The Summer Triangle is a group1 of three very bright stars visible from the Northern Hemisphere, particularly during the summer months. It consists of the stars Vega, Deneb and Altair. The Summer Triangle, coined by the well-known British astronomer Sir Patrick Moore, is particularly prominent before midnight in the month of August.
How to Find the Summer Triangle
First, you need to locate the well known pan-shaped constellation known as the Great Bear2 in the northern sky. The two outer stars of the pan are known as The Pointers, and they can be used to find the position of the North Pole. The two inner stars can be used to find the Summer Triangle. By tracing a line through the stars and extending the line northwards, you finally reach a very bright star. This star is Vega, one of the brightest stars in the northern hemisphere. To Vega's left is another bright star - this is Deneb. To the south of both stars is the final member of the trio - Altair.
Vega, or alpha Lyrae3 is a very prominent star in the small constellation Lyra 'the Harp'. Vega is 25 light years away from Earth, and the name Vega comes from the Arabic word meaning 'Falling Eagle'. It is a main sequence star 3 times larger and 50 times more luminous than our Sun.
Deneb, or alpha Cygni, is a completely different type of star to Vega. It's a supergiant star, which is another way of saying that it is enormous. It has a diameter over 200 times greater than our Sun, and the same luminosity4 as 85,000 suns. It looks similar to Vega in our sky only because it is much further away – about 1,500 light years. Deneb is one of the biggest stars in the entire Milky Way galaxy, but its luminosity and size indicate that it is a dying star. Some time within the next few million years, it will go supernova, i.e. it will explode suddenly and violently leaving a huge dust cloud and a small, dead neutron star in its wake.
Deneb forms the tail5 of the constellation Cygnus 'the Swan' – an easy constellation to recognise because the bright stars of this constellation form a pattern that looks like a swan in flight. The 'head' of the swan points south of Deneb.
Altair, or alpha Aquilae, is the most southerly of the three stars of the Summer Triangle. It is also the most similar to our Sun, being only 1.5 times larger, and 10 times more luminous. It's also closer to us than either Vega or Deneb – only 17 light years away. Altair6 is part of the Aquila 'the Eagle' constellation.
Behind the Summer Triangle
The Summer Triangle crosses over the great plane of stars in our home galaxy, known as the Milky Way. The Milky Way is best viewed during clear, moonless nights, and in places where light pollution is low. It may take a few minutes for your eyes to adjust to its light. It appears as a dim phosphorescent band, which in reality is millions of stars, many thousands of light-years away from us. A dark band, known as the Great Rift, cuts through the Milky Way in the region of the Summer Triangle. In reality, this is a distant dust cloud which is obscuring light from more distant stars.
Interesting deep sky objects in the region of the Summer Triangle include the binary star Albireo, the Ring Nebula (M57), the Dumb-bell Nebula (M27), the Crescent Nebula (NGC 6888), the Butterfly Nebula, and the North American Nebula.
Lyra - the Harp - was formally known as the Vulture, and the three constellations of Cygnus, Aquila and Lyra were associated with the Stymphalian birds, who fought with Hercules7 during one of his twelve labours. The small constellation of Sagitta, also within the Summer Triangle, represents one of the arrows of Hercules.
In Chinese mythology Vega and Altair are a fairy weaver and a cowherd, two lovers separated by a great river (the Milky Way). Once each year, a huge flock of magpies form a bridge over Deneb to allow the lovers to spend the night together. This night is Qi Qiao Jie, the Chinese equivalent of Valentine's Day.