Constellations · 5 of 5

The Summer Triangle

Which two lovers does the Milky Way keep apart?

Three brilliant stars, three constellations, one love story written across the Milky Way.

You’ll be able to find Vega, Deneb and Altair, and the Milky Way that runs between them.

10 steps · about 8 min

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The first three stars out on a northern summer evening, nearly overhead: Vega, Deneb and Altair. Three bright stars from three constellations, joined into one shape — the Summer Triangle.

Start at Vega, the brightest corner and the fifth-brightest star in the sky, at the head of little Lyra, the lyre. Its tag is the surprise: it spins so fast it is squashed, and we see it pole-on, so it took interferometry to notice.

North-east of Vega: Deneb, the Swan's tail. Compare their tags. Deneb is so far that nobody can pin its distance down, and to look this bright from there it must shine like 50,000 to 200,000 Suns — the most luminous star most people ever see.

South of Vega, Altair closes the triangle. It was the first star burning hydrogen as the Sun does, not a swollen giant, to have its surface imaged, and its tag is what the picture showed: it spins so fast that it bulges at the waist.

Turn the Triangle sideways: each corner keeps its place across the sky and slides out to its distance. Vega and Altair stop close by; Deneb, even at the near end of its range, runs on dozens of times farther.

At the other end of the Swan, its beak: Albireo. Binoculars split it into two stars, one gold and one blue, the prettiest pair in the summer sky. Gaia's measurements say they may not orbit each other; that is still argued over, but they are no chance alignment, and look born together.

Run four hours and watch the Triangle.

Now a dark sky. The Milky Way runs down the Triangle's long side, Deneb in the stream and Altair on its bank, and between them it splits along a dark lane: the Great Rift, dust clouds in our own arm blocking the stars behind.

In China the band is the Silver River, with the Weaver Girl and the Cowherd on its banks. On the seventh night of the seventh month magpies bridge it: Qixi in China, Tanabata in Japan, still celebrated. More in The Chinese Sky.

The Triangle shows from town; the Swan needs a darker sky.

Vega, Deneb and Altair, the evening's first
A shape on the sky, not in space
Albireo splits gold and blue

The Summer Triangle

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