A relationship between a property of the calcium K line in late-type stars and the luminosity of the star. When the strong calcium absorption line is observed at high resolution it is often found to have a weak emission at its centre. The strength of this line has been found to correlate with the star’s brightness. The effect is named after the American astronomer Olin Chaddock Wilson (1909–94) and the Indian astronomer (Manali Kallat) Vainu Bappu (1927–82).