These densities - These densities giving number of stars...

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These densities, giving number of stars along the main sequence and implying the rate of evolution for stars beyond it, are combined with theoretical evolutionary tracks for stars of various mass and initial metallicity. At this point we may either do an evolutionary synthesis - starting with a hypothesized population (age, SFR, metallicity) and predict a spectrum as a function of time; or use population synthesis - fitting the observed spectrum to infer SFR, age, metallicity, perhaps with various "astrophysically reasonable" constraints connecting various components in the H-R diagram. Some typical ingredients for this approach were illustrated in Keel 1983 (ApJ 269, 466, fig. 12): It is also instructive to examine a library of stellar spectra, such as that of Jacoby, Hunter, and Christian 1984 (ApJ Suppl. 56, 257; also available on disk as IRAF images) or Gunn, Stryker,
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and Tinsley 1981 (ApJ 249, 48) - at lower spectral resolution but covering a longer wavelength baseline. Similar atlases have been attempted in the UV using IUE data - Wu et al. 1093, IUE Ultraviolet Spectral Atlas (NASA IUE Newsletter 22, 1), Heck et al. 1984, IUE Low Dispersion Reference Atlas (ESA: Noordwijk), Fanelli et al. 1990 (ApJ 364, 272). Recently, Leitherer and a 12-line list of co-authors (PASP 108, 996, 1996) have compiled a very extensive data collection of galaxy-evolution inputs, with the tables appearing in the AAS CD-ROM series (disk 7, 1996 part 2). Many of the difficulties found in dealing with galaxies may be traced to a lack of very high-metallicity stars in our neighborhood for inclusion in such compilations, and the often- subtle changes in late stages of stellar evolution with metallicity. Typical spectral synthesis results look like this (again shamelessly taken from Keel 1983): in which the observed spectrum is at the top, the model spectrum composed from solar- neighborhood stars below, and the difference at the bottom. The jump near 5600 Å is due to splicing together data with different grating settings; this kind of analysis tests one's ability to do absolute spectrophotometric calibrations. The fitting itself may use various algorithms and
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These densities - These densities giving number of stars...

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