Stellar spectra are integral to nearly all aspects of modern astrophysics, as they allow us to learn about a star's physical properties as well as its radial velocity and evolutionary stage. The following animations show a generative data-driven model that predicts what a stellar spectrum should look like for a given set of stellar properties (e.g., temperature, surface gravity, and elemental abundances). The model (2nd-order polynomial in the 5 labels) was trained on data from the APOGEE Survey at the wavelength-pixel level, closely following the procedure of Ness et al. 2015.
The first movie shows how the spectrum changes as a star evolves off the main sequence and ascends the red giant branch ("RGB"). The metallicity is fixed to solar metallicity, while logg varies from 3.5 to 0.5 and Teff simultaneously varies such that the star moves along an isochrone. The second movie shows how the same region of the spectrum changes with metallicity at fixed Teff and logg. Clearly, both spectra show deeper absorption lines as the metallicity increases (with all other parameters fixed) or as the star moves up the RGB at fixed composition. So how can one tell the difference between a cool, low-logg star and a warmer, higher-logg star that is more metal-rich? The key is to look at the relative strengths of the absorption lines. As the metallicity at fixed stellar parameters increases, so does the optical depth of the stellar atmosphere, which subsequently increases the strength of all the absorption lines somewhat uniformly. However, when a star ascends the RGB, the physical mechanism for changing absorption line strength is the decrease in effective temperature, which alters the ionization state of the atmosphere (as described by the Saha Equation ). This sets the the atomic line strengths — at higher temperatures, more atoms can become ionized due to thermal collisions. When this happens these atoms can no longer absorb photons, thus decreasing the strength of absorption lines. Therefore, decreasing Teff increases the strength of absorption lines, but not all lines are affected equally because different atoms will have different ionization states at a given temperature.