Science: New-found hotbed of star formation: Heather Couper and Nigel Henbest report on theories which suggest that a black hole controls the centre of our galaxy

One of the most beautiful sights on a summer's night is the ghostly band of the Milky Way meandering among the stars. In mid-June you will see it high in the south-eastern sky, making its billowing passage through the constellations of Lyra, Cygnus, Aquila and Scorpius. Sweep it with binoculars and it will seem to dissolve into myriads of stars - all far more distant than those scattered over the rest of the sky.

Until recently, little was known about the galaxy. The problem was that we could not see the wood for the trees. The stars weren't the problem; it was the 'galactic undergrowth' - tiny dust-grains of soot wafted off the surfaces of cool stars - that was in the way. You can see it, heaped into vast obscuring clouds, blotting out the Milky Way in the constellation of Cygnus.

Although light cannot penetrate the dust, other radiations can. Astronomers have been able to take advantage of advances in electronics and computer technology to explore the galaxy and the result is a pan-galactic overview of the Milky Way that we have never had before.

But we have not cleared up all the mysteries. For instance, it is increasingly emerging that many galaxies go through at least one violent stage during their lives, when they buffer colossal explosions. The giveaway is that their centres are disturbed or distorted, even after the explosive activity has ended. So what is happening at the centre of our galaxy? Again, this is an area in which long-wavelength vision is shedding light. Astronomers have known for nearly 40 years that our galaxy's 'downtown' regions are disturbed - there are fast-moving jets, expanding rings of clouds and features unlike any found elsewhere in the Milky Way. Recently, it was discovered that the innermost 10 light years of the galaxy's centre is a hotbed of star formation, the heart of which is marked by a very small - but brilliant - source of radio waves called Sagittarius A*. Many astronomers believe this is an accretion disc (where the stuff of torn stars is heated to incandescence before leaving our universe for ever) and the glaring throat of a huge black hole. The gravity of this black hole would be capable of 'driving' all the activity we see around it. But other experts reckon that much of the disturbance we see could be explained by a recent outburst of star formation.

Most astronomers believe that a black hole must contribute to some of the activity at the galactic centre. The clincher would be to measure the precise size of Sagittarius A*; if it is less than a few light-hours across it is almost certainly an accretion disc.

Last year, Don McCarthy and his colleagues at the University of Arizona thought they detected a rapid, 10-minute variation in the infra-red output from Sagittarius A*. A source that varies on this timescale must be minuscule on the cosmic scale - as small as the orbit of Venus around the Sun. McCarthy hopes that this summer's observations will confirm these variations and the size of Sagittarius A*.

THE MOON AND PLANETS

Instead of the usual four phases, the Moon manages to fit in five this month (see Diary).

Mercury is making one of its evening appearances this month, and if the weather is settled, there will be a good chance to spot it. On 1 June it sets as late as 11pm, and will still be visible in the evening twilight up to a week later.

Venus, also visible in the twilight, is unmistakable. It is currently shining 40 times brighter than the brightest stars in the sky. On 12 June, it will be grouped with the crescent Moon. The other planet on evening view - Jupiter - is in the constellation of Virgo. At the beginning of June it sets at 3.30am, but as early as 1.30pm by the end of the month.

Saturn and Mars are both morning planets. Saturn, in Aquarius, rises first at 1am mid-month. Although not especially bright, it is prominent in a barren region of sky. Mars rises an hour and a half later, and makes a pretty grouping with the Pleiades during the last week of June.

THE STARS

A glance at the stars of the northern sky this month affords a glimpse into the future and the past. The star that marks north is, of course, Polaris - but it was not always so. Currently, Polaris lies almost directly 'over' the Earth's North Pole. Because we spin 'underneath' it, Polaris appears to stay still in the sky, marking north.

But the Earth is not quite round. The Sun and the Moon pull on the slight bulge at its equator, making the Earth's north-south axis swing round like a child's top on the verge of falling over. Over a 26,000-year period, the Earth's poles sweep out a cone in the sky. This means we see different 'pole stars' as time goes by - usually none at all.

For instance, the pole star that stood guard over the building of the pyramids - in 2800 BC - was Thuban in Draco. By the time of the Greek empire, more than 2,000 years ago, it had been replaced by orange Kochab in Ursa Minor. Polaris itself will have a significant successor in Alderamin (in Perseus) in about AD 8000. But in around AD 15000 there will be a truly spectacular north star - Vega, one of the brightest in the sky.

'The Guide to the Galaxy' by Heather Couper and Nigel Henbest is published by Cambridge University Press, price pounds 17.95, on 9 June.

Diary (all times BST)

June 1 5.03am moon at last quarter

8 9.27am new moon

16 8. 57pm moon at first quarter

23 12.33pm full moon

25 Mercury at inferior conjunction

30 8.31pm moon at last quarter

(Graphics omitted)

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