2-2 Eighty-eight constellations cover the entire sky

Looking at the sky on a clear, dark night, you might think that you can see millions of stars. Actually, the unaided human eye can detect only about 6000 stars. Because half of the sky is below the horizon at any one time, you can see at most about 3000 stars. When ancient peoples looked at these thousands of stars, they sometimes imagined that groupings of stars traced out pictures in the sky. Astronomers still refer to these groupings, called constellations (from the Latin for “group of stars”).

You may already be familiar with some of these pictures or patterns in the sky, such as the Big Dipper, which is actually part of the large constellation Ursa Major (the Great Bear). Many constellations, such as Orion in Figure 2-2, have names derived from the myths and legends of antiquity. Although some star groupings vaguely resemble the figures they are supposed to represent (see Figure 2-2c), most do not.

Figure 2-2: R I V U X G
Three Views of Orion The constellation Orion is easily seen on nights from December through March. (a) This photograph of Orion shows many more stars than can be seen with the naked eye. (b) A portion of a modern star atlas shows the distances in light-years (ly) to some of the stars in Orion. The yellow lines show the borders between Orion and its neighboring constellations (labeled in capital letters). (c) This fanciful drawing from a star atlas published in 1835 shows Orion the Hunter as well as other celestial creatures.

(a: Eckhard Slawik/Science Source; c: Stapleton Collection/Corbis)

The term “constellation” has a broader definition in present-day astronomy. On modern star charts, the entire sky is divided into 88 regions, each of which is called a constellation. For example, the constellation Orion is now defined to be an irregular patch of sky whose borders are shown in Figure 2-2b. When astronomers refer to the “Great Nebula” M42 in Orion, they mean that as seen from Earth this nebula appears to be within Orion’s patch of sky. Some constellations cover large areas of the sky (Ursa Major being one of the biggest) and others very small areas (Crux, the Southern Cross, being the smallest). But because the modern constellations cover the entire sky, every star lies in one constellation or another.

The star names shown in Figure 2-2b are from the Arabic language. For example, Betelgeuse means “armpit,” which makes sense when you look at the star atlas drawing in Figure 2-2c. Other types of names are also used for stars. For example, Betelgeuse is also known as a Orionis because it is the brightest star in Orion (a, or alpha, is the first letter in the Greek alphabet).