the gas bubble detaches from the star cluster and begins to blur. Its expansion is weakly dependent on the central energy source (explosions of supernovae). The average age of star clusters in M83 at this distance (20 pc) is $\log (t/{\rm yr})=6.76$. Our data obtained from equation (\[equation:ts\_gen\]), and coefficients in Table \[table:coeff\] for clusters in NGC 628 (an example of a galaxy which is similar to M83 by luminosity and morphology) in the range 20-400 pc, give a close age $\log (t/{\rm yr})=6.9\pm0.3$ at a distance of 20 pc.

The data for other four galaxies (NGC 3184, NGC 3726, NGC 5585, and NGC 6946) at the ranges of $S$ 40-200 pc and 40-500 pc give $\log(t/{\rm yr})=6.2-7.4$ at the distance 20 pc. Unfortunately, the errors of $\log(t/{\rm yr})$ exceed 0.4 dex for all galaxies except NGC 6946.

We applied the standard technique for studying an extension of hierarchical star formation [@efremov1998; @grasha2017] to our data. Separation $\Delta S$ and age difference $\Delta t$ between all identified cluster pairs based on their coordinates and $U-B$ colour photometric ages were calculated. Our star clusters are randomly distributed in the galactic plane, thus the number of pairs with separation $\Delta S<500$ pc 