When it comes to classifying stars based on their spectral characteristics, astronomers use a system known as the Morgan-Keenan (MK) system, which categorizes stars into seven main spectral classes. Each class is designated by a letter of the alphabet, ranging from O to M, with O-type stars being the hottest and most massive, while M-type stars are the coolest and least massive. Understanding these spectral classes can provide valuable insights into a star's temperature, luminosity, and evolutionary stage.

The seven spectral classes are O, B, A, F, G, K, and M, with each class further divided into subclasses based on specific spectral features. O-type stars are extremely hot and blue in color, while M-type stars are cool and red. B-type stars are blue-white, A-type stars are white, F-type stars are yellow-white, G-type stars are yellow, and K-type stars are orange. These spectral classes are determined by the surface temperature of the star, with O-type stars having temperatures of over 30,000 Kelvin, while M-type stars have temperatures below 4,000 Kelvin.

By analyzing the spectral lines in a star's spectrum, astronomers can determine its temperature, chemical composition, and luminosity. O-type stars have strong ionized helium lines in their spectra, while M-type stars have strong molecular bands of titanium oxide. The presence of certain elements and compounds in a star's spectrum can provide clues about its age, mass, and evolutionary stage. For example, the presence of heavy elements like iron and nickel can indicate that a star is older and has gone through multiple stages of nuclear fusion.

In addition to determining a star's spectral class, astronomers also use spectral classification to study the properties of galaxies, nebulae, and other astronomical objects. By analyzing the spectra of these objects, scientists can gain insights into their composition, temperature, and distance from Earth. Spectral classification is a powerful tool that allows astronomers to unravel the mysteries of the universe and understand the processes that shape the cosmos. As technology advances and new telescopes are developed, our knowledge of spectral classification will continue to expand, leading to new discoveries and breakthroughs in the field of astronomy.