The Colours of the Stars (L)

Abstract

In the introduction of exercise B15 we have already noticed the colour differences between the stars. For scientific work these colours are studied quantitatively by photography (or by photoelectric measurements). According to whether we use blue-sensitive plates or orthochromatic, yellow-sensitive plates with a yellow filter, we obtain photographic or photovisual magnitudes. Their effective wavelengths correspond approximately to λ = 4400 Å and to λ = 5450 Å. The higher the temperature of the star, the greater the ratio i (blue)/i (yellow) and the smaller the colour index c = m _pg - m _pv. — The colour index, thus, is a measure for the ‘yellowness’ of the star and for its temperature. — We study some photographs of binaries, whose components have very different temperatures. An objective grating has been placed before the objective, giving two diffraction images for each star, their magnitude being about 1^m.0 lower than the main image. By shifting the plate a little, a number of consecutive exposures have been obtained. — As to the function of the objective grating, cf. exercise A19, § 12.