Astronomical Colorimetry

Astronomical Colorimetry 

a division of practical astrophysics concerned with determining the color of celestial objects, principally stars. The introduction of various color indexes into astronomical practice in the early 20th century made it possible to characterize quantitatively the color of an object. On the one hand this can be done in terms of the wavelength which, in the emission spectrum of the object, exerts the most active effect on the radiation sensor, that is, the eye, a photographic emulsion, or a photocathode (the so-called acting, effective isophotic wavelength). On the other hand it can also be determined in terms of the ratio of illuminations or luminous fluxes from the object in two or several sufficiently distinct but wide regions of the spectrum—for example, in regions of the spectrum to which the eye and an insensitized photographic emulsion are respectively sensitive. In the 1940’s methods were widely introduced for electrophotometric comparison of luminous fluxes arriving either at one photocathode from the object in two or several regions of the spectrum selected by light filters, or at photocathodes of different spectral sensitivity. The ratio of photocurrents is converted to a logarithmic scale and expressed in stellar magnitudes.

The most widespread astronomical colorimetry is based on measurements in three spectral regions: V (visual; effective wavelength λ_eff= 550nm [1 nm = 10 Å]), B (blue; λ_eff = 450 nm), and U (ultraviolet; λ_eff = 360 nm). The color of a star is characterized by the differences B-V and U-B, expressed in stellar magnitudes. These differences are assigned the value zero for white stars of spectral class AO (under specified conditions). Colorimetric determinations are being successfully developed in a large number of spectral intervals in both the visible and infrared regions of the spectrum. An example of this is the Johnson system U, B, V, R, I, J, K, L, M, in which the λ_eff values for the last six intervals are 640, 840 nm, 1.16, 2.14, 3.36, and 5.0 micrometers respectively. Multicolor colorimetry approximately describes the energy distribution in the spectra of faint stars where spec-trophotometric measurements are difficult to take. There exists almost a one-to-one relationship between the B-V color of a star and its temperature, as well as the spectral class. But that relationship is distorted by selective absorption (reddening) of the light as it passes through interstellar space and also by absorption in the spectral bands of molecules in the atmospheres of colder stars, so that the color indexes actually observed often deviate from the indexes determined with the aid of that relationship.

REFERENCE

Martynov, D. Ia. Kurs prakticheskoi astrofiziki, 2nd ed. Moscow, 1967. Chapter 3, sections 20, 21.

D. IA. MARTYNOV