Final answer:
Physical properties such as temperature, luminosity, and size are used to classify stars on the H-R diagram. Stars are sorted into spectral classes (O, B, A, F, G, K, M, L, T, and Y), each representing a range of temperatures. Luminosity classes further categorize stars by brightness within their spectral class.
Step-by-step explanation:
Physical properties are fundamental in the classification of stars, particularly when creating an H-R diagram, which allows astronomers to categorize stars based on their temperature, luminosity, and size. Stars are sorted into spectral classes, ranging from the hottest O stars to the coolest Y stars. Each class reflects a range of temperatures with subclasses indicating finer variations. In addition to temperature, the spectra of stars permit calculations of chemical compositions, pressures in atmospheres, and insights into star sizes.
The most widely used system to classify stellar spectra takes into account the presence of specific absorption lines which vary with surface temperature. For instance, stars with the same temperature can differ in atmospheric pressure, helping to identify if a star is a giant or a dwarf. The spectral classification, O to Y, also denotes a star's color, from blue to red, correlating with temperature. Stars are further categorized into luminosity classes (I to VI), which indicate how bright a star is relative to others in its spectral class.
By examining the H-R diagram, astronomers can infer that stars in different parts of the diagram share certain physical properties like radius and mass. For instance, main sequence stars, which form a continuous band across the H-R diagram, have a specific mass-luminosity relationship, allowing their mass to be estimated from their luminosity.