STELLAR EVOLUTION. A star cannot shine unchanged for ever. It must have a life-history, an evolution of some sort.
For a star is not a mere lump of inert matter ; it is actively radiat ing energy—heat and light—into the unsounded depths of space, and at a tremendous rate. This energy, according to our present understanding of nature, can be no new creation—it must have come from some storehouse within the star. Such a store of energy cannot be inexhaustible; unless it is in some way replenished from without, the star must ultimately cease to shine.
Though a star's life is thus probably of limited duration, it is doubtless so long that the span of human history is too short to afford hope of detecting even a trace of change by direct observa tion. Yet, as a visitor to a forest might, in a few moments, see trees in all stages of youth, maturity and decay, we may reason ably expect to find among the stars in the heavens examples of every stage of their evolution. The problem becomes, therefore, that of picking out, among the various types of stars known to observation, those which, on some reasonable physical theory, should represent the earlier and later stages of the process. Such a theory must depend on some knowledge, or at least some hypothesis, concerning the nature of the store of energy which the stars are expending ; and hence the problems of the source of stellar energy and the course of stellar evolution are inseparable.
The Contraction Theory.—Of the known stores of energy, cal culable by familiar physical principles, which a star possesses the greatest is the gravitational potential energy, arising from the mutual attraction of its parts. These attractive forces tend to compress the star. If they were not balanced by the internal pressure of the gases of the interior the star would contract, the gravitational forces would do work, and a corresponding amount of potential energy would be liberated, appearing as heat of com pression in the gas. If, at a given instant, the internal pressures just balanced those due to gravity, nevertheless, as the star lost heat by radiation from its surface, the interior would tend to cool and the gas-pressure to diminish. Contraction would then begin
and continue till enough heat had been produced to raise the pressure and restore the balance. The evolution of a star, on this conception, would be a gradual shrinkage—part of the heat liber ated by transformation of the gravitational potential energy main taining the internal equilibrium and the rest keeping up the radia tion.
This first rational theory of stellar evolution owes its origin to Hermann von Helmholtz. What proportion of the liberated energy is stored inside the star, and how much remains available to keep it shining, can be calculated from the known laws of gases. Much of the internal energy is the kinetic energy of motion of the atoms of the gas, to which it owes its high temperature. Part is energy of ionization. The collisions between the atoms are so violent that the outer electrons which normally belong to their structure are detached and fly about independently. To remove them against the electrical attraction of the residue of the atoms demands a large expenditure of energy which would be released again if the gas should cool and the collisions become less violent. Finally, the radiation imprisoned within the star, flying from atom to atom and gradually working its way out to the surface, carries an amount of energy which, especially in the stars of greatest mass, is a considerable fraction of the whole.
All told, it appears that more than half of the energy liberated by contraction, and sometimes a good deal more, must be stored inside the star in these three forms; but that, under actual con ditions, a surplus will be available to meet the loss by radiation.
Lane's Law.—As the star contracts its internal temperature rises. In the simplest case, when the laws of a "perfect" gas hold good and the density at all points within the star is increased in the same proportion by the contraction, it can easily be proved that the temperatures at corresponding points vary inversely as the radius of the star (Lane's Law, first stated by J. Homer Lane in 1870) ; thus this law should indicate the order of magnitude of the change in more complex cases.