Spectral Types of Stars

Definition

Stars are classified using a spectral classification system, the most commonly known one being the OBAFGKM scale.
The scale categorises stars from the hottest (O) to the coolest (M) – an easy way to remember this is “Oh Be A Fine Girl/Guy, Kiss Me”.
Each of these classes is then subdivided from 0 to 9, indicating the hottest and coldest stars within each class.

O-type stars are the hottest, with temperatures exceeding 25,000K. These stars are rare and predominantly blue.
B-type stars are blue-white stars with temperatures between 11,000 - 25,000K.
A-type stars are white or bluish-white with temperatures ranging from 7,500 - 11,000K. Our own Sun is a type A star.
F-type stars are yellowish-white with temperatures between 6,000 - 7,500K.
G-type stars are yellow, similar to our Sun, with temperatures between 5,200 - 6,000K.
K-type stars are cool, orange stars with temperatures between 3,700 - 5,200K.
M-type stars or red dwarfs, are the coolest stars with a temperature less than 3,700K.

The Hertzsprung-Russell Diagram is an important tool used to illustrate the relationship between luminosity of stars and their surface temperatures.
The H-R Diagram allows us to identify patterns and different types of stars including main sequence stars, giants, supergiants, and white dwarfs.
The stars on the main sequence follow the pattern: the hotter a star, the brighter it is.

Understanding a star’s spectral type allows astronomers to learn about its temperature, composition, age, and distance.
This knowledge helps in understanding the universe, particularly how stars evolve, the processes involved, and the role stars play in producing elements.

Each spectral classification also exhibits unique absorption lines, which act as ‘fingerprints’ of the stars, helping astronomers determine their chemical composition.
The patterns of these lines provide insight into the physical and chemical processes occurring in the star’s atmosphere.