The rising gas is hotter and brighter than the cooler falling gas.
Solar granules take a few minutes to rise and disperse.
For other stars, we rely on the asymmetries of the spectral lines to detect granulation. The conventional observational tool for measuring asymmetries is the line bisector , constructed by connecting the midpoints of horizontal line segments across the spectral line at a set of depths. Only the shape of the bisector has significance, since the absolute zero is unknown owing to the motion of the star through space. Bisectors show systematic variations with position in the H-R diagram . Cooler stars and fainter stars show weaker granulation as shown in this figure.