This month is a good time to see the constellations and deep sky objects around the summer triangle. The summer triangle is an asterism that first appears in the eastern evening sky at the beginning of summer. Now, nearing the end of summer, it is high overhead at dusk.
An asterism is simply an easily recognizable pattern of stars that may or may not correspond to a constellation. In this case, the summer triangle has three very bright stars that are in three different constellations. The stars are Vega in the constellation Lyra, Deneb in the constellation Cygnus and Altair in the constellation Aquila.
Vega will be the brightest star in the evening sky and very close to your zenith (i.e. right overhead) at dusk. It is the brightest star in the small constellation Lyra. In Lyra, the Ring Nebula, M57, and Epsilon Lyrae, the “double double,” are easy to find with a telescope but require high magnification to see any details.
The second star in the summer triangle is Deneb, which is 23 degrees, or a little over two fist widths, to the east of Vega. The name Deneb is derived from the Arabic dhaneb, meaning tail, and Deneb is the tail of Cygnus the swan. The body and long neck of the swan follow the Milky Way toward the south to Alberio, the star making the head of the swan.
Alberio is near the center of the summer triangle and is one of the prettiest double stars in the sky. This double star is one of the easiest to resolve in a telescope with low magnification. The brighter companion is yellow and makes a sharp contrast to the dimmer, blue companion. Besides a swan, Cygnus can be seen as the Northern Cross, with Deneb as the head of the cross and Alberio the foot.
Because it is aligned with the Milky Way, the background of Cygnus is rich in dim stars when viewed with binoculars, and several open clusters can be seen. On very dark nights, the bright patch of Milky Way that makes the North American nebula is visible just to the east of Deneb, but the shape of North America won’t show up except in photographs. The Veil nebula is a portion of a supernova remnant and can be seen on dark nights along the eastern wing of the swan with large binoculars or wide-field telescopes.
The third star in the summer triangle is Altair, a bit over 30 degrees to the south of Vega. Altair is the brightest star in the constellation Aquila, the eagle, which is flying north along the Milky Way. Because it is also along the Milky Way, there are lots of stars and clusters to see as you scan the region with binoculars. Near the tail of the eagle, but just outside the boundaries of Aquila, is M11, the Wild Duck cluster, one of my favorites.
This monthVega will be the brightest star in the evening sky. Use Vega to help find Deneb and Altair, and then the constellations Cygnus and Aquila. I find it interesting that the swan is flying right toward the eagle, but I am unfamiliar with any ancient stories from European mythology relating the two.
Venus has been the morning star for a while now, but there are other things to see if you get up before dawn. Mercury will be at its greatest distance from the sun in the morning sky on Sept. 12, but it might be more interesting to see on Sept. 10 when it is within a half degree of Regulus. Mercury will be following Venus about 10 degrees closer to the sun on these mornings. Mars will follow Mercury by about 3 degrees but is much dimmer so might get lost in the morning light. Mars will be less than a quarter degree from Mercury on Sept. 16, and on Oct. 5, Mars will be similarly close to Venus.
There is a star party at Mesa Verde on Sept. 16 and one at Chimney Rock on Sept. 22. As cooler weather sets in, these might be the last of the season, so let’s hope for clear skies.
Charles Hakes teaches in the Physics and Engineering Department at Fort Lewis College and is the director of the Fort Lewis Observatory. Email him at firstname.lastname@example.org.