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values to within the limits of accuracy expected. In a more indirect way the same confirmation is shown in Fig. 7 by the close agreement of the squares, representing Cepheid variables, with the theoretical curve. The Cepheid as a' Standard Candle'Cepheid variables of the same period are closely similar to one another. A Cepheid of period 5 1/3 days found in any part of the universe will be practically a replica ofThis property renders the Cepheid extremely useful to astronomers. It serves as a standard candle -- a source of known light-power. In an ordinary way you cannot tell the real brightness of a light merely by looking at it. If it appears dim, that may mean either real faintness or great distance. At night time on the sea you observe many lights whose distance and real brightness you cannot estimate; your judgement of the real brightness may be wrong by a factor of a quintillion if you happen to mistake Arcturus for a ship's light. But among them you may notice a light which goes through a regular series of changes in a certain number of seconds; that tells you that it is such-and-such a lighthouse, known to project a light of so many thousand candlepower. You may now estimate with certainty how far off it is -- provided, of course there is no fog intervening. So, too, when we look up at the sky, most of the lights that we see might be at any distance and have
any real brightness. Even the most refined measurements of parallax only succeed in locating a few
of the nearer lights. But if we see a light winking in the Cepheid manner with a period of 5 1/3 days,
we know that it is a replica of With the Cepheids serving as standard candles distances in the stellar universe have been surveyed far exceeding those reached by previous methods. If the distances were merely those of the Cepheid variables themselves that would not be so important, but much more information is yielded.
Fig. 11 shows a famous star-cluster called The astronomer, more than other devotees of science, learns to appreciate the advantage of not being too near the objects he is studying. The nearer stars are all right in their way, but it is a great nuisance being in the very midst of them. For each star has to be treated singly and located at its proper distance |
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