Draft:Stellar Population Synthesis

The combined light of stellar populations encodes information about the stars, gas, and dust within the population. Stellar Population Synthesis, or SPS, is the process by which the physical properties of stellar populations are extracted from observed light, color, and spectra. This process first involves modeling the light output from different possible elements of a stellar population, including different stellar types, dust, and gas. These models can then be compared to observations to try to disentangle the contributions from individual elements of a stellar population. SPS is often used to understand the evolution of stellar clusters or galaxies by tracing the times at which stars were formed, their composition when they were formed, and whether they are still forming stars. . Popular SPS softwares include Prospector, Bagpipes , and CIGALE.

Stellar Classifications
Full article: Stellar classification Main sequence stars are classified based on their spectra into seven main classifications: O, B, A, F, G, K, and M (as well as low-luminosity L and T stars). Each stellar type has different masses, luminosities, and main sequence lifetimes, with O stars dying after about 10 million years and M stars having lifetimes longer than the current age of the Universe.

Galaxy spectra can be considered to be a mix of contributions from stellar spectra of different types. These spectra may be impacted by the chemical composition of the stars (“metallicity”), as well as absorption or emission from gas and dust surrounding the stars. By disentangling these contributions, one can obtain the relative contributions of each stellar type within the galaxy.

Single Stellar Populations (SSPs)
Stars in galaxies form out of large clouds of gas and dust. This means the stars in any given cluster tend to share chemical composition and to have formed around the same time. In other words, the stars formed in one cluster could be considered a Single Stellar Population, or SSP. These single units can then be combined to understand the formation of many populations of stars over the history of a galaxy.

Single Stellar Populations are the basis of stellar population synthesis. To construct an SSP, one first assumes an initial mass function, or a number of stars of each type formed in any particular cluster. The stars in an SSP share a chemical composition and are formed at the same time. These stars are then evolved along the main sequence for some amount of time. After 10 Myr, the most massive stars (O stars) will leave the main sequence and become supergiants, eventually exploding into supernovae. As more time passes, more stars will leave the main sequence and evolve into giant or supergiant stars.

The age of an SSP can be determined by its appearance on the Hertzsprung–Russell diagram. The most massive, brightest stars will leave the main sequence first, moving right along the HR diagram. By tracing the location of this main sequence turnoff in the HR diagram, one can determine the age of the SSP.

Stellar Population Synthesis
Galaxies are composed of multiple clusters of stars which formed at different times. Galaxy spectra are often modeled with multiple SSPs of different ages but the same chemical composition. By tracking the ages of different SSPs, one can reverse-engineer a star formation history for the galaxy, as well as current properties of the galaxy (e.g. star formation rate, stellar mass, amount of dust and gas, and the presence of an AGN). These physical properties can then provide insight to the physical processes relevant to galaxy formation and evolution.

Galaxies with lots of hot, blue stars are actively star-forming, as they must continuously replenish their supply of massive stars which die very quickly. These galaxies are typically spiral or irregular galaxies. Galaxies which are no longer star-forming will appear to be “red and dead,” as only cool, low-mass stars remain after the more massive stars have died. These galaxies tend to be elliptical and very massive, having formed most of their stars in the early Universe