ABSTRACT
In this section, we are concerned with interstellar extinction and the clues it provides to dust properties.
The interstellar extinction curve is obtained from stellar photometry. The wavelength resolution in such measurements is usually poor, typically λ/∆λ 10, but the broadness of the observational bands enhances the sensitivity. The standard photometric system is due to H.L. Johnson and W.W. Morgan and rooted in optical astronomy. It was later expanded into the infrared; there the choice of wavelengths was dictated by the transmission of the atmosphere of the Earth. The observational bands are designated by letters: U,B,V,R,I, . . .. Table 10.1 lists their approximate center wavelengths λc and the conversion factors, wλ, for translating magnitudes into Jansky and back; by definition,
1 Jy = 10−23 erg cm−2 s−1 Hz−1.
