The lifecycles of stars

Stars undergo a definite life cycle, though not all stars go through the same sequence. Some stars exist for a comparatively brief period; others live for thousands of millions of years.

As a proto-star shrinks so its centre becomes hotter until its temperature becomes greater than seven million degrees K. Then thermonuclear reactions begin inside the centre of the star. If and when the temperature in the star's central regions reaches the astonishing figure of 18 million degrees, then a new reaction takes over and becomes the dominant factor.

This is a very complex process, which is sometimes referred to as the CNO reaction because not only is carbon present but nitrogen and oxygen are formed during it, only to be used up again before the reaction is finished. The nitrogen is formed from a combination of carbon and hydrogen, while the oxygen is a product of the nitrogen and hydrogen. But the reaction begins with carbon and hydrogen and ends with carbon and helium, so the carbon finishes as it started; what is used up is the hydrogen.

Reactions like the CNO and the proton-proton one which occurs at lower temperatures emit vast amounts of energy and one may wonder whv thev do not blow the star to bits. After all, proton-proton reactions are what occur in a hydrogen bomb and this results in a colossal explosion. The reason why a star remains stable throughout its life or at least for the greater part of it - is due to the way the electrified or ionized gases in'the star react. They are of course, pulled down to the star by gravity, but on the other hand the pressure of the radiation - i.e. of the photons from such reactions is very strong. This will press the gas outwards. But the most significant outward thrust is given by the great heat generated. This causes the pressure to rise so that the gases surrounding the central regions expand.

Fortunately the expansion of these gases does not go on for ever; if it did, every star would blow up soon after birth and we and our Sun would have ceased to exist ages ago. The expansion stops because the ionized gases cool as they expand, and thus they reach a point where the pressure between the gas atoms drops. Then gravitation takes over and the gases contract, their temperature and pressure rise and the cycle is repeated. Of course, during all this, the star is shining, radiating its energy out into space, and thus some of the heat is dissipated. The star reaches a balance, generation of energy inside is countered by radiation, by the emission of neutrinos, and by the cycle of expansion and contraction. Together these all act as natural regulators. The star stays stable and, if it is well-behaved like our Sun, the expansion and contraction remain small, at least for a great part of the star's life

In the CNO and in the protonproton reactions, hydrogen is converted into helium, and the helium is not 'burnable'; we can think of it as a kind of nuclear 'ash' clogging up the central regions of a star where it has fallen because it is heavier than the surrounding hydrogen. In small or moderately sized stars, the hydrogen round this ashy core begins to burn in nuclear reactions, and as more helium ash forms, so the burning takes place nearer the star's surface. Meanwhile the core contracts, heats and as more ash forms, finally collapses. At this, the outer layers of gas expand and the star becomes a red giant.

If, on the other hand, the star is very large, the nuclear ash is replenished for a time with fresh hydrogen falling in towards the centre due to the bigger gravitational pull in a more massive star. But in the end the star leaves the main sequence; however, the helium is under such pressure that it fuses to carbon and a new stable period ensues. Yet this is short-lived, and again shrinkage of the core and burning lead to a red giant or supergiant phase. After this both massive and non-massive stars cease to be huge red stars and enter on the last phases of their lives. What then happens is very different and depends entirely on their mass.