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Our middle-aged neighbouring star


© Jisas/Lockheed/S.P.L/Cosmos, Paris

The sun is actually a middle-aged star. But even though halfits life is over, there's no cause for alarm. The heavenly body that gives us light still has a brightfuture ahead of it.

by Pierre Lantos

Once upon a time there was a vast empty space within our galaxy, nearer to the edge than the centre. We should probably never have known anything about it had it not become the birthplace of a star our sun. For the space was not entirely empty; it c'ontained an enormous cloud of hydrogen molecules and particles of dust. This cloud became unstable, then collapsed and broke up into smaller and smaller clouds. Our sun and its planets were born of one of these tiny clouds.

When such a cloud collapses under the effect of its own weight, a lot of energy is released, and the surrounding temperature — at first only a few dozen absolute degrees — rises and eventually reaches several thousand degrees. Contraction comes to an end when the star's core temperature reaches a million degrees and nuclear reactions begin. Tens of millions of years elapse between the time when the hydrogen cloud begins to destabilize and the birth of the adult star. This is a very brief youth for a star like the sun whose life will last for ten thousand million years or more.

A nuclear furnace

When they tried to explain the origin of solar energy, nineteenth-century astronomers advanced three hypotheses: that it was due either to chemical reactions, to the contraction of the star or to meteorites falling into the sun. By the end of the century the most lucid of them had realized that none of these processes could supply the energy the star needs over a very long period. The great experimental and theoretical discoveries made in the first half of the twentieth century revealed that solar energy is actually produced by nuclear fusion reactions that convert hydrogen into helium. These reactions take place in the core of the sun where 700 million tons of hydrogen are changed into helium each second and four million tons of matter are transformed, as Einstein showed, into energy.

Today several experimental methods can be used to investigate the opaque interior of the sun. One of them consists of studying the elementary particles known as neutrinos that are released by the fusion of the gas nuclei and are capable of passing effortlessly through the whole volume of the sun. But although the neutrinos are very speedy messengers, the energy released by nuclear fusion in the central part of the star takes some ten million years to reach the surface, moving centimetre by centimetre in a random fashion. It finally appears in the form of light, an immense source of life for our planet.

Spots and flares

The main observable phenomena on the sun's surface are sunspots, which are darker than the rest of the disc. In Antiquity some observers saw them with the naked eye. They are signs of the sun's magnetic field, which, created deep inside the star, structures its entire atmosphere. When, during a total eclipse of the sun, the extended part of this atmosphere — the corona — can be observed, it is clear that the solar atmosphere is not uniform.

Above the spots, in the solar corona, the magnetic field can stock energy and suddenly release it. Solar flares which occur frequently in weak form can nonetheless have a strong disturbing influence on the interplanetary environment. Like the spots that produce them, they follow an eleven-year cycle, which is due to the oscillation of the mechanism creating the magnetic fields inside the sun. When solar activity is at its height, the earth's space environment is bombarded with a shower of highly destructive protons emitted during flares, intraplanetary shock waves, and an increase in ultraviolet rays and X-rays that are thousands of times more intense than when the sun is calm.

At such times satellites have a higher rate of breakdowns and their orbits are greatly disturbed. Astronauts travelling to the moon or to other planets would be in serious danger of radiation during a big flare. On the earth's surface, we are well protected from the sun's caprices by our planet's atmosphere and magnetic field. It should be noted, however, that on rare occasions particle emissions from the sun may disturb the earth's magnetic field, as can be seen by an increase in the northern lights, and have severe effects in telephone and computer cables and electricity distribution grids. During flares, radio communication may also be difficult, and even impossible, in certain regions.

As already noted, the sun will live to an age of some ten thousand million years. At 4.5 thousand million years old, it is now in middle age. It will continue to burn its hydrogen for about the same amount of time and remain stable. But then, in its last thousand million years, the sun will dilate, and its surface temperature, around 5,800°K at present, will be reduced by half. It will then become what astronomers call a "red giant", i.e. it will be at least fifty times bigger and 300 times brighter than it is today. Life on earth will become infernal. Then, lacking fuel, the sun will shrink and lose its brightness, ending up as one of the ghost stars that wander through the universe.

So, in five billion years, "Stop the world, we have to get off".

Learn more about light in the Courier


Pierre Lantos

Research director at the French National Centre for Scientific Research (CNRS) and an astronomer at the Paris-Meudon observatory, Pierre Lantos is the author of Le Soleil (Tne Sun), published in 1994.