We present starburst models for far-infrared/sub-millimeter/millimeter ( FIR/sub-mm/mm ) line emission of molecular and atomic gas in an evolving starburst region , which is treated as an ensemble of non-interacting hot bubbles which drive spherical shells of swept-up gas into a surrounding uniform gas medium .
These bubbles and shells are driven by stellar winds and supernovae within massive star clusters formed during an instantaneous starburst .
The underlying stellar radiation from the evolving clusters affects the properties and structure of photodissociation regions ( PDRs ) in the shells , and hence the spectral energy distributions ( SEDs ) of the molecular and atomic line emission from these swept-up shells and the associated parent giant molecular clouds ( GMCs ) contains a signature of the stage of evolution of the starburst .

The physical and chemical properties of the shells and their structure are computed using a a simple well known similarity solution for the shell expansion , a stellar population synthesis code , and a time-dependent PDR chemistry model .
The SEDs for several molecular and atomic lines ( ^ { 12 } CO and its isotope ^ { 13 } CO , HCN , HCO ^ { + } , C , O , and C ^ { + } ) are computed using a non-local thermodynamic equilibrium ( non-LTE ) line radiative transfer model .

By comparing our models with the available observed data of nearby infrared bright galaxies , especially M 82 , we constrain the models and in the case of M 82 , we provide estimates for the ages ( 5 - 6 Myr , 10 Myr ) of recent starburst activity .
We also derive a total H _ { 2 } gas mass of \sim 2 - 3.4 \times 10 ^ { 8 } M _ { \odot } for the observed regions of the central 1 kpc starburst disk of M 82 .