Stargazing in May

Spring has sprung – and it shows in the sky. Gone are the in-your-face constellations of winter, to be replaced by the more subtle groupings for the softer days. One is the constellation of Boötes: a kite-shaped pattern of stars just below the "tail" of the Great Bear (Ursa Major). In legend, Boötes was "the herdsman", and the name of its brightest star – Arcturus, visible high in the south – means "bear driver", because this brilliant star appears to herd the bear around the sky as the Earth spins.

If you have a clear, dark sky, look at the colour of Arcturus. It's orange. With our light-polluted skies, we can hardly see that the stars are coloured: but their hues range from baleful red to brilliant blue-white. Red stars are the coolest, blue stars are the hottest.

Arcturus, a nearby star 37 light years away, is becoming elderly. As a result, is has developed middle-age spread, and its layers of gas have cooled to become orange-red, glowing at a temperature of just 4000C. The distended star is 110 times brighter than the Sun, and Arcturus would engulf all the planets out to the orbit of Mars if it were placed inside our Solar System. Arcturus has little control over its inflated layers. As a result, it swells and shrinks. Eventually, it will puff away its atmosphere, curl up and die – as will our Sun in 5 to 7 billion years. No need to panic yet.

Follow the curve of the Great Bear's tail through Arcturus downwards, and you hit a very different beast of star. Spica is the brightest star in Y-shaped Virgo, the second-largest constellation in the sky. It takes a leap of the imagination to see this group of stars as a maiden holding an ear of corn, but in early autumn, the Sun passes against the background of the stars of Virgo, reminding us of its association with the harvest.

Spica is a hot, blue-white star thousands of times brighter than the Sun, blazing at a temperature of 22,500C. It couldn't be more different from Arcturus. It's orbited by a stellar companion that raises huge tides on the unfortunate star. As a result, Spica is distorted into a shape like a celestial rugby-ball.



What's Up

Saturn rules supreme over the evening skies, tucked under the belly of Leo. The celestial lion is one of the few constellations whose outline actually resembles its namesake, but Saturn's extra "dot" turns her into a pregnant lioness!

To the right of Leo, the last of the brilliant winter constellations, Gemini and Auriga, are now disappearing into the western twilight. To the east, we're starting to see the large dim constellations of summer: Ophiuchus and Hercules. And the southern sky is enlivened by the bright stars Spica and Arcturus.

There's more planetary action as The Morning Star, Venus, reaches its maximum brilliance in early May, shining over ten times brighter than any star. Well to its right, and rather higher, is the second brightest planet, Jupiter. There's some excitement for telescope-owners on 28 May, when Jupiter sails past the most distant planet, Neptune.

You don't need a telescope, though, to see shooting stars from around now. These meteors, streaming from the constellation Aquarius, are fragments of Halley's Comet, burning up in Earth's atmosphere.

Heather Couper and Nigel Henbest



Diary

9 5.01am Full Moon

17 8.26 am Moon at Last Quarter

24 1.11pm New Moon

31 4.22am Moon at First Quarter

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