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Ephemeris Extra – Sightseeing around the Summer Triangle

August 21, 2016
Deep-Sky treasures around the Summer Triangle

The Summer Triangle and some Deep-Sky treasures within and nearby. Created using Cartes du Ciel (Sky Charts).

Published in the Grand Traverse Astronomical Society’s August 2016 Stellar Sentinel Extras section, updated from an article first published in 1998

One of the signs that summer is here is the appearance of the Summer Triangle in the evening sky. Three of the brightest stars of summer are arranged in a large beautiful triangle. The Summer Triangle isn’t an official constellation. It is an informal one, which is called an asterism. The Big Dipper is another famous asterism.

Each of the triangle stars belongs to its own separate constellation. At the top of our chart is Deneb, the dimmest of the three in our sky at magnitude +1.25. However it is actually the brightest of the three. It suffers only due to its distance of 2,600 light years. Its absolute magnitude, its magnitude if it were 10 parsecs or 32.6 light years away, is -8.4, as bright as the quarter moon. Deneb is situated at the tail of the Cygnus the swan. The word Deneb means tail. It’s actually part of a longer Arabic phrase the means “Tail of the Hen”.

Brightest from our point of view is Vega in Lyra the harp. Vega means either “Falling Eagle” or “Falling Vulture”. Vega is close to the sun, at 25 light years. It’s apparent magnitude is +0.03, with an absolute magnitude of +0.5.

Closest of the three is Altair in Aquila the eagle which is only 16.7 light years away, at magnitude +0.77 with an absolute magnitude of +2.2.  It spins rapidly, once in 8.9 hours, creating a decidedly oblate shape with an equatorial radius of 2 times the Suns radius and a polar radius of 1.6 times.

The area in and around the Summer Triangle is a wonderful hunting ground for a telescope. They include binary stars and deep sky objects.
Alberio, the star at the beak of Cygnus the swan is a beautiful maize and blue pair, which I have dubbed the “U of M Star” (University of Michigan’s Maize and Blue) at the public viewing nights. Sorry State fans, I don’t know of a green and white binary.  It’s a wide pair, but requires more than a pair of binoculars.

Another neat binary star is Epsilon Lyrae, just to the left of Vega. It shows as a wide pair of equally bright stars in binoculars. A telescope reveals, upon close inspection, that each is again a bin ary. Epsilon Lyrae is the famous double-double star, And splitting them is a good test of telescope and atmospheric seeing.

Just about centered between the two stars at the south end o f the parallelogram of Lyra can be found one of the really special faint wonders of the sky. It is M57, 57th object of Charles Messier’s list of objects that look like comets but aren’t. It is better known as the Ring Nebula. It is one of a class of objects known as planetary nebulae. Planetary nebulae have nothing to with planets, but many of them look like Uranus or Neptune, being faint small greenish or bluish in color. Anyway, the ring, and all planetaries are the result of the SlimStar™ plan of rapid mass loss. It is something small stars like the Sun do when out of fuel and are in the final collapse to white dwarfhood. The outer layers of the star are pushed out to form many wonderful shapes. The Ring Nebula apparently is a torus about half a light year in diameter and about 2,300 light years away. Though small and bright in telescopes, it is invisible in finder telescopes.   See the Hubble image of it on page E-4.

Another planetary is M27, also known as the Dumbbell Nebula is located just north of the tip star of Sagitta the arrow. This is a tough one to find, and like M57 is invisible in finders. The Dumbbell is large, but with a low surface brightness. The two glowing lobes of gas first visible give the object its name. Much finer detail is visible to the careful observer. It’s distance is thought to be about 1,360 light years, which makes its diameter nearly 3 light years.

The greatest globular star cluster in this area is M13 the Great Hercules Globular Star Cluster which is located along the western edge of the ‘Keystone’ part of Hercules. The fuzzy blob that is visible in binoculars and small telescopes begins to resolve itself into stars in telescopes of 6 inch diameter and larger. M13 contains upwards of a million stars packed in a diameter of 168 light years and is located some 22,200 light years away. Globulars are ancient clusters that trace there origins to the formation of the Milky Way.  Other globulars in this area are M92, also in Hercules, M56 in Lyra, M14 in Ophiuchus, the serpent bearer, M15 in Pegasus, and perhaps M71 in Sagitta the arrow. M71 may be a sparse globular or very compact galactic cluster.

Galactic or open clusters here are M11, off the tail of Aquila the eagle, M39 and M29 in Cygnus. M11, which is actually in Scutum the shield is among the finest of its type. It takes a telescope of 6 inches diameter to completely resolve it.  M11 is also called the Wild Duck Cluster, perhaps due to its vaguely triangular appearance.

The Summer Triangle provides a wealth of objects to view and study, Some are among the finest of all the heavens.


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