One of the challenges amateur star observers face is learning to identify constellations. When the average person hears that the sky is divided into 88 constellations, the initial reaction is often, “Forget it. It’s hopeless. I’ll never remember most of them.” The purpose of this article is to convince aspiring star gazers otherwise. Identifying a handful of constellations is the first step (and arguably the most important one) in learning to navigate around the night sky.
The Big Dipper
For observers in the Northern Hemisphere, a good place to start is finding the Big Dipper, the most noticeable group of stars in the constellation Ursa Major (the Great Bear). Under clear skies, the Big Dipper is visible at some time of night from all locations north of the equator. One way to identify the Big Dipper is to find north, using a compass if necessary. Look for seven stars that form the pattern shown on this diagram. You will notice that the Dipper’s shape somewhat resembles a ladle or large spoon, and most of its stars are second and third magnitude, meaning they are fairly bright.
To an observer facing north, the Dipper appears to move in a counterclockwise circle around the north celestial pole. The north pole of the sky is marked by the second magnitude star Polaris, better known as the North Star. Once you are comfortable finding the Big Dipper, you can use it as a jump off point to find many other constellations as the seasons pass.
The Spring Sky: Arc to Arcturus, and speed on to Spica, but don’t miss Leo.
The Big Dipper is directly over Polaris for most of spring. If you follow the curve of the Dipper’s handle, it will point to a bright orange star called Arcturus in the constellation Bootes, the Bear Driver. Extending the same line past Arcturus will lead to the bright bluish white star Spica in the constellation Virgo. Northwest of Virgo is the constellation Leo, the Lion. Leo can also be found by drawing a line backwards through the pointer stars of the Dipper’s bowl. The line will end near the star Regulus, a bluish white first magnitude star that marks the Lion’s heart.
The Summer Sky: The Summer Triangle, the Scorpion, and Sagittarius
One highlight of the summer sky is three constellations whose brightest stars form a large upside down triangle in the sky. These stars are Vega in the constellation Lyra (the Harp), Deneb in the constellation Cygnus (the Swan), and Altair in the constellation Aquila (the Eagle). Southwest of Altair (the farthest south of the three stars of the summer triangle) is the constellation Scorpio with the bright red star Antares at its center. Scorpio does resemble a scorpion and is visible in the southern sky all summer long. East of Scorpio is the constellation Sagittarius, the Archer. This constellation contains dozens of deep sky objects visible through binoculars or a small telescope. The center of the Milky Way galaxy is located in Sagittarius.
The Autumn Sky: The Great Square of Pegasus, Auriga, and Taurus
The Great Square of Pegasus rises in the northeast during late summer. Due north of Pegasus is a hazy patch of light. This is M31, the Andromeda Galaxy, one of the few galaxies visible to the naked eye. Although it is 2.5 million light years away, the Andromeda Galaxy and milky Way are both part of the same galactic cluster known as the Local Group. The next noticeable constellation to rise after Pegasus is Auriga, the Charioteer, with its bright yellow star Capella. An hour or two after Auriga rises, Taurus the Bull becomes visible in the eastern sky. This constellation contains the bright red star Aldebaran, and two naked eye star clusters, the Hyades, which form the V shape of the Bull’s horns, and M45, a.k.a. the Pleiades (the Seven Sisters).
The Winter Sky: Gemini, Orion, and the Dog stars
The rising of Taurus tells us that the next constellation in the zodiac will rise soon. This is Gemini, the Twins. The two brightest stars in Gemini are named Castor and Pollux. Gemini is visible from autumn through the following June for observers in the Northern Hemisphere. The main highlight of the winter sky, however, is Orion (the Hunter), located south of Taurus and Gemini.
Orion straddles the celestial equator and can be seen from most locations on Earth for at least 6 months out of the year. Orion contains two first magnitude stars, the red variable star Betelgeuse, which means House of the Twins in Arabic, and the blue giant star Rigel, which means foot in Arabic and Hebrew. Orion’s Belt contains three fairly bright stars which divide the constellation neatly in half. Below the second star in the Belt is a group of stars called Orion’s sword. The middle star of the Sword appears fuzzy to the naked eye. That’s because it is really a cluster of stars called M42, the Great Orion Nebula.
Drawing a line through Orion’s belt to the southeast points to Canis Major (the Large Dog). Canis Major contains Sirius (the Dog Star), the brightest star in the night sky at magnitude -1.47. Sirius is a brilliant white star visible from all latitudes south of Greenland. North of Sirius, just above the celestial equator is Canis Minor (the Small Dog), which contains the bright yellow star Procyon, which literally means “before the dog”, as it rises a short time before Sirius.
Key constellations for observers in the Southern Hemisphere to learn include Carina (the Keel), which contains Canopus, the second brightest star in the night sky; Centaurus (the Centaur), which contains the first magnitude stars Alpha and Beta Centauri; and the nearby constellation Crux, the Southern Cross, which contains two first magnitude stars, Acrux and Becrux. One other first magnitude star visible from latitudes south of Florida is Achernar, which marks the southern end of the constellation Eridanus (the River), southwest of Orion. The Large and Small Magellanic Clouds, named after the 16th century explorer Ferdinand Magellan, are satellite galaxies of the Milky Way. They are located in the faint constellations Dorado (the Goldfish) and Tucana (the Toucan), respectively. The Large Magellanic Cloud contains NGC 2070, better known as the Tarantula Nebula, one of the brightest star clouds in the known universe.