LEVITATION

Some stars have outer layers which are sufficiently quiescent for diff usion to occur and to modify the relative abundances of the chemical e lements. Levitation occurs when the forces due to radiation pressure a re larger than those due to gravitation. The paper describes some rece nt work on the calculation of the radiative forces using atomic data o btained in the course of the work of the Opacity Project.Large abundan ce anomalies are observed for the HgMn stars, which lie on or close to the main sequence and have effective temperatures in the range 11000 to 15000 K. Some results are given for calculated abundances of elemen ts of the iron group in the HgMn stars.