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star, light, times, density, companion, suns, sun and mass

SIRIUS, the "dog star," is the brightest star in the heavens. It is situated in the constellation Canis Major (q.v.). The my thology and early references to it are discussed in the article CANIS MAJOR. In 1844 Bessel found that Sirius was not moving uni formly through the heavens, being sometimes ahead and some times behind the mean position. He deduced that (in addition to uniform proper motion) it had a motion in an elliptic orbit in a period of about 5o years. This indicated that an unseen star must be present, forming with Sirius a double star. In 1862 Alvan Clark, who was testing a large new object-glass, unex pectedly detected the companion as a faint point of light amid the glare of Sirius itself.

Sirius is one of the nearest stars and its parallax has been measured with considerable accuracy. Its distance is 2.7 parsecs =8.8 light years=51 million million miles. Its apparent brilliance is due more to its nearness than to actual luminosity; absolutely it is about 28 times as bright as the sun—a luminosity which is not at all exceptional. Its mass has been determined by Kepler's laws from the double star orbit and is 2.4 times that of the sun. Its spectrum shows that the surface temperature is much higher than the sun's (about against and its light is correspondingly whiter. It is a typical star of the spectral class Ao, in which the Balmer series of hydrogen is most strongly developed.

The companion is a remarkably interesting body. It revolves at a distance from Sirius nearly equal to the distance of Uranus from the sun in a period 49 years. Although it has one-third the mass of Sirius it gives less than 10 of the light. A more con venient comparison is with the sun ; it has 0.85 of the sun's mass, but only of the sun's light. It seemed natural to assume that it must be radiating feebly like a star on the verge of extinction, but in 1914 W. S. Adams made the surprising discovery that its spectrum was not much different from that of Sirius itself. This indicates a white light coming from a surface at high temperature and radiating more intensely than the sun's surface. If the small total light is not due to low emitting power, it must be due to small dimensions. Reasoning in this way the radius is found to be 18,80o km. ; i.e., intermediate between the earth and the next larger planet Uranus. The mass, which is o.85 of the sun's or 250,000 times that of the earth, has to be squeezed into this small sphere; and the resulting density is 61,00o times that of water or about a ton to the cubic inch.

The result at first seemed incredible, but the cause of the sup posed breakdown of this natural inference remained a mystery.

In 1924, however, it was discovered that, owing to the breaking up of the outer systems of electrons in the atoms under the high temperature prevailing in the interior of a star, stellar matter would not offer the resistance to high compression that terrestrial matter does. It was therefore quite conceivable that densities might be attained in the stars far transcending the greatest known terrestrial densities. It was suspected that this might be the ex planation of the result for the companion of Sirius. Accordingly, in 1924-25 W. S. Adams carried out a special test, depending on the Einstein effect in the star's spectrum, in order to check the high density. This effect is a lengthening of the wave-length of light which has come from a place of high gravitational potential, manifesting itself as a displacement of the spectral lines towards the red. It is thus theoretically possible to calculate the potential at the surface of a star (i.e., the mass divided by the radius) if the magnitude of the Einstein effect can be measured. In all ordinary stars the shift is too small to measure ; it is believed to have been found in the sun, but this can scarcely yet be regarded as certain. But if the foregoing dimensions of the companion of Sirius are right, the Einstein shift will be 3o times as great as on the sun. This quantity should be very easily measurable, were it not for special difficulties which arise from the faintness of the star and the interference of the scattered light from Sirius. Adams applied the test and his results are believed to be trust worthy; they definitely confirm the large spectral shift predicted. It appears therefore that the companion of Sirius is a genuine example of matter compressed to enormously high density— more than 2,000 times the density of platinum.


the high density of the companion of Sirius: A. S. Eddington, Monthly Notices of Roy. Ast. Soc., 84, P. 322 (1924) ; W. S. Adams, Proc. Nat. Acad. Sci. Washington, II., p. 382 (1925). The remarkable physical conditions arising from the high density are discussed by R. H. Fowler, Monthly Notices, 87, p. 114 (1926).

(A. S. E.) SIRMUR or SARMOR, an Indian State, within the Pun jab. It is also called Nahan, after the chief town. It occupies the lower ranges of the Himalaya, between Simla and Mussoorie. Area 1,198sq.m. On the northern frontier Chor Peak and station are about 12,000ft. above the sea. Pop. (1931), 148,568. Esti mated gross revenue, £45,000.