Three Types of Astronomical Study
Direct astronomical investigation of the properties of stars relies principally upon only three types of study. Astrometry, the measurement of stellar positions, yields parallaxes, proper motions, and apparent orbital motions (of binary stars). Photometry, the measurement of the quantity of light received at all wavelengths or within specific wavelength regions, gives apparent brightnesses (apparent magnitudes), colors, and variability of stellar brightnesses. Spectroscopy, the dispersion of light into its component colors to determine the stellar energy distribution as a function of wavelength, produces spectral types, luminosity classes, strengths of absorption and emission features from which chemical composition may be deduced, and Doppler shifts. More specialized interference techniques applied to starlight can produce data on stellar angular sizes and some idea of differences in the brightness across the photospheres of distant giant stars.
Understanding the physical nature of stars relies upon only a small number of properties that can be directly inferred from these three observational techniques. The first observational factor is distance (although distance is not really a property of a star, in the same sense that a person is the same person whether here or far away). The knowledge of a star's distance is needed to reduce observed factors (apparent brightness, apparent size, and so on) to absolute quantities (absolute brightness or luminosity, radius, and so on).