Parallax

Between the distances of I and 5 parsecs, 23 stars have been found, of which seven are double stars. These include the bright stars a Centauri, Sirius and Procyon and three stars below the roth magnitude.

(2) If a star is moving at an inclination i to the tangent plane of the celestial sphere, its angular velocity i can be compared with its linear velocity V sin i in the line of sight obtained from spectroscopic observations. For some double stars and for several open clusters of stars i is known and accurate values of the parallax have been determined for Capella, the Taurus and the Great Bear clusters.

(3) For the more distant stars, trigonometrical methods are in applicable, but the parallax can be derived from the apparent magnitude of the star, if there are any means of knowing the absolute magnitude of the star; i.e., the magnitude the star would have at the standard distance of 10 parsecs. For Cepheid Vari ables this can be inferred with considerable accuracy from their periods, and for many stars a reasonable guess can be made from the spectral type and proper motion. The formula connecting ab solute magnitude and apparent magnitude with parallax is M=---- tn+5+51og in and expresses the condition that the light received from a star varies inversely as the square of the distance.

(4) Spectroscopic Parallaxes.—Stellar spectra are arranged in a sequence which represents the effective temperatures of the stars. In 1914 Adams and Kohlschiitter found differences in the intensities of certain lines in stars of the same type of spectrum, dependent on the absolute luminosities of the stars, or slight dif ference in the spectra of great and small stars of the same surface temperature. By photometric comparison of neighbouring lines,

one invariable, and the other varying with the size of. the star, the absolute magnitude of a star is determinable, and the paral lax by means of the formula given above. This method has been applied extensively to most of the brighter stars in the Northern hemisphere, using stars of known parallax as standards.

Average Parallaxes.

The solar system is moving through space with a velocity of 19.5 km./sec., carrying it four times the earth's distance from the sun in one year. This produces a general drift in the angular movement of the stars away from the apex or point in the sky to which the movement is directed. Were the stars at rest, this would give a ready means of determining their distances. As the stars are all moving, the method gives the average distance of a group of stars examined, on the assumption that their peculiar motions are eliminated. In this way the mean parallaxes of stars of successive apparent magnitudes of different galactic latitudes and of different spectral types are obtained. Thus the mean parallax of 5th magnitude stars : i.e., of stars just visible to the naked eye—is ".0i8; and of the Loth mag nitude stars; i.e. of stars giving of their light—is "40027.

As spectroscopic observations have given the mean peculiar velocities of stars of different types, the average proper motions perpendicular to the direction of the solar motion may also be used as a criterion of parallax. There is a very great diversity in the parallaxes of individual stars included in these mean values.

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