with the relation between stellar and gas absorption, depending on the age, and can be used as an evolutionary indicator.

The diagrams in Fig. \[figure:ubv\_sep\] show that clusters with close H$\alpha$ emission sources ($S<$ 2 arcsec) have a wide range of colours. This is a result of a significant absorption in young dusty star clusters.

Fig. \[figure:ubv\_sep\] shows a correlation between the separating distance from the nearest H[ii]{} region and $U-B$ colour index of a star cluster: clusters with 2 arcsec $<S<$ 8 arcsec become redder with increase of separations. Here we do not observe objects with extremely red colour indices. Apparently, the absorption in these star clusters without nearby H$\alpha$ emission is close to zero [@whitmore2011].

Remark the absence of any dependence between separation and $B-V$ colour index for star clusters with 2 arcsec $<S<$ 8 arcsec (Fig. \[figure:ubv\_sep\]). The reason is that young stellar systems with ages from 10 to 200-300 Myr have an approximately constant $B-V$ colour index unlike $U-B$. It is clearly illustrated in Fig. \[figure:ubv\_age\] where simple stellar population (SSP) models with Salpeter IMF and the interval of masses, ranging from $0.1 M_{\odot}$ to $100 M_{\odot}$, are shown. Here we used the grid of isochrones 