How to Find the Summer Triangle
A Summer Asterism Made from Three Different Constellations
May 26, 2009
One of the iconic symbols of the summer night sky is the Summer Triangle. For Northern midlatitudes, the summer triangle is already above the eastern horizon by mid-May and remains above the horizon until the end of the year. However it is referred to as the summer triangle because it is most prominent in the summer months, appearing overhead on summer evenings.
What Is the Summer Triangle?
The summer triangle is made of three bright stars: Vega, Altair, and Deneb. These three bright stars are part of three different constellations: Lyra, Aquila, and Cygnus, respectively. Vega is the brightest of the stars at magnitude 0.03, Altair is next brightest at magnitude 0.76, and Deneb is third at magnitude 1.25. The triangle made by the stars spans a large swath of sky, stretching to 38 degrees at its longest point.
How to See the Summer Triangle
The best way to spot the summer triangle is to start looking for the first stars to appear after sunset. In May and June, look east as the sky begins to grow dark and look for the first star to appear in the oncoming night. This star will be Vega. From approximately July through September, Vega will be nearly overhead at twilight. Facing east, look to the right (or southeast) of Vega to find Altair and to the left (or northeast) of Vega to find Deneb.
To be certain that the star you picked as Vega is the correct one, watch as the sky grows darker and more stars appear. Vega should be next to a parallelogram of stars that forms the rest of Lyra the Lyre or Harp. Deneb is part of a constellation named Cygnus the Swan. Cygnus forms a cross shape with Deneb as the top star on the cross. Altair, as part of Aquila the Eagle, is part of the tail of the constellation, which also looks somewhat like an airplane shape.
What Else Is There to See by the Summer Triangle?
Once you've found the summer triangle, you can use it as a key to finding other wonders in the summer sky. Inside the summer triangle is a portion of the Milky Way Galaxy. Use the bodies of Cygnus the Swan and Aquila the Eagle to follow its trail across the sky. These two birds appears to be soaring along the length of the Milky Way.
One of the best double stars in the sky can be picked up easily with any telescope at the base of the cross shape of Cygnus. Albireo shows two stars, one yellow and one blue, when viewed with optical aid. These stars are about 380 light-years away from Earth.
Within the summer triangle are two of amateur astronomer's most favorite nebulae. The first is M57 in Lyra. M57, also known as the Ring Nebula, lies midway between the two stars that make up the line of the parallelogram that is farthest from Vega. The other nebula, M27, also known as the Dumbbell Nebula, lies on an imaginary line connecting Altair and Deneb. M27 is about one-third of the way to Deneb from Altair.
One last favorite object by the summer triangle is the petite constellation known as Delphinus the Dolphin. It lies just east of M27, below that imaginary line between Altair and Deneb. Its shape is reminiscent of a kite with a tail.
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