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PlE Ilac clu LIFE ON OTHER WORLDS collision is extremely remote and would make the number of planetary systems extremely small.

Later scientists who took up this 'hot' hypothesis and carried out detailed calculations of the outcome of such a 'catastrophe, found • great difficulty in keeping the displaced material in solar orbit at all.

In addition, the fact that the temperature involved would be over a million degrees centigrade held out the possibility that the matter would explode rather than condense into planets.

CONDENSED GAS Ten years after Leclerc, Kant proposed that the whole universe was originally gas that condensed into regions of higher density. One of these became the solar system, which — because of its spin — flattened into a disc. Then, due to gravity, the matter coalesced into the Sun and the different planets.

From the standpoint of explaining all the facts, the 'cold' theory of Kant seemed unable to explain why the planets should have the lion's share of the solar system's angular momentum and this seemed a severe criticism of the 'nebular hypothesis'.

However, Engels in his 'Dialectics of Nature' hails the theory as a big step forward from the standpoint of method.

No longer was the solar system God-given, but matter in motion, coming into being over a period of millions of years in a process of change.

At the recent 14th General Assembly of the International Astronomical Union in Brighton, Professor Fred Hoyle took the nebular hypothesis as his starting point and attempted to resolve the angular momentum problem as a material 'sorting out' subordinate to the main process.

MAGNETIC THEORY Hoyle invokes magnetic theory which, of course, was not developed in Kant's day, and calculates that the necessary distribution could have occurred if a moderate magnetic field has operated over a period of about 10,000 years —a short period in astronomical terms.

However, even where planets exist, the development of life depends on many other factors, not least of which is the nature of the parent star itself.

Putting the stars into some sort of order required a long period of observation and classification. The recognition that the light from a star contained vital information about its constitution was the necessary key that finally opened the door.

Towards the end of the last century, it appeared that stars fell into certain categories according to their spectra (see part 3) and the different types reflected differences in temperature.

The seven main types were classified in order of decreasing temperature by the letters O, B, A, F, G, K, M. Our Sun falls in the G category.

PATTERN

Meanwhile Russell at Princeton and Hertsprung in Europe came upon another striking pattern.

They found that the overwhelming majority of stars of a particular type had the same inherent brightness (luminosity) and are said to lie on the 'main sequence'.

Summing up their results: if a star lies on the 'main sequence, then the hotter it is, the brighter and larger it is and the shorter its lifetime.

For example, a B-type star will be about 17 times as heavy as the Sun and five times as hot, whereas an Mtype will be about one-third the size and one-third cooler.

The main sequence is the home of a star as long as its fuel is hydrogen which is converted to helium by a continuous thermonuclear ('hydrogen bomb') reaction.

Once a star's internal heat and pressure build up to the point at which the reaction can take place, it moves on to the main sequence and stays there until the hydrogen in the core is exhausted.

BURN UP

The period on the main sequence must be long if life is to develop. The age of the earth is about 4,600 million years, about a third of which passed before the necessary chemicals for life had evolved.

The big stars burn up so fast that some of them stay on the main sequence for only about one million years, whereas the smallest M-types remain there for more than 100,000 million years. Yet these small stars may be too cool to support life on their planets, if any.

During the 1950s, Huang in the United States carried out an investigation into the possibilities of life-bearing planets orbiting the different types of stars and came to the conclusion that the most probable candidates were the smaller F-types, the Gs and the larger Fortunately, we fall right in the centre of this range!

In other words, if a star is very hot, it is too shortlived; if too cold, suitable planets could only exist in a very narrow band close to their sun.

Huang's candidates strike a happy medium. In our solar system, Mars lies just within the zone of toleration, as does Venus.

Yet, given a suitable star and the existence of at least one planet within it, there are still further conditions-on the planet itself-which must be satisfied for life to evolve which is in any way similar to that on Earth.

continued. imperialist repression is the stark reality.

The official unemployment figure of 59,587 — about 3 per cent of the population—does not give a true picture of the conditions in S Ireland.

PITIFULLY

Many youth who either cannot find a job or refuse to work for the pitifully low wages offered emigrate to England.

Others, on leaving school, may never have a job-they are not included in the Ministry's figures.

Today the youth in S Ireland find themselves in the centre of a growing militancy on the part the working class against Lynch's capitalist republican government.

Such an upsurge is personified in the 21-week strike of cement workers for a £7 increase in wages.

Their determination was only finally broken by the actions of the Polish government in exporting cement to Ireland to help break the stoppage.

In the face of such a challenge from the working class thẻ Lynch government faced a serious crisis within its own ranks earlier this year —two ministers were sacked from the government for alleged gun-running.

In the build-up of these events the working-class youth have surged into the forefront and their movement towards political action has been crystallized in the recent launching of the Irish Young Socialists, sister organization to the Young Socialists in this country, which embraces both Nand S Ireland.

RESPONSE

Led by YS National Secretary nan Simmance, a special recruitteam visited Dublin from Britain to join the Irish Young Socialists in the building of their revolutionary youth movement.

Outside factories and on the housing estates the response they received was enthusiastic.

The young people they spoke to were keen to help build new branches and work towards the IYS projected rally in Dublin on September 20, Young Socialists which will attended from all over Ireland.

At a plastics factory a group recru

PAGE FOUR September