Lecture 3


  • Pulsating Stars
  • The Cepheid as a 'Standard Candle
  • The Contraction Hypothesis
  • Subatomic Energy
  • Evolution of the Stars
  • Radiation of Mass
  • We have seen that spatially the scale of man is about midway between the atom and the star. I am tempted to make a similar comparison as regards time. The span of the life of a man comes perhaps midway in scale between the life of an excited atom (p. 74) and the life of a star. For those who insist on greater accuracy though I would not like to claim accuracy for present estimates of the life of a star -- I will modify this a little. As regards mass, man is rather too near to the atom and a stronger claimant for the midway position would be the hippopotamus. As regards time, man's three score years and ten is a little too near to the stars and it would be better to substitute a butterfly.

    There is one serious moral in this fantasy. We shall have to consider periods of time which appall our imagination. We fear to make such drafts on eternity. And yet the vastness of the time-scale of stellar evolution is less remote from the scale of human experience than is the minuteness of the time-scale of the processes studied in the atom. Our approach to the 'age of the stars' will be devious, and certain incidental problems will detain us on the way.

Pulsating Stars

The star Cephei is one of the variable stars. Like Algol, its fluctuating light sends us a message. But the message when it is decoded is not in the least like the message from Algol. Let me say at once that experts differ as to the interpretation of the message of Cephei. This is not the place to argue the matter, or to explain why I think that rival interpretations cannot be accepted. I can only tell you what is to the best of my belief the correct story. The interpretation which I follow was suggested by Plummer and Shapley. The latter in particular made it very convincing, and subsequent developments have, I think, tended to strengthen it. I would not, however, claim that all doubt is banished.

Algol turned out to be a pair of stars very close together which from time to time eclipse one another; Cephei is a single star which pulsates. It is a globe which swells and contracts symmetrically with a regular period of 5 1/3 days. And as the globe swells and contracts causing great changes of pressure and temperature in the interior, so the issuing stream of light rises and falls in intensity and varies also in quality or colour.

There is no question of eclipses; the light signals are not in the form of 'dots' and' dashes'; and in any case the change of colour shows that there is a real change in the physical condition of the source of the light. But at first explanations always assumed that two stars were concerned, and aimed at connecting the physical changes with an orbital motion. For instance, it was suggested that the principal star in going round its orbit brushed through a resisting medium which heated its front surface; thus the light of the star varied according as the heated front surface or cooler rear surface was presented towards us. The orbital explanation has now collapsed because it is found that there is literally no room for two stars. The supposed orbit had been worked out in the usual way from spectroscopic measurements of velocity of approach and recession; later we began to learn more about the true size of stars, first by calculation, and afterwards (for a few stars) by direct measurement. It turned out that the star was big and the orbit small; and the second star if it existed would have to be placed inside the principal star. This overlapping of the stars is a reductio ad absurdum of the binary hypothesis, and some other explanation must be found.

  By PanEris using Melati.

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