Since stars in faraway galaxies are difficult to resolve individually, the composition of the stellar populations that lie within them can only be identified by matching the spectroscopic signal from the entire galaxy to the theoretical integrated spectra for hypothetical populations of stars. Evolutionary population synthesis does exactly that - assuming a certain initial mass function and metallicity, a stellar population’s positional distribution on an Hertzsprung-Russell diagram can be given as a function of time by means of population synthesis methods. Each H-R diagram distribution can then be mapped to a unique integrated spectrum under certain assumptions, ultimately leading to the evolution of the population’s spectral signal as a function of time.

Our studies of evolutionary population synthesis started in 2000, and were largely responsible for including the effects of binary systems in such investigations (Yunnan Model, 2004). It was 4-5 years before other research groups started following this path. Including binary interactions in evolutionary population synthesis is important, because such interactions can produce very hot objects, skewing the integrated spectra towards the short-wavelength end.

Lately, we have been refining the Yunnan Model to include other factors, to optimize it for the purposes of studying nearby galaxies, galaxy parameter determination, galaxy formation and evolution, HII regions, etc.