The Formation of Massive Stars

o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o We review bservati nal and the retical evidence bearing n the rigin f highmass stars. The c nventi nal the ry f pr t stellar infall successfully acc unts f r bjects between ab ut 2 and 10 M . After f rmati n, these stars underg quasistatic c ntracti n with radiative interi rs. In this stage, they c rresp nd t Herbig Ae and Be stars. M re massive bjects have n pre-main-sequence phase and disrupt their infalling envel pes thr ugh str ng winds and radiati n pressure. Nevertheless, there is spectr sc pic evidence f r ng ing c llapse in a number f systems ass ciated with c mpact H II regi ns. The striking c ncentrati n f massive stars t ward the centers f y ung clusters pr vides an imp rtant clue. The indicati n is that stars well in excess f 10 M arise fr m the c alescence f already existing cluster members f l wer mass. C alescence may ccur after the merger f dense m lecular c res that c ntain the l wer-mass stars.