The Summer Triangle is not a constellation, but rather three bright stars seen easily in the sky from late spring through late winter in the evening sky. It is composed of Vega, the brightest of the three at magnitude zero, Altair at a "high" first magnitude shining at 0.8, and Deneb, the dimmest of the three at magnitude 1.3. Vega anchors the diminutive constellation of Lyra the Harp, that contains an impressive four First Light Guide Objects. Altair is the brightest star of Aquila the Eagle and is the furthest south of the three. Deneb forms the "tail" of Cygnus the Swan, and this larger constellation also has 4 First Light Guide objects within it.
More than that, the Summer Triangle can be used to find other constellations beyond the three with bright stars. But first let's look at where it is often seen in the sky (see first chart below). To determine if indeed you have found the Summer Triangle, measure the three sides using the Stargazing Basics measurement techniques found here. Vega to Deneb is about 25 degrees, and is the length of the shortest side. Vega to Altair is next-longest at around 35 degrees - use the 25 and 10 degrees hand shapes to confirm this. And lastly is the longest side, Altair to Deneb, slightly longer at nearly 40 degrees.
Cygnus is the largest constellation within the Summer Triangle. From Deneb, draw a line through the triangle, about halfway between the Vega -> Altair line. This line forms the "body" of Cygnus the Swan, and the last star at the end is Albireo (near the "e" in Summer below), the "beak" of the swan. Closer to Deneb, the "wings" spread out at right angles from this line. Some people view the 4 "body" stars and three central "wing" stars as an asterism called "The Northern Cross." These are also some of the easiest stars to see in Cygnus; the length of the cross is about 20 degrees, and the distance across the sides is around 15 degrees. The larger 'wings' are shown in the graphic below:
Find Lyra within the Summer Triangle
Lyra the Harp is a small constellation - it is one of the smallest in the northern celestial hemisphere. It also contains mostly dim stars of 3rd magnitude and dimmer. However, Lyra is punctuated by one of the brightest - and nearest - stars. Vega is a zero magnitude star, placing it as the fifth brightest in the whole of the night sky. Looking slightly within the bounds of the Summer Triangle from Vega, keen-eyed observers will notice a small parallelogram shape of two additional 3-rd magnitude stars farther from Vega and two 4-th magnitude stars closer to Vega. Even if you can't see these naked eye, a set of 7x35 or larger binoculars will show nearly all of the constellation including these dimmer stars, even under light polluted skies. This is because the farthest star from Vega is only about 7 degrees away, almost within the field of view of most binoculars. Check out where to find Lyra, here:
Draco the Dragon is a long, winding constellation. But finding the head of the dragon is easy to do from the Summer Triangle. Draw an imaginary line from Altair through Vega. From Vega, continue that line another 15 degrees past that bright star. The "head" of Draco is composed of 4 stars, the brightest one being 2-nd magnitude. This will be the first star 2-nd magnitude star encountered on the way from Vega; the next star in the head is a slightly dimmer 3-rd magnitude and is a mere 4 degrees away. Continuing past these two stars another 15 degrees takes the observer to another 3-rd magnitude star - Eta Draconis - that is useful to find a couple of First Light Guide Objects in the sky. Even if you can spy all of Draco, look for these three brightest stars from the Summer Triangle. There is much to be found from them - check out this chart:
Hercules the Strong Man, in contrast to Lyra, is one of the largest constellations in the night sky, 5-th largest overall. Despite its large size, it is also composed of many dim stars, the brightest at a high third magnitude of 2.8. However, the "Keystone" shape - a trapezoid that forms the "body" of Hercules - is largely identifiable from most light polluted areas. Draw an imaginary line from Deneb through Vega. Curve or angle that line just slightly towards Draco in the north. The center of the Keystone is about 20 degrees from Vega. The closest and southernmost star of this grouping of four will be the most difficult to see at 4-th magnitude. The others are of 3-rd magnitude, and should be easier to spot. Two First Light Guide objects are near this shape, so it is the useful portion of Hercules to locate. See the chart below to see where the Keystone is, along with the rest of Hercules:
Ophiuchus the Physician is a large constellation. While not quite as large as Hercules in total area, the brighter 2-nd and 3-rd magnitude stars composing the constellation can make it appear larger than the strong man. It is often said to look like a tall, narrow house shape. Star at Deneb in the Summer Triangle, and follow the line down the "Northern Cross" shape through Albireo towards the south. Go a LOOOONG way - almost half the sky, about 70 degrees - to the red giant star Antares towards the south (it will appear orange to man observers). This actually skips past all of Ophiuchus, so now look back up above Antares, and somewhat to the west of there - in between Antares and Vega.
Three stars along the "bottom" of the house are low second magnitude stars (a fourth one close to the western-most one is 4th magnitude), and are less than 20 degrees across. Now look up to the "roof" stars - the peak is around 25 degrees north of the base stars, and is the brightest star in the constellation. The two stars forming the lower parts of the "roof" appear about a magnitude dimmer, but at high 3-rd magnitude are visible from most areas. See the chart here for a better look at this "Doctor in the House" shape:
Serpens is an interesting constellation, as it is the only one split into two in the night sky, bisected by Ophiuchus, the constellation listed above. For the First Light Guides, observers will only need to find the eastern half of Serpens. So once Ophiuchus has been identified, look at the lower left stars of the "house" shape; from there, draw a line up somewhat towards bright Altair of the Summer Triangle. Look carefully; there are three dimmer 3-rd magnitude stars here stretching about 20 degrees up from that "lower left corner" of the "house" shape of Ophiuchus. These are the stars we are interested in finding for our First Light Guides; if you have trouble seeing them, try using binoculars of 7x35 or larger. Each of the three stars of Serpens are only about 6 to 8 degrees apart, so they can be found and identified without too much trouble this way. See the chart below to get a better sense of where Serpens is on both sides of Ophiuchus - remember, the left/east side is the side to focus on:
Scutum is one of the smallest constellations in the entire sky - it's 84th of 88! But it contains two First Light Guide objects. However, both its small size and dim stars make it a challenge to find without starting from a brighter and larger constellation: Aquila the Eagle. Beginning at Altair - the bright star at the "beak" of Aquila - follow the 3-rd magnitude stars that go in approximately the same direction as the Deneb -> Albireo stars in Cygnus the Swan covered earlier. The "hop" from Altair to the first 3-rd magnitude star is ~8 degrees. Outside of this "hop" in equal directions are two 3-rd magnitude stars about 20 degrees apart; these form the "wings" of the eagle. The next hop to the second 3-rd magnitude star. Near this second 3-rd magnitude star is a third 3-rd magnitude star. This is the "tail" of Aquila. This is really the best starting point for the First Light Guide objects here, because Scutum is so much fainter. But the 4-th and 5-th magnitude stars are shown in the shape along with Aquila in this chart:
Sagittarius is the Archer, and the "bow" of the archer is one way to pick out this constellation, but there's a more-often used way: Identifying "The Teapot" asterism within the constellation. But start back at the Summer Triangle. Sagittarius is best seen when the bright star Vega is on the meridian. But look at the constellations of Cygnus and Aquila. Look at the 'spines' of the two birds; these were identified previously. For Cygnus it's the length of the 'cross' shape; for Aquila it's the third magnitude stars. Look at the chart below; the plane of the Milky Way can be seen running between these sections of stars. While the Milky Way itself is not visible from most light polluted areas, observers can follow a line parallel to the 'spines' of the birds towards the the southern horizon. This leads the observer to the Teapot asterism of Sagittarius, that will be east of the first magnitude star Antares of Scorpius. Formed mostly from high third magnitude stars, the Teapot is looking nearly at the central portion of the Miky Way galaxy; see the chart for the general shape of the whole constellation, with the Teapot mostly in the central section:
Scorpius is not too hard to find for most observers; first magnitude and orange-looking red giant Antares is the brightest star in the constellation. It can be found south of - or below - Ophiuchus, and west of Sagittarius. The constellation does resemble a scorpion, and many ancient cultures saw it as such; the body is slightly above and to the west (right) of Antares towards Arcturus, and the "stinger" heads down towards the horizon, them curls back up just to the west (right) of Sagittarius. The end point of the stinger is Shaula, a brighter 2-nd magnitude star, and the 24th brightest in the whole sky. Look for these two stars first, then try and round out the shape of the scorpion from there, that has another dozen or so stars of 2-nd and 3-rd magnitude. Unfortunately for many northern hemisphere observers, the low elevation above the horizon may make some of these stars harder to see depending on local light pollution. But Antares and Shaula should be visible with careful observation - see this chart:
Vulpecula is a constellation purported to represent a fox, but the dim stars make it difficult for most observers to discern easily. But one of the best First Light Guide objects is within its borders, so although most observers will not need to actually identify the constellation (most of it's bright stars are at 4-th magnitude!), it does help to know where it is in relation to the Summer Triangle.
Note the line that would to from Deneb in the north towards Altair in the south. Next, not the line from Vega to Altair. Just to the south of Albireo - the "foot of the cross" in Cygnus's Northern Cross - is where Vulpecula lies. No need to actually identify the constellation; most observers simply need to know what direction it is in relation to the other Summer Triangle stars. See the chart to get a sense of where Vulpecula is:
And that is how to find 11 constellations from the Summer Triangle! Don't miss all the great objects you can find and see with a small telescope that reside within these shapes in the sky.
Check out the First Light Guides starting page to learn what you can find and observe by hopping to Cygnus, Lyra, Draco, Hercules, Ophiuchus, Serpens, Scutum, Aquila, Sagittarius, Scorpius and Vulpecula from the Summer Triangle.