O B A F G K M

Bright Spectral type

HOT Color index (B-V) COOL

Fig. A.2 An example of an HR diagram: the main sequence is circled

HOT Color index (B-V) COOL

Fig. A.2 An example of an HR diagram: the main sequence is circled

This classification, solely based on the temperature of stars, divides them into 9 categories, of which the first 7 (O, B, A, F, G, K, and M) cover about 99 per cent of the stars in the sky. Table A.1 describes the different categories in the Harvard classification. This classification is now extended to the coolest objects (stars of very low mass and sub-stellar objects) with the introduction of types L and T.

Each class is divided into 10 sub-classes, numbered 0-9, from the hottest stars to the coolest (a G0 star is hotter than a G9 star).

The same spectral classes are used to describe every star, whatever their type. However, to differentiate between objects (the effective temperature of a G0 giant is not the same at than of a star of the same G0 spectral class that is on the Main Sequence), the spectral type is followed by a number, in Roman numerals, describing the nature of the object, with the following convention:

Table A.1 The Harvard spectral classification (after Léna, 1996)

Spectral type

Teff of star

Colour of star

Spectroscopic signature

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