By the early 1990's, it became possible to explain most of the spectral and color
features of galaxies by linear combinations of SSP models. The model assumes a star
formation rate, then takes an amount of stellar mass and converts it into a ZAMS
population (zero-age main sequence). The population is then evolved using stellar
evolution codes. At any particular timestep, the code is stopped, the stars counted
by mass bin and a stellar spectral library is used to sum the integrated
An example of one such model is shown below for a burst population (i.e.
all the stars are made in a single burst). The model time (in Gyrs) is
shown on the left hand side of each spectrum.
However, despite initial successes, there was a terrible non-uniqueness problem to
SSP models. The age-metallicity degeneracy.