rises steeply the closer to the horizon your object is. At 20 degrees up it's about 300 km, at 15 degrees it's close to 400 km, 10 degrees is around 500 km, a mere 5 degrees above the horizon is 700 km, and something right in line with the horizon is seen through approximately 1100 km of atmosphere.
Other Fall Twinkling Stars
In the northwest, Arcturus will be sinking over the evening, dropping into into a viewing angle with thicker atmosphere. In late fall into winter, the brightest star in the sky, Sirius, will be rising above the horizon in the southeast. Both are bright stars that will twinkle rapidly through our thick and turbulent atmosphere.
