D-BRANES AND FAT BLACK-HOLES

The application of D-brane methods to large black holes whose Schwarzs child radius is larger than the compactification scale is problematic. Callan and Maldacena have suggested that despite apparent problems of strong interactions when the number of branes becomes large, the open string degrees of freedom may remain very dilute due to the growth of the horizon area which they claim grows more rapidly than the average number of open strings. Such a picture of a dilute weakly coupled str ing system conflicts with the picture of a dense string soup that satu rates the bound of one string per Planck area. A more careful analysis shows that Callan and Maldacena were not fully consistent in their es timates. Tn the form that their model was studied it can not be used t o extrapolate to large mass without being in conflict with the Hawking -Bekenstein entropy formula. A somewhat modified model can reproduce t he correct entropy formula. In this ''improved model'' the number of s tring bits on the horizon scales like the entropy in agreement with ea rlier speculations of Susskind.