Our Sun: nothing special, but quite amazing

Incredible. The sun's out at last. But thanks to the rescue of the space probe Soho last week, we know that's not half as incredible as the story of the great yellow orb itself

A LONG TIME ago, on the edge of an insignificant galaxy in a nondescript corner of the universe, a star was born. As stars go, it was nothing to write home about. It was not particularly big, nor was it that small. It was not too bright, neither was it unusually dim. In fact it was just about as average as you can get - for a star.

Anybody living on a planet orbiting Proxima Centauri, the star's nearest neighbour, would perceive this distant point of light to be no different to the many millions of other astronomical bodies in the Milky Way. But if they could get a little nearer to this star, they would soon see it in a different light.

Coming closer, the star would be the most striking object forthousands of millions of miles. Within 100 million miles the star would be transformed into a wondrous, yellow orb. At this distance, the star would emanate a God-like glow, bathing everything in its view with warmth and light.

FOR MILLIONS of Britons, last week provided a long overdue opportunity for basking in the brilliant glory of our own star - the Sun. It was also the week when scientists regained contact with a lost space probe which is discovering that there may be more to this very average stellar body than the astronomy textbooks would have us believe. The Sun is, indeed, a typical middle-aged, middle-of-the-road star, like billions of others. However, until its demise, the Soho satellite was rapidly expanding our knowledge about the very extraordinary ways in which an "ordinary" star can behave.

Soho - the Solar and Heliospheric Observatory - is a pounds 660m set of scientific instruments designed to investigate the highly unusual nature of our nearest star from space. Since its launch two years ago it has provided stunningly detailed observations about the bizarre events that take place within the hot, gaseous environment of the Sun. It was, therefore, a tragedy for space scientists when earlier this year they lost contact with Soho after it had begun to spin out of its control.

For weeks the experts at the American National Aeronautics and Space Administration (Nasa) and the European Space Agency (Esa) tried to make contact with Soho. The first signs of a breakthrough occurred at the end of July when a radar echo from Earth located Soho's position a million miles from Earth. Then, last week, Soho sent back a burst of signals, each lasting between two and 10 seconds.

The contact, no matter how transitory, was a welcome piece of good news to the Nasa scientists. With more gentle coaxing over the next few weeks, they hope to be able to thaw out the space probe's frozen instruments. If this is possible, the scientific quest to learn more about our mysterious Sun from a unique vantage point in space can begin once more.

The importance of the Sun cannot be overestimated. Without it, life on Earth would never have existed. Apart from the obvious physical benefits, it has proved to be an inspiration for the human imagination throughout history. Just about every ancient society had a Sun God. The ancient Egyptians called him Ra, the Romans called him Sol, who was identical to the Greek god Helios, son of Hyperion and Theia. He charged across the sky in his golden chariot, appearing in the east and disappearing over the horizon in the west at the end of each day.

A more rational approach to the Sun began with Galileo, who was the first person to look at it through a telescope (and is said to have suffered blindness as a result). Copernicus, the Polish astronomer of the 16th century, successfully challenged the official church position with his thesis showing that the Earth moved around the Sun, rather than the other way round. With this, Copernicus gave birth to the heretical idea that the Earth was a mere satellite of the Sun, the centre of its own solar system.

For many years scientists continued to puzzle over what could be the fuel that made the Sun so hot. The Victorians thought it might have been coal - understandably given that it was this fuel that had made the Industrial Revolution possible. But this would have meant that the Sun could survive only for a few tens of thousands of years.

William Kelvin, the great 19th century physicist, believed the Sun's energy came from gravity, which would have given it enough fuel to shine for a few hundred thousand years. This idea was soon superseded in the age of Einstein and 20th century physics, when nuclear energy was discovered as the real source of the Sun's immense power.

Why the Sun shines is now well established. Deep in its gaseous core the temperature soars to 15 million degrees Centigrade, which is hot enough for nuclear fusion, when the Sun's hydrogen fuel is converted into helium with an explosive release of energy. Scientists estimate that the Sun has been burning fuel like this for about five billion years, and will continue to do so steadily for another five billion years before it eventually expands into a giant red star.

The Sun shines because it emits packets of light - called photons - but one of the intriguing aspects of this process is that although it takes several thousand years for a photon to travel from the Sun's inner core to its surface, a distance of about 435,000 miles, the photon, once released, takes a mere eight minutes to travel the remaining 93 million miles to Earth. But this is not the only mystery of the Sun.

SOLAR SCIENTISTS speak in hushed tones about the ``neutrino problem''. Neutrinos are sub-atomic particles that are emitted from the Sun when it converts hydrogen to helium. The laws of physics, however, say there should be far more neutrinos coming from the Sun than the estimates scientists have come up with. It is perhaps the biggest outstanding problem of solar physics and one that has repercussions for finding the ``missing mass'' of the universe - the matter that is there but cannot be seen.

The Soho space probe, if it can be revived, may be able to elucidate the issue. It has already shown how solar flares, which burst out from the Sun's surface for many millions of miles, produce huge seismic waves which ripple around the Sun. These quakes are thousands of times bigger than the biggest earthquakes. As one Nasa scientist commented: "The energy released is equal to completely covering the Earth's continents with a yard of dynamite and detonating it all at once."

SOHO MIGHT also be able to shed further light on the mysterious coronal mass ejections, which Joe Gurman, the US project scientist on the mission, describes as "a million tons of matter moving at a million miles an hour". One such ejection is said to have knocked out a telecommunications satellite last year, and an even bigger one, which sent a stream of electrically charged particles through the Earth's atmosphere, caused a power blackout throughout the Canadian state of Quebec in 1989.

These violent ejections rely on solar activity, which completes its cycle once every 11 years. We are heading for the next "solar maximum" in about four years' time, when coronal mass ejections are expected to become more common and more violent. Gurman believes it will pose a stern test for current telecommunications satellites many of which, he believes, are not sufficiently protected against the problem.

A space probe such as Soho may help scientists to make reasonable predictions about when such solar activity is going to happen. That may help those of us on Earth who will over the coming years become increasing reliant on satellite communications. It underlines the fact that the Sun is a violent place, prone to the unexpected. Like all middle-aged bodies, it is vulnerable to the odd mid-life crisis.

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Hot facts about a super star

Official stellar classification: Yellow dwarf, type G2.

Age: About 4.7 billion years.

Diameter: 1,392,000 km.

Surface temperature: 5,500C.

Temperature at centre: 15,400,000C.

Composition: 70 per cent hydrogen, 30 per cent helium, less than 1 per cent anything else.

Average distance from centre of Earth to centre of sun: 149,598,023 km (92,955,902 miles).

Mass: About 2000000000000000000000000000 (2x1027) tonnes.

Weight loss: About 4 million tonnes per second.

First recorded total eclipse: 17 July, 709 BC.

Next total eclipse in London: 5 May, 2600 (if you can't wait, there's a 99 per cent eclipse on 14 June, 2151).

Maximum possible duration of an eclipse: 7 minutes 31 seconds.

Maximum duration of eclipse perceived by passengers flying on Concorde: 74 minutes.

Maximum error of a correctly aligned sundial: 16 minutes.

Number of times a year a sundial reads the exact time: Four.

Average hours of sunshine per day at Kew: 4.18.