Sir Patrick Stewart, a giant of modern theater and cinema, has died at 86, according to his spokesperson, Kelly Bush Novak. The actor passed away Wednesday at his home in Los Angeles after a "brief and difficult struggle with Alzheimer's and Lewy body dementia." Stewart, forged in London theater, brought a Shakespearean touch to two major fantasy film franchises: Star Trek, where he played Captain Picard in series and films, and the X-Men saga, where he was Professor Charles Xavier, a role he reprised last year while filming the new installment of The Avengers: Doomsday, yet to be released.