The Morgan–Keenan (or MK) system is used in modern astronomy to classify stars according to their spectral type and luminosity class and is named after William Wilson Morgan and Philip C Keenan, who introduced it 1943.

Spectral Type

The spectral type of a star depends upon its effective temperature, which is the temperature that an idealised black body, with the same surface area as the star, would need to have in order to produce the same total energy output.

The MK system uses the letters O, B, A, F, G, K, and M to designate a star’s spectral type, by subdividing the range of possible stellar temperatures, from the coolest, M, to the hottest, O. Each letter is subdivided into numbers, with 0 being the hottest and 9 the coolest.

For example, the Sun is a G2 star in this classification and has an effective temperature of 5,778 Kelvin. The red giant star Aldebaran, in the constellation of Taurus, has an effective temperature of around 3,910 Kelvin, and is classed as a K5 star, whereas, Alcyone, the brightest of the blue giant stars in the Pleiades open cluster, has an effective temperature of around 12,700 Kelvin, and is classed as a B7 star.

The letters date from earlier classification systems and originally ran alphabetically, but they were reordered and letters missed out as our understanding of stellar characteristics improved. A common mnemonic used to remember the seemingly odd choice of letters in the MK system is:

Oh Be A Fine Girl (or Guy), Kiss Me.

The colours and effective temperature ranges are as follows:

• O – greater than 30,000 Kelvin – colour description: blue
• B – 10,000 to 30,000 Kelvin – colour description: blue white
• A – 7,500 to 10,000 Kelvin – colour description: white
• F – 6,000 to 7,500 Kelvin – colour description: yellow white
• G – 5,200 to 6,000 Kelvin – colour description: yellow
• K – 3,700 to 5,200 Kelvin – colour description: orange
• M – 2,400 to 3,700 Kelvin – colour description: red

Luminosity Classification

This designation of spectral type is then appended by roman numerals indicating the luminosity class of the star. This is determined by the width of certain absorption lines in the star’s spectrum, which vary with the density of the star’s atmosphere. This luminosity classification, therefore, distinguishes between different types of giant stars and main sequence stars (or dwarf) stars. (See star types)

For the above examples, the MK classifications of the Sun, Aldebaran and Alcyone are G2V, K5III and B7III, respectively, indicating that Aldebaran and Alcyone are both giant stars. However, one is red giant and one is a blue giant, hence the difference in the spectral class part of their designations.

The different luminosity classes are as follows:

• 0 or Ia⁺ hypergiants or extremely luminous supergiants.
• Ia – luminous supergiants
• Iab – intermediate luminous supergiants
• Ib – less luminous supergiants
• II – bright giants
• III – normal giant stars
• IV – subgiants
• V main-sequence stars, also known as dwarf stars

Some prefixes are also used, such as:

• sd prefix – subdwarf prefix
• D prefix – white dwarf prefix

Spectral Peculiarities

Finally, additional letters may be added as suffixes to indicate spectral peculiarities. For example, the full MK classification of Alcyone is B7IIIe, with the ‘e’ indicating that emission lines are present in the star’s spectrum.