Absolute Measures of Stellar Radiation

Abstract

Observations at the Cambridge Observatories showed that the broad-band V and B photometric systems may be reproduced by observations in narrow bands about 200 A broad centred at 5410 A and at 4390 A respectively. At the Royal Observatory, Cape of Good Hope, measurements were made of the radiation flux outside the Earth's atmosphere in four narrow (∼200 A) wavelength bands from stars of known magnitude and colour index, by comparing bright stars with an artificial star, composed of a calibrated lamp and blue filter. A 4-inch refractor was used, with interference filters and a d.c. photometer. The results are given in Table I. They are in good agreement with some other recent measurements. Systematic errors are discussed in detail. Narrow-band visual magnitudes were measured for 221 stars, and compared with broad-band visual magnitudes determined by Johnson, by Hogg and by Eggen, and with early and recent Cape results. When linear colour equations are removed the standard deviations of the residuals are between ±0 m .014 and ±0 m .036. The internal random errors of the best individual series are believed to be ±0 m .010 (narrow-band) ±0 m .007 (Johnson) and ±0 m .007 (Cape). It follows that the large differences usually found between similar broad-band visual magnitudes cannot be explained by differences of band-width and equivalent wave-length combined with differences of stellar spectra. There is good agreement between the present narrow-band colour indices and gradients on the Greenwich system measured at Mt Stromlo. The relation between absolute and Greenwich gradients was found to be ϕ = GG + 1.11, in excellent agreement with the best previous results.