When you are out on a clear night, be sure to look for the three bright and beautiful blue-white stars of the Summer Triangle asterism, which shine high in the eastern sky every July. Once you have it identified, you can find some treasures within it.
Summer Triangle stars
Find an open area and face east. Almost straight overhead is the bright star, Vega. It’s the fifth brightest star in the entire night sky and one of the first stars to appear after dusk. Now look for the other two corners. Altair is not as bright as Vega and sits about 3.5 outstretched fist diameters (34°) to the lower right of it. The third star, Deneb, is about 2.5 fist diameters (24°) to the lower left of Vega and higher up than Altair.
Vega (constellation Lyra)
Can you see the four fainter stars forming a small parallelogram just below Vega? This shape is the body of the musical harp that makes up the constellation of Lyra. Vega marks the top of the instrument’s neck.
Vega’s visual magnitude, or brightness, is the zero reference point for the scale we use to define stars’ brightness values. Objects brighter than Vega have values lower than zero, and vice versa. For example, Antares, the bright, reddish star sitting over the southern horizon in Scorpius, has a value of about 1.0, making it 2.5 times dimmer than Vega. (It’s a logarithmic scale.)
Deneb (constellation Cygnus)
The bright star Deneb marks the tail of the great Cygnus. A faint star about two fist diameters (22°) to its right, in the middle of the Summer Triangle, is Albireo, a colorful double star that marks the swan’s head. If you are in a dark location, you should also be able to see that the Milky Way runs right through Cygnus, as if she is about to land for a swim on that celestial river!
Altair (constellation Aquila)
The most southerly of the triangle’s corners is marked by Altair — the head of the great eagle Aquila. At only 16.8 light-years away, Altair is one of the nearest bright stars — so close that its surface has been imaged! The star also seems to be spinning 100 times faster than our Sun.
Constellations inside the Summer Triangle
Have a look for two little constellations in the area. Sagitta comprises five faint stars, aligned left to right, located midway between Altair and Albireo. The three stars on the constellation’s right end form the feathers.
Below Sagitta and to the left of Altair, there’s cute little Delphinus. Four stars form a diamond-shaped body, and another star to the lower right of that makes the tail!
There’s one more small constellation inside the Summer Triangle, but its dim stars make it difficult to see from the city. It’s called Vulpecula. It is made up of only two magnitude 4.5 stars and is located north of Sagitta, near Albireo.
Deep-sky objects inside the Summer Triangle
Grab your binoculars and look about midway between Vega and Altair for a little grouping of stars called the Coathanger. It’s composed of a rod made by a line of six stars plus a hook made up of four stars. (Hint: For North American observers, it’s oriented with the hook downwards to the right.) Its fancier names include Brocchi’s Cluster, Al Sufi’s Cluster, and Collinder 399.
In the constellation Sagitta, near the bright Altair, there’s a fairly bright globular star cluster named Messier 71 or the Angelfish Cluster. Under a dark sky, binoculars should show it as a small, faint, fuzzy star. In a backyard telescope, it will resemble a mound of sugar sprinkled on black velvet.
One of Vulpecula’s claims to fame is the spectacular planetary nebula known as the Dumbbell, also known as the Apple Core Nebula, Messier 27, NGC 6853. From a dark location, aim your telescope 3° to the celestial north-northwest of Sagitta’s arrow tip and look for a small, faintly glowing cloud of gas that resembles an apple core.
Bottom line: The Summer Triangle asterism, which shines nearly overhead in the evening sky during July, is composed of the bright stars Deneb, Vega, and Altair. The easiest way to find this star pattern in the sky is to use the Sky Tonight astronomy app.
Text Credit: Chris Vaughan
