Final answer:
The spectral classification of stars sorts them into classes based on temperature, indicated by their spectrum's absorption lines. The classes O, B, A, F, G, K, and M (plus L, T, and Y for cooler objects) and subclasses numbered 0 to 9 help describe stellar characteristics. Luminosity classes I to VI further categorize stars by size and luminosity.
Step-by-step explanation:
Spectral Classification of Stars
The classification of stellar spectra is essential for understanding the characteristics of stars. Astronomers sort stars into a spectral class based on the absorption lines observed in their spectra, which are indicators of the star's surface temperature. The seven standard spectral classes, which are listed from hottest to coldest, are O, B, A, F, G, K, and M. To accommodate the discovery of cooler star-like objects, classes L, T, and Y have been added. The different classes and subclasses, which range from 0 to 9, help describe the temperature and characteristics of the stars. A notable characteristic of type A spectra is the presence of Balmer lines of hydrogen, which are most prominent in the A-type stars and reduce in strength as temperatures move away from this type. Cool stars, such as K and M types, exhibit strong lines from neutral sodium. Furthermore, each spectral class can fall into one of six luminosity classes, designated by Roman numerals, to indicate if they are main-sequence stars, giant stars, or supergiant stars. This classification helps astronomers understand not just the temperature, but also the size and luminosity of the stars.
For example, our Sun has the spectral type G2, placing it within the G category which is indicative of its moderate temperature and color. The spectral classification system is a foundational tool for astronomical research and understanding the different types of stars and brown dwarfs in our universe.