Figure 13-4 δ Cephei A Pulsating Star (a) As δ Cephei pulsates, it brightens quickly (the light curve moves upward sharply) but fades more slowly (the curve declines more gently). The increases and decreases in brightness are nearly in step with variations in (b), the star’s radial velocity (positive when the star contracts and the surface moves away from us, negative when the star expands and the surface approaches us), as well as in (c), the star’s surface temperature. (d) The star is still expanding when it is at its brightest and hottest (compare with parts a and b).