Stars are the largest self-contained nuclear power generators

Counting the stars visible to the naked eye takes patience, persistence (and a certain kind of madness!) and many ancient astronomers have done it. About 6,000 stars are visible to the naked eye. The Greek astronomer Hipparchus (146-127 B.C.) not only counted these stars but also classified them, based on their brightness. The brightest ones (about 20 or so) were called stars of “first magnitude”, the second brightest of “second magnitude” and so on. The stars barely visible to the naked eye were sixth magnitude stars. Typically, stars of second magnitude are about 2.5 times fainter than those of first magnitude, stars of third magnitude 2.5 times fainter than those of second magnitude and so on. Thus, the sixth magnitude stars are about 100 times fainter than the brightest stars. With powerful telescopes, we can now see stars which are about 2,000 million times fainter than first magnitude stars.

As seen from earth, a star might appear faint either because it is intrinsically less bright or because it’s at a large distance. But how can we find the intrinsic physical properties of a star that is so far away? The answer lies in the analysis of the spectrum of the starlight.

The light we receive from a star is a mixture of electromagnetic radiation of different wavelengths. For example, the visible light we receive from the sun can be split into different colours (corresponding to different wavelengths) by passing it through a prism. By similar — but more sophisticated — methods, we can split the light from the star into various wavelength bands. The spectrum is almost like the star’s signature — one can understand its intrinsic properties by studying its spectrum. For example, we know from the study of the spectrum that the surface temperatures of stars are fairly high and vary considerably from star to star. The bluish stars can be as hot as 30,000 °C while the surface temperature of a star like the sun is about 6,000 °C. The spectrum also allows us to determine the chemical composition of the stars and it turns out that stars are gaseous spheres made of hydrogen and helium with small traces of all the other elements.

The most important feature about these stars, of course, is that they radiate enormous quantities of energy. Every square metre region of the sun’s surface, for example, emits a power of 62,000,000 Watts. The source of this power has been a mystery for scientists till the early part of this century. The conventional sources of energy are simply inadequate. If the sun was made up of the best quality coal which was burning — with oxygen being supplied! — the energy output could have lasted for only about 1,500 years. More realistically, one can imagine the gravitational energy being converted into heat. If the sun contracts under its own weight, the gas molecules will be compressed to a higher pressure and temperature and will provide a source of heat. Assuming the sun was a very large sphere to begin with, its contraction to the present size would have maintained the energy supply for about 20 million years. That might seem a long time, but it is not long enough. Geologists have found evidence that the earth’s crust had existed, and was illuminated by the sun, for a much longer period.

The reason that the sun — and the other stars — have been shining for such a long time in splendour is because they use one of the most efficient sources of energy, that is, nuclear power. In nuclear reactions, the mass of a substance is directly converted into energy. Such a process is nearly 20 billion times more efficient than the best chemical fuel. This is the secret of star power.

A simple way of understanding the nuclear processes in the stars is as follows. Consider, for example, the nucleus of a helium atom which consists of two protons and two neutrons. The mass of a helium nucleus, however, turns out to be slightly less than the sum of the masses of its constituents. This means that if we take the constituent particles and “fuse” them together to form a helium nucleus, we will be able to release energy in the process by the famous relation E = mc2. Such a nuclear fusion can take place only at high temperatures which exist in the central regions of stars. The sun is exploding millions of hydrogen bombs each second to give us light!

Stars are thus the largest self-contained nuclear power generators. In fact, it is the triggering of this nuclear power which signals the birth of a star. Astronomers have identified several regions in our galaxy where star formation is actively going on right now. Such a process will begin with the contraction of a gas cloud under its own gravity. As the cloud gets smaller, the inside pressure and temperature will go on increasing. Eventually, it can rise to a value which is high enough for nuclear fusion to take place. The nuclear power not only provides the star with its energy but also halts further contraction. Most stars we see are in such a state of equilibrium.

Though nuclear power can provide the star with the energy to shine for a long, long time, it cannot do it forever. Sooner or later, the nuclear fuel will be exhausted and the star cannot shine any more. What happens to the star after that is a fascinating story which we will discuss in the next episode.

T. Padmanabhan is an astrophysicist at the Inter-University Centre for Astronomy and Astrophysics, Pune.