AbstractWe examine the star forming phenomenon as it can be encountered in galaxies in the Sloan Digital Sky Survey, which possibly contains the largest homogeneous sample of star forming galaxy spectra to date.After eliminating all spectra with an insufficient signal-to-noise ratio, without strong emission lines, and without the [OII] λ3727 Å line, which is necessary for the determination of the gas metallicity (which excludes galaxies with redshift 0.024–0.025), our sample contains ~6000

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... ra of star forming galaxies.Through a detailed stellar population analysis employing evolutionary synthesis methods we determined the stellar composition of these galaxies, that is, the masses, ages and metallicities of their partial stellar populations.We find that most, possibly all, galaxies of our sample contain, apart from the presently bursting, ionising young generation (≤ 107 yrs), old (≥ 109 yrs) and intermediate (between 107 and 109 yrs) populations, whereas the old population dominates the stellar mass (but not the light).We also find that high (stellar) mass galaxies have higher gas metallicities and lower present star formation rates relative to their total (stellar) masses, than low mass galaxies, indicating a higher chemical evolution degree for high mass galaxies.Furthermore, we find that gas enrichment mechanisms in star forming galaxies do not vary with galactic mass, being the same for low- and high-mass galaxies on average. Gas enrichment mechanisms seem to present a greater variety at the high-mass end, though, indicating a more complex assembly history for high-mass galaxies.