Wolf-Rayet (WR) stars are a late stage in the evolution of massive stars (M ≥ 25 M⊙), characterized by strong stellar winds (dM/dt ~ 10−5 M⊙/yr). Ionizing radiation from the central star heats the expanding outer envelope of material, leading to recombination emission lines of helium, carbon, nitrogen, oxygen, and/or hydrogen in the WR star spectrum. This outflow of material enriches the surrounding ISM, which is further enriched when the WR star likely explodes as a type Ib or Ic supernova. WR stars are also likely progenitors for long soft gamma-ray bursts, and they are excellent tracers of the present sites of massive star formation in our Galaxy. The current Galactic WR star catalog is very incomplete. I discuss three methods of selecting strong WR star candidates from crowded fields in the Galactic plane: image subtraction, narrowband (NB) color, and broadband (BB) color. Using these methods, an extensive near-infrared narrowband survey begun in 2005-2006, and extended by me, has yielded 28% of the known Galactic WR stars to date; I add 59 new WR stars to the total in this thesis. I then compare two recent models of the Galactic population of WR stars, discuss the implications with respect to how many WR stars remain to be found, and use these results to inform an analysis of the remaining 834 strong carbon-rich WC star candidates from the survey. I also provide a listing of these 834 WC star candidates throughout our Galaxy, and map them; a central result of this thesis. Finally, I present selection criteria which may be used to identify [WR] stars (central stars of planetary nebulae which display WR spectral features), and proof of concept observations which led to 7 new confirmed [WC] stars.

Seven very luminous galaxies were examined to determine the contribution of active nuclei to their luminosity, the nature of their stellar population, and the validity of previous measurements of their H₂ content. Spectra of the 2.3 μm bands of CO were used to measure the stellar velocity dispersion, and hence the mass in the central regions. The bolometric luminosity, ionizing continuum luminosity, CO index, and 2.2 μm luminosity were also determined. Models of young stellar populations were compared to the observed properties of the galaxies. It was found that four of the seven galaxies have properties which are inconsistent with those of stellar populations. The active nuclei of these four galaxies produce more than half the total light of the galaxies, but less than one-third of the 2.2 μm light. The galaxies containing AGN also have young stellar populations. There is no evidence for the existence of active nuclei in the other three galaxies. The stellar populations that dominate the starlight in the galaxies have a wide range of properties. Stellar populations range in age from 10 to 65 Myr. The timescale for star formation ranges from 5 Myr to 100 Myr. The local initial mass function is ruled out for one of the four galaxies for which stellar population analysis could be done. The molecular gas masses for some very luminous galaxies determined from the ¹²CO J = 1 → 0 line are in error. The previously determined molecular gas masses are larger than the dynamical masses. The I(CO)/M(H₂) conversion factor found for Milky Way giant molecular clouds is evidently not correct for some very luminous infrared galaxies.