The sky — astronomy

The Summer Triangle: Vega, Deneb and Altair

The Summer Triangle is a pattern of three bright stars – Vega (in Lyra), Deneb (in Cygnus) and Altair (in Aquila) – that rides high in the evening sky of the northern summer. It is an asterism (an informal star pattern), not a constellation. The easiest way to find it is to start with brilliant Vega.

The three stars and their constellations

The Summer Triangle is an asterism, an easily recognised pattern of stars that is not an official constellation. Of the 88 constellations recognised by the International Astronomical Union (IAU), it borrows stars from three: Lyra, Cygnus and Aquila. See the 88 constellations for more.

Vega (Alpha Lyrae) is the brightest of the three and usually ranks as the fifth-brightest star in the night sky. It lies about 25 light years away. A light year is the distance light travels in one year. Because of precession (the slow wobble of Earth’s axis), Vega was near the north celestial pole thousands of years ago and will be again in roughly 12,000 years. See Polaris, the pole star and precession.

Deneb (Alpha Cygni) marks the top of the Northern Cross. It is a distant, very luminous supergiant, hard to measure, with estimates of its distance in the thousands of light years that differ considerably. That is why it looks a little fainter than Vega, even though it actually gives off vastly more light.

Altair (Alpha Aquilae) is the nearest of the three, about 17 light years away. It is known for spinning extremely fast, completing a rotation in well under a day, which makes it noticeably flattened. A dimmer star sits on each side of it, which helps in identifying it.

How to find the triangle

Look in the late evening from late spring through autumn, from a dark place away from city lights. For general tips, see how to watch the sky.

Start with Vega. In summer at mid-northern latitudes it is very high, often nearly overhead, and shines with a blue-white light.
Look for Deneb to the east-northeast of Vega. It is the brightest star in Cygnus and forms the head of the Northern Cross.
Look for Altair to the south-southeast of Vega. It shines with a white light and sits in the middle of a short row of three stars.

Join the three points and you get a large, roughly right-angled triangle. It spans many fists held at arm’s length, so you need a wide view rather than binoculars.

In the southern hemisphere the triangle is seen on winter evenings, lower over the northern horizon and oriented differently from the view in the north.

The Milky Way through the triangle

The Summer Triangle is a great signpost for the Milky Way. Under a properly dark sky the pale band runs through the triangle, along the line from Deneb to Altair. In Cygnus the band appears forked by dark dust clouds – known as the Great Rift. These are clouds of interstellar dust that block the light of the stars behind them. To learn about our galaxy, read the Milky Way.

If you have small binoculars, look also at Albireo, a double star at the beak of the Swan, whose two components show a lovely contrast of golden and bluish colours.

The weaver and the herdsman

In East Asia the three stars belong to an old tale. In the Chinese tradition Vega is the Weaver Girl (Zhinü) and Altair is the Cowherd (Niulang). They are separated by a heavenly river – the Milky Way – and may meet only once a year, on the seventh day of the seventh lunar month. In the story, flocks of magpies form a bridge across the river, and some tellings link Deneb to that bridge. The two small stars beside Altair are often described as the couple’s children.

The festival is called Qixi in China and Tanabata in Japan. This is a cultural tradition, not an astronomical fact, but it shows how people around the world have looked at the same stars and given them their own stories.

In short

The Summer Triangle is a simple way to begin finding your way around the sky: three bright stars, three different constellations and the Milky Way running between them. On a clear summer evening, step outside, find Vega and let your eyes follow the rest.

Questions and answers

Which stars form the Summer Triangle?

Vega in Lyra, Deneb in Cygnus and Altair in Aquila. Each is among the brightest stars of its constellation.

Is the Summer Triangle a constellation?

No. It is an asterism, an informal and easily recognised star pattern. The IAU recognises 88 official constellations, and the triangle uses stars from three of them.

When can I see the Summer Triangle?

In the northern hemisphere it is visible on evenings from late spring to autumn and is highest in summer. In the southern hemisphere it appears lower in the sky on winter evenings.

Which is the brightest star in the Summer Triangle?

Vega. It is one of the brightest stars in the night sky and the easiest to recognise, which makes it the best starting point.

The Milky Way: Our Galaxy in the Sky How to Stargaze: A Beginner’s Guide Polaris: Why the North Star Stays Put The 88 Constellations: A Map of the Whole Sky Precession: The Slow Wobble of Earth’s Axis

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