The Summer Triangle [Image: Jimmy Westlake/Steamboat Pilot&Today / EarthSky]

The Summer Triangle is a stellar treat for northern mid-latitudes summer sky watchers. It graces the sky all night long in summer, and its three bright stars are visible even in urban areas. Under dark skies you can also see the Milky Way within the asterism.

Asterism vs constellation
There are 88 official constellations. The International Astronomical Union (IAU) defined them so that they cover the whole sky without overlaps or gaps. Asterisms are patterns whose stars belong to one or more constellations or sometimes star clusters. They're usually more recognizable than the constellations, but they have no official status. The best known asterism is the Big Dipper ("the Plough" in the UK), which is part of Ursa Major, the Greater Bear.

Stars of the Summer Triangle
The header image shows the three stars of the triangle. Each star is the brightest in its constellation and among the brightest stars in the night sky. Vega is the Harp Star in Lyra, the lyre of Orpheus. Deneb is the tail of Cygnus the swan, and Altair is the “soaring eagle” in Aquila the eagle. You may see the bluish tinge to these hot stars.

Vega and Altair have a number of similarities. They're hotter, more massive and much younger than the Sun. Vega is the hotter of the two. Although it's farther away than Altair, it's noticeably brighter. They're both still in their hydrogen-burning phase, but will burn out long before reaching anything like the age of the Sun.

Both stars rotate very fast, making them greatly flattened at the poles. While the Sun takes a leisurely 25 days to rotate, Vega spins once every 12.5 hours and Altair every 9-10 hours. Altair is spinning so fast that if it were going any faster it would fly apart.

Deneb doesn't appear as bright as the other two, but that's misleading. It's a lot farther away from us than Vega and Altair. Astronomers aren't certain how far, but it may be around 80 times farther away than Vega, say 2000 light years. Deneb is a blue supergiant that many astronomers think is well on its way to being destroyed in a supernova explosion. Yet how can Deneb seem so bright over such a distance? It's because it's thousands of times brighter than the Sun. If it were as close to us as Altair, you could probably read by its light.

Besides the stars, the Summer Triangle has another prominent feature. The Great Rift of the Milky Way runs through it. When we look at that part of the Milky Way, we're seeing an edgewise view of the Galactic disk. The “rift” isn't really a break in the disk, it's a wide dust lane. [Image: Sky & Telescope]

Summer Triangle – some history
The Summer Triangle doesn't have a classical origin. It was shown on some star maps in the nineteenth century, but not named, and has been known to navigators as the “navigator's triangle” into the twentieth century. The name Summer Triangle appears early in the last century, but wasn't widely used until the 1950s. H.A. Rey's delightful and original constellation drawings popularized it in his book The Stars, as did the writings of astronomer Patrick Moore in Britain.

However summer triangle is somewhat misleading. It's true that in the northern mid-latitudes it's only in the summer that can you see it all night long. However it can be seen for some part of the night at any time of the year. It's easily visible in the autumn. The New Handbook of the Heavens in 1941 didn't give it a name, but described it in the chapter on autumn stars.

Locating the Summer Triangle
A good way of finding the triangle is too look for Vega which is high in the sky at nightfall, and one of the brightest stars visible in the sky. Deneb is to its lower left and Altair completes the triangle. If you look to the east after it gets dark, the Summer Triangle should stand out. Keep in mind that you're looking for a very large triangle.

The Summer Triangle is also visible in some parts of the southern hemisphere, though not beyond about 40° south. When the triangle is visible in August, it's winter in the southern hemisphere. The triangle is low in the northern sky, and it's upside down compared to the northern hemisphere view.

Mythology
Although the Summer Triangle doesn't date from classical times, the bright stars have been well known for millennia and each has many associations with Graeco-Roman mythology.

Aquila represents Zeus's eagle who carried his thunderbolts, and also occasionally aided his seductions. “Altair” comes from the Arabic, meaning “flying eagle”. Cygnus represents Zeus himself disguised as a swan to seduce Leda, the queen of Sparta.

Vega has been represented as a swooping eagle. However it was called the Harp Star by Pliny, author and natural philosopher of ancient Rome. This is because Lyra represents the lyre – a harplike instrument – played so sweetly by Orpheus that the lord of the Underworld was persuaded to allow him to take his beloved wife Eurydice back to the land of the living. It's a famous story, so you probably know that it didn't end happily.