It remains a major challenge to derive a theory of cloud-scale ( \la 100 pc ) star formation and feedback , describing how galaxies convert gas into stars as a function of the galactic environment .
Progress has been hampered by a lack of robust empirical constraints on the giant molecular cloud ( GMC ) lifecycle .
We address this problem by systematically applying a new statistical method for measuring the evolutionary timeline of the GMC lifecycle , star formation , and feedback to a sample of nine nearby disc galaxies , observed as part of the PHANGS-ALMA survey .
We measure the spatially-resolved ( \sim 100 pc ) CO-to-H \alpha flux ratio and find a universal de-correlation between molecular gas and young stars on GMC scales , allowing us to quantify the underlying evolutionary timeline .
GMC lifetimes are short , typically 10 { - } 30 ~ { } \mbox { $ { Myr } $ } , and exhibit environmental variation , between and within galaxies .
At kpc-scale molecular gas surface densities \Sigma _ { H _ { 2 } } \geq 8 ~ { } \mbox { M$ { } _ { \odot } $ } ~ { } \mbox { $ { pc } $ } ^ { -2 } , the GMC lifetime correlates with time-scales for galactic dynamical processes , whereas at \Sigma _ { H _ { 2 } } \leq 8 ~ { } \mbox { M$ { } _ { \odot } $ } ~ { } \mbox { $ { pc } $ } ^ { -2 } GMCs decouple from galactic dynamics and live for an internal dynamical time-scale .
After a long inert phase without massive star formation traced by H \alpha ( 75 { - } 90 per cent of the cloud lifetime ) , GMCs disperse within just 1 { - } 5 ~ { } \mbox { $ { Myr } $ } once massive stars emerge .
The dispersal is most likely due to early stellar feedback , causing GMCs to achieve integrated star formation efficiencies of 4 { - } 10 per cent .
These results show that galactic star formation is governed by cloud-scale , environmentally-dependent , dynamical processes driving rapid evolutionary cycling .
GMCs and H ii regions are the fundamental units undergoing these lifecycles , with mean separations of 100 { - } 300 ~ { } \mbox { $ { pc } $ } in star-forming discs .
Future work should characterise the multi-scale physics and mass flows driving these lifecycles .