Science: Flashed by a little green man: Heather Couper and Nigel Henbest look at those rapidly spinning stars called pulsars

Close to the tiny constellation of Delphinus is the even smaller Vulpecula, the fox. To the unaided eye, it contains no features of interest, but in 1967 it was the scene of an extraordinary discovery.

Two Cambridge astronomers, Antony Hewish and Jocelyn Bell-Burnell (now head of physics at the Open University), were using a new radio telescope to pick up rapid flickering from astronomical objects. They were looking at very distant galaxies to see how they 'scintillated' when seen behind the stream of the solar wind. That is when the signals started: regular 'blips', repeating every 1.337 seconds, from the direction of Vulpecula.

The astronomers checked their equipment and investigated whether the signals could be electrical interference from Earth. After they realised the signals were real, and definitely coming from space, they had to face another possibility: could they be messages from extra-terrestrial intelligence? Only half tongue-in-cheek, they nicknamed their source LGM-1, for 'Little Green Man-1'.

Only when Bell-Burnell turned up three more 'LGMs' did astronomers realise they were observing a natural phenomenon - the odds on four civilisations beaming towards Earth are rather steep. Theoretical astronomers had already predicted what kind of object should act this way: a spinning neutron star, or 'pulsar'.

Nearly 30 years on from the discovery of the first pulsars, astronomers have found over 400 more, the remains of stars that have exploded as supernovae. The only part that survives is its core, a highly compressed ball of neutrons not much bigger than a large city. A pinhead of matter taken from one of these superdense denizens of the galaxy would weigh a million tonnes - and to escape the gravitational pull from its surface, you would need to travel at half the speed of light.

When pulsars are formed, they spin very quickly, sometimes several times a second, and all the rotational energy of the exploded star is taken up by the compressed relic. As the pulsar spins, it beams radiation like a lighthouse. If we happen to lie in the line of sight of the pulsar's beam, we see the flash. Because the alignment is so crucial, there must be many pulsars we do not see. Astronomers reckon there must be at least a million in our galaxy alone.

A pulsar spins with fantastic regularity. The fastest of all - the 'Millisecond Pulsar' that spins 42 times a second - keeps time to around one second in a million years, an accuracy comparable to an atomic clock. But as pulsars continue to radiate, they gradually slow down. A pulsar's fate is to end up as a non-radiating neutron star.

Meteors

The Perseid meteors are debris from a comet called Swift-Tuttle, burning up as they enter the Earth's atmosphere. Last year, the Earth was due to pass through a particularly dense stream of debris, but, in fact, just missed.

There is a chance that the Earth will run into the thick of the comet debris this year instead. Look out on the nights of 11-13 August.

The planets

Look to the west after sunset for a glimpse of brilliant Venus as it slides downwards into the twilight glow. Jupiter, recovering from its bashing by Comet Shoemaker- Levy, is also dropping into the twilight in the south-west.

As the two most brilliant planets slip from view, the evening sky becomes the preserve of their much dimmer sibling, Saturn. Lying towards the south-east, it glows a distinct yellowish colour in an area bereft of bright stars.

On 3 August, the crescent Moon lies only six moonwidths above Mars, which is rising at about 1am.

The stars

Sagittarius, the archer, gets to its highest point in our skies this month (which, sadly, for such a spectacular constellation, is actually low in the south in Britain). The stars of Sagittarius lie directly in line with the centre of our galaxy, its most densely populated region. As a result, the stars appear piled up on top of each other and the band of the Milky Way looks exceptionally dense and bright.

As well as stars, the raw materials of stars - gas and dust clouds, and clusters of young stars - also congregate in Sagittarius. There are too many of these 'deep-sky objects' to mention here, but a sweep of Sagittarius with binoculars is rewarding.

Diary (all times BST)

7 August 9.45am New Moon

12 10pm Maximum of Perseids meteor shower

13 Mercury at superior

conjunction

14 6.57am Moon at first quarter

21 7.47am Full Moon

24 Venus at greatest eastern elongation

29 7.41am Moon at last quarter

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