The Summer Triangle
By mid-July the Summer Triangle is high in the Eastern sky for those of us at mid Northern latitudes (+40°) giving us the opportunity to observe some familiar deep sky targets and seek out a few of the more challenging targets for small telescopes.
If you’re not familiar with the Summer Triangle, here’s how you find it. Start at the constellation Cygnus the Swan and locate the first magnitude star Deneb, then draw a line to the bright star… Vega (mag. 0.03) in the constellation Lyra. Next draw a line to the 0.7 magnitude star Altair in the constellation Aquila the Eagle. Connect Altair with Deneb and you have the Summer Triangle. British Astronomer Sir Patrick Moore popularized the Summer Triangle in the 1950’s, although he didn’t invent it.
The bright stars that make up the triangle are interesting themselves and make a great place to start. Deneb (Alpha Cygni), is the brightest star in the constellation Cygnus and is the 19th brightest star in the night sky. Deneb is 200 times the diameter of our Sun and its mass is estimated to be around 20 solar masses. Deneb is the prototype of a class of variable stars known as Alpha Cygni variables, which undergo non-radial fluctuations which cause its brightness and spectral type to change slightly. A 2008 study puts Deneb at a distance of approximately 1,550 light-years. Vega (Alpha Lyrae), at magnitude 0.03 is the fifth brightest star and the second brightest in the northern hemisphere. At only 25 light-years from Earth, Vega is one of the most luminous stars in the solar neighborhood. Vega is also a suspected variable star. It is rotating rapidly with a velocity of 274 km/s at the equator. This is causing the equator to bulge outward because of centrifugal effects, and, as a result, there is a variation of temperature across the star’s photosphere that reaches a maximum at the poles. We see Vega from the direction of one of these poles. Altair (Alpha Aquilae), the twelfth brightest star in our night sky is located 16 light-years away and one of the closest stars visible to the naked eye. Its mass is 1.8 times the mass of our Sun and 11 times as luminous. In 2005 Altair was identified as a Delta Scuti variable star. Altair’s rate of rotation is extremely rapid, around 9 hours. This rapid rotation forces Altair to be oblate; its equatorial diameter is over 20 percent greater than its polar diameter. Owing to this rapid rotation, Altair’s surface gravity and effective temperature are lower at the equator, making the equator less luminous than the poles.
Deep Sky Objects
The more familiar deep sky objects within the triangle are the planetary nebula M57 the “Ring Nebula” (actually lies along the line from Vega to Altair), M56, a globular cluster, M71, another globular cluster in the constellation Sagitta (this object may be difficult in some telescopes depending on your observing location), M27, the “Dumbbell nebula”, a planetary nebula in Vulpecula, and M29, an open cluster in Cygnus. Some of the more challenging and therefore less observed deep sky objects are listed below. All of them are open (sometimes called “galactic”) clusters.
NGC 6866, around mag. 7.2.
NGC 6819, mag 7.3.
NGC 6791, around mag. 9.
NGC 6910, mag 7.4 (The Rocking Horse Cluster).
IC 4996, mag 7.3.
NGC 6883, mag.8. This object could be a serious challenge.
NGC 6871, mag.5, sparse.
NGC 6834, around mag.8.
NGC 6823, mag.7.1, an emission nebula (NGC 6820) surrounds this cluster.
NGC 6802, mag.8.0.
NGC 6830, mag. 7.9.
Remember, when observing these deep sky objects to pump up the power once you locate them. In Sky & Telescope’s article, “In Caroline Herschel’s Footsteps” by Tony Flanders, it mentions that when observing NGC 6866, Caroline found resolving the cluster into individual stars to be difficult at low magnification but easy at 70X. She was using a 4.7” telescope.