  We present a general star formation law where star formation rate depends
upon efficiency $\alpha$, timescale $\tau$ of star formation, gas component
$\sigma_{g}$ of surface mass density and a real exponent $n$. A given exponent
$n$ determines $\tau$ which however yields the corresponding star formation
rate. Current nominal Schmidt exponent $n_{s}$ for our model is $2<n_{s}<3$.
Based on a gravitational instability parameter $Q_{A}$ and another
dimensionless parameter $f_{P}\equiv (P/G\sigma_{c}^{2})^{1/2}$, where $P$ =
pressure, $\sigma_{c}$ = column density of molecular clouds, we suggest a
general equation for star formation rate which depends upon relative competence
of the two parameters for various physical circumstances. We find that $Q_{A}$
emerges to be a better parameter for star formation scenario than Toomre
Q-parameter. Star formation rate in the solar neighbourhood is found to be in
good agreement with values inferred from previous studies. Under closed box
approximation model, we obtain a relation between metallicity of gas and the
efficiency of star formation. Our model calculations of metallicity in the
solar neighbourhood agree with earlier estimates. We conclude that metallicity
dispersion for stars of same age may result due to a change in efficiency
through which different sample stars were processed. For no significant change
of metallicity with age, we suggest that all sample stars were born with almost
similar efficiency.
