spectral class

A star's spectral class, indicated by a single-letter code, is an overall classification based upon its spectral features. The classes, which are ordered by the temperature associated with their features:

A set of stellar spectral types belongs to each of these classes, indicated by a digit following the class, such as "G2". (Note that a star may be referred to as a "G-type star" which is natural English to indicate it has one of the types within the G class, but can leave the impression that spectral type and spectral class are synonymous.) The individual types indicate the features and color in more nuanced fashion, since these aspects vary gradually with the temperature, e.g., an O1 and a B9 star are similar.

The spectral features associated with each spectral class and type are the result of the temperature of the outer layers of the star, basically, its photosphere: two stars of very different in size, mass, and internal structure may yet have the same class and type. The order above (O B A F G K M; well-known phrase to help remember them: Oh be a fine girl kiss me) is by the temperature that produces the class's spectral features, from hotter to colder and the types are arranged so that lower-numbered digits are hotter, i.e., an O3-type star is hotter than an O5-type star. The color of the star is also associated with its temperature, so there is also a correspondence between the color and the class.

The terms early and late (early-type star or late-type star) are used to mean hotter and colder, e.g., "O", "B", and "A" are termed early classifications, and a star with a temperature of 3600 K would be termed an early M-class star. The early/late terminology arose at a time when it was imagined that stars cool over their lifetime.

If the star is on the main sequence, its structure, temperature and spectral class roughly correspond to a mass range and a star's class and type are associated with additional characteristics:

Post-main-sequence stars include giant stars, stars producing more energy than during their main-sequence, which enlarges them to the point that they have a cooler photosphere for the given mass: a star that is G class during its main sequence will later spend time as an M class giant. Pre-main-sequence stars also differ, and white dwarfs still shine, typically beginning very hot and cooling over their lifetime. The constituents of the star's outer layers do affect the spectral features of the star, but such differences among most main sequence and many post-main-sequence stars are minor and the same above list of spectral classes are used among them. But white dwarfs have more distinct spectral features and their own set of spectral classes, indicated by "D" followed by a letter. For very cool stars (including brown dwarfs), additional classes have been defined:

A couple of additional classes are commonly used for specific molecular lines found in some asymptotic giant branch (AGB) stars:

Spectral classes, especially the coolest three, are sometimes cited for planets, in particular, gas giants, which are typically comprised of the same materials as a star, and show the same temperature-based colors and spectral features.