Science: Seven silver-blue sisters wreathed in cloud: Star clusters will vie for attention next month, but the loveliest is still the Pleiades, say Nigel Henbest and Heather Couper
Monday 25 January 1993
This tiny clutch of blue-white stars, clearly visible now for most of the night, really does look like a glittering, silver swarm; and while the Pleiades (the 'Seven Sisters') are undoubtedly the most lovely of all, they have some competition in February. Several rivals are on view, and are worth more than a second glance, particularly if you have binoculars.
Unlike the constellation patterns, made of unrelated stars that lie in roughly the same direction in space, clusters are genuine star groups. They consist of stars born relatively recently in cosmic terms: tens of millions of years ago.
Since stars are born in tight clumps (as you can see in the centre of huge gas clouds such as the Orion Nebula) their closeness in a cluster is a reflection of their youth. As stars grow older, they shuffle off the gravitational bonds of their neighbours and plot a solo course around the galaxy.
The youngest star clusters visible in February ride high in the sky between the constellations of Perseus and Cassiopeia. The 'double cluster' makes a glorious sight through binoculars; each of its parts is about the size of the full Moon, and they look identical - except that, at 3 million years old, 'chi Persei', the left-hand cluster, is 2 million years younger than its neighbour, h Persei.
Each cluster contains thousands of fiercely burning young stars, of which a couple of hundred can be seen through a moderate telescope. They are so luminous that they can easily be picked out at 7,000 light years away.
On the other side of the sky lie two older clusters. Age mellows them: their stars modify their energy output, and they begin to pursue their own paths. Middle- aged clusters, such as the Hyades in Taurus and Praesepe in Cancer, lose the 'jewels on black velvet' look of their younger cousins.
Both clusters were born 630 million years ago, and are heading away from the Sun. In fact, the motion of the V-shaped Hyades - at 149 light years away, the closest to the Sun - offers astronomers a means of measuring distances in space. Praesepe, 520 light years away, appears fainter and more compact; it is a fine sight through binoculars.
But, at last, to the Pleiades: 410 light years away and 70 million years old, the cluster has just the right combination of age and distance to create a bright, compact appearance. First logged by Chinese astronomers in 2357 BC, it has played a big part in the beliefs and rituals of many cultures. The Aztecs used to time their most gory human sacrifices to coincide with the highest rising of the Pleiades at midnight.
To the naked eye, the Seven Sisters look more like six, although the keen-eyed have reported seeing up to 13. With binoculars or a small telescope you can see dozens of sparkling bluish stars (a sign of their high temperatures) among the 200 or so. Long-exposure photographs reveal all the stars, and the soft nebulosity that surrounds the cluster.
Until recently, the nebulosity was a puzzle. Unlike the gas wreathing Orion Nebula, it could not be left-over 'natal' gas; the Pleiades are far too old for that. Research at infrared wavelengths, which examines the distribution of lukewarm gas and dust in space, has revealed that the Pleiades are ploughing into a completely unrelated gas cloud. But where did it come from?
The Pleiades themselves can shed some light. As they travel through space, the stars create a 'wake' where they have tunnelled through the gas, and this helps astronomers trace the origin of the cloud itself. At present they believe a gigantic gas cloud, located more than 1,000 light years away, is responsible. Fifteen million years ago it underwent a massive burst of star formation. Its most energetic stars quickly exploded as supernovae, scattering the relics of the original gas cloud. One clump hit the Pleiades, filling the cluster with the nebulosity that enhances its stunning appearance.
If you have never seen Mercury, now's your chance. It is an 'evening star' this February, and draws steadily away from the Sun during the second and third weeks to set an hour and a half after sunset (at 7pm) on the 24th. You will be able to spot it as a bright, untwinkling star, below and to the right of Venus, which continues to dominate the evening sky. Setting four hours after the Sun for the whole month, Venus becomes even brighter - increasing in magnitude from -4.1 to -4.5 - as it draws closer to Earth. On the 24th, the planet will be at its brightest, and situated just north of a crescent Moon.
Mars, which has been making up a striking trio with Gemini's bright twins, Castor and Pollux, is moving away from Earth and fading. Because Earth has just overtaken Mars in its orbit, the latter appears to move backwards. On the 15th, however, Mars will resume its 'direct' motion and drift back towards Castor and Pollux.
Jupiter, which rises at 9pm mid-month, is poised to become the planet for the spring. At magnitude -2.3, it is considerably brighter than Mars, although it is still low in the sky in the evenings.
Saturn passes behind the Sun ('undergoes superior conjunction') and will not be visible.
Sirius, the brightest star, stands at its highest point. It never looks as spectacular from our high northerly latitudes as it does from farther south, where it rises higher. But its low altitude, combined with its brightness, leads to some scintillating atmospheric effects.
Because of the continual shifting of Earth's atmosphere, the stars appear to twinkle: we see them through different, moving thicknesses of air. The shifting air cells, each about 50cm in diameter, act as lenses to band the starlight, too.
This refraction splits the starlight into its constituent colours, from red through blue. As a result, the brightest stars appear to flash all the colours of the rainbow when we see them low in the sky and through the greatest thickness of the atmosphere.
6 11.55pm Full Moon
9 Saturn at superior conjunction
13 2.57pm Moon at last quarter
21 1.05pm New Moon; Mercury at greatest elongation
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