AE Ursae Majoris
AE Ursae Majoris is a short-period, high-amplitude delta Scuti star, a pulsating variable close to the main sequence. The delta Scuti stars, once given the moniker "dwarf Cepheids", are stars slightly more massive than our Sun that pulsate with periods of 1 to 6 hours. Their pulsations are all driven by the same physical mechanism as that of the Cepheids and RR Lyrae stars, namely the "kappa" or "opacity" mechanism. Radiation is periodically trapped in a spherical shell within the star at a depth and temperature where helium transitions from its singly- to doubly-ionized state. When that transition occurs, the material becomes more opaque, absorbing radition from lower layers in the star. This creates an increase in pressure that pushes the overlying layers of the star outwards until the pressure is balanced again. When the outer layers fall back in, the cycle repeats. AE UMa itself is a member of the "SX Phoenicis class" (low-metallicity Population II delta Scuti stars), and has a period of 0.086 days (about 2 hours), and also has a second period of about 0.067 days, leading to the strong variation in amplitude shown here. A number of observers in the Short-Period Pulsators section have been observing AE UMa for several years; these particular observations are by S. Dvorak (DKS).