Eye Astronomy #3: The Constellations
by Dale E. Lehman
Aside from the five brightest planets (see Eye Astronomy #2), passing airplanes, and the increasing number of artificial satellites orbiting the Earth, every point of light you see in the night sky is a star. To the unaided eye under dark conditions, over 9,000 stars are visible. At any given time you only see about half the sky, though, the rest being hidden by the Earth below your feet. And few of us are lucky enough to live under dark skies.
The closest star visible to the eye is Alpha Centauri. It’s 4.25 light years away. (One light year is the distance light travels in a year, about 5.9 trillion miles!) And the farthest? That would be a star at the edge of visibility without a telescope, a star with no name, just a designation: V762 Cas, which is 16,000 light years away!
The designation “Cas” indicates the constellation Cassiopeia. So, what is a constellation? Two things, really. Looking at the night sky, our ancient ancestors played a game that today we still play, although on paper: connect the dots. In those sparkling dots in the sky, they saw pictures, and those pictures are constellations. Every culture played this game, finding in the stars characters from their religious traditions or nature or life in general. They found kings and queens, gods and goddesses, bears and lions and loons, cradles and boats and dragons.
Western civilization adopted the Greek constellations, then added a few of its own when its astronomers ventured into the southern hemisphere to explore hitherto unknown (to them) skies. Which brings us to the second thing a constellation is: a region of the sky. Modern astronomers divide the sky into 88 constellations, each with a distinct boundary. Every object in the sky lies within a constellation, even those objects that can only be seen telescopically.
But for most of us, constellations are still pictures, traced out by the brightest stars. Cassiopeia happens to be one of the brighter and more obvious constellations. To the Greeks, she was a mythical queen seated on her throne. To us, she’s a great “W” or “M” in the northern sky. See if you can find her in this star chart:
If you need help, here’s the same chart with the “dots” connected:
There she is, in the upper right. In the lower left, you might have noticed the famous “Big Dipper.” The Big Dipper isn’t a constellation. It’s an asterism, a group of stars that form a picture. The Big Dipper asterism is part of the larger constellation, Ursa Major, the Great Bear.
Near in the center of the image, you’ll find the Little Dipper asterism, which is part of Ursa Minor, the Little Bear. The star at the end of the “handle” of the Little Dipper is Polaris, the North Star. As the sky turns—really, it’s the Earth turning, but it looks to us like the sky turns—the other stars wheel around Polaris, which stays nearly in the same spot. (Polaris isn’t exactly above the north pole, so it does trace out a tiny circle in the sky, but you won’t notice it.)
Learning to recognize the constellations is an important step in your astronomical journey. Not all of them are as obvious as Cassiopeia and Ursa Major, but because the stars seem to stay in the same place relative to each other, at least over thousands of years, the constellations always appear in the same groupings. This makes all but the faintest ones easy to find. Ursa Major and Cassiopeia are always roughly on opposite sides of Polaris, for example. Cephus, the King—that house-like shape at the top of the image—is always beside Cassiopeia. (The women in the audience might enjoy the fact that he’s not as bright a constellation as his queen!) And so forth.
In future installments of Eye Astronomy, we’ll tour the northern hemisphere constellations so you can learn your way around the night sky. Until then, look up and see if you can connect some of the dots on your own.
