The correlation between colour index and absolute magnitude for stars on the lower main sequence; also known as colour–luminosity relation. If the absolute magnitudes of stars intrinsically fainter than the Sun are plotted against colour index, there appears a tight relationship which defines the main sequence. The slope of the relationship depends on the colour index used. If B − V is used for the colour then the relationship is much steeper than for V − I.
Subjects: Astronomy and Astrophysics.