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    Naturevolume 43pages6465 (1890) | Download Citation



    THE DUPLICITY OF α LYRÆ.—At the meeting of the Royal Astronomical Society on November 14, Mr. A. Fowler exhibited some photographs of the spectrum of α Lyræ which indicate that it is a spectroscopic double of the β Aurigæ and ζ Ursae Majoris type. The photographs were taken with the 10-inch refractor belonging to the Royal College of Science, with two prisms of 7½ each in front of the object-glass, and form part of a photographic study of stellar spectra recently commenced at Kensington by Prof. Lockyer with a special object. The evidence of duplicity of this kind of binary star depends upon the fact that when the two components are moving in opposite directions in the line of sight, the lines that are common in their spectra are displaced towards opposite ends of the spectrum, in accordance with Doppler's principle, and therefore appear double. When the motion is at right angles to the line of sight, there is, of course, no such displacement, and the lines therefore appear single. Hence, during a complete revolution the lines will twice reach a maximum separation and twice appear single. The principal lines in the spectrum of α Lyrae are due to hydrogen. These do not exhibit a duplication, because the separation is less than their thickness. A variation in their width, however, is very obvious. The K line of calcium is the next strongest, and is sufficiently fine and distinct to render the duplicity very apparent. Fourteen photographs of the spectrum have been taken from October 3 to November 4. The maximum separation of the K line was recorded on October 8 as 7-8 tenth-metres. On October 17, 28, and November 1, 8 p.m., the same line appeared single. At 8.30 p.m. and 10 p.m. on the last-named date, the separation was respectively 2˙3 and 3˙8 tenth-metres. A discussion of the data obtained from all the photographs shows that they are fairly satisfied by assuming a circular orbit, the plane of which passes through the sun, and the remarkably short period of revolution of about 24˙68 hours. This period does not appear inconsistent with the relative orbital velocity of 370 miles per second indicated by the photograph taken on October 8, and is confirmed by the three photographs taken at short intervals on November 1. If 370 miles per second be taken as the maximum relative orbital velocity, the distance between the components is about 5,000,000 miles. The total mass will therefore be about 22˙5 times that of the sun, and as there is no appreciable difference in the intensity of the K lines, the masses of the components are probably about equal. In the cases of β Aurigæ and ζ Ursæ Majoris, Prof. Pickering found, respectively, periods of 4 and 52 days, and maximum orbital velocities of about 150 and 100 miles per second.

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