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There are a few important aspects that have not been mentioned above, for the sake of a more-linear presentation. Most of them have been treated extensively in Stasinnska (2004), and will be mentioned here only briefly, with some updates if necessary.

1.5.1 Correction for reddening, undderlying stellar absorption and aperture effects

Before being analysed in terms of abundances or star-formation rates or being compared with the results of photoionization models, the intensities of observed lines must be corrected for various effects. The presence of dust between the zone of emission and the observer attenuates the collected radiation and modifies its colour. It appears that the dust-extinction curve, which was once considered universal, is actually a one-parameter function characterized by the value of the total-to-selective extinction RV = AV/E(B — V) (Fitzpatrick 1999, 2004). The canonical value of RV, which is generally used for extinction corrections, is 3.1 or 3.2. However, the measurement of RV using 258 Galactic O stars yields a distribution around this value with a dispersion of ±0.5. Values of RV as small as 1.6 or as large as 5 are found (Patriarchi et al. 2003).

The extinction curve of a single star caused by interstellar dust is different from the obscuration curve of an extended nebula (Calzetti 2001) because in the latter case the observed radiation includes some scattered light. From a sample of starburst galaxies observed from the far ultraviolet to the near infrared, an obscuration curve for galaxies was derived (Calzetti 1997, Calzetti et al. 2000).

The amount of reddening is usually determined by fitting the observed Balmer decrement to the theoretical one at the appropriate temperature. For emission-line galaxies, in giant HII regions (and also in unresolved planetary nebulae), the observed Balmer lines are affected by the underlying absorption from the stellar component. This absorption can be determined empirically together with the extinction by fitting several Balmer lines to the theoretical Balmer decrement (Izotov et al. 1994). It can also be modelled, using spectral-synthesis techniques to reproduce the stellar absorption features other than Balmer lines and reading out from the model spectrum that best fits the data the stellar absorption at the Balmer lines' wavelengths (Cid Fernandes et al. 2005). This procedure applied to a sample of 20 000 galaxies from the SDSS (see Stasinska et al. 2006 for a description of the sample) shows a spectacular correlation between absorption equivalent widths at the Balmer-line wavelengths and the discontinuity at 4000 A, ^„(4000), as shown for the first time here (Figure 1.5). Since ^„(4000) is relatively easy to measure, this provides an empirical way to estimate the underlying stellar absorption at Ha and Hp. The data shown in Figure 1.5 can be modelled by the expressions

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