The Master’s thesis explores the chemical homogeneity of the stars of the Orion Molecular Complex, based on measurements of Oxygen abundance in individual stars. Stellar spectroscopy is introduced, followed by the description of a blackbody spectrum and the formation of absorption lines. Theoretical Voigt profile of absorption lines is described in detail by taking pressure and thermal broadening into account. Doppler shift and rotational broadening of lines that originate from the star dynamics are described as well. Furthermore, the Echelle Spectroscopy technique, HR and Kiel diagram, and the spectral classification of stars, based on similarities in their spectra, are presented. The process of the stellar life is described; from the formation in molecular clouds and the main sequence phase, to the evolution into giants and finally deaths that enrich the surrounding environment with the elements of stellar nucleosynthesis. Theoretical part is concluded with the presentation of physical and chemical features of the Orion Complex. In practical part, spectroscopical observations of 30 stars from the molecular cloud areas in Orion and Monoceros, covering the area of wavelengths from 395 to 680 nm, have been used. Mainly, these are hot main sequence stars, including a few giants. It is described how raw spectra have been processed in IRAF programme and how iSpec programme has been used to determine the radial velocities, atmospheric parameters (effective temperature, gravitational acceleration on the surface, micro- and macro-turbulence velocities, the projection of the rotational velocity and the limb darkening coefficient) and most importantly, the Oxygen abundances of observed stars. Oxygen abundance have been determined under the assumption that Orion complex is chemically homogeneous in metallicities and abundances of $\alpha$ elements – therefore, the Oxygen abundance should not vary between different spectral types. Considering Oxygen abundance, the Orion Complex is proven homogenous. The determined absolute Oxygen abundance in the observed stars is $A$(O)$=(8,33\pm0,49)\,$dex, while the determined abundance, relative to iron is [O/Fe]$=(-0,26\pm0,49)\,$dex. The average spread of determined Oxygen abundances of all stars is equal to 0,54\,dex. Lastly, the relations between the determined Oxygen abundances and uncertainties of individual parameters have been researched. Among all the parameters, what affects the Oxygen abundances the most, is the accuracy of determining the effective temperature and the gravitational acceleration on the surface.
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