LOS ANGELES -American bluegrass pioneer Ralph Stanley has died at the age of 89, according to his grandson, who said the singer and banjo player had been battling skin cancer. Part of the first generation of bluegrass musicians, Stanley came to mainstream attention through the 2000 Coen brothers movie “O Brother, Where Art Thou?,” for which he provided a Grammy Award-winning a cappella rendition of traditional folk song “O Death.” “He went peacefully in his sleep due to a long, horrible battle with skin cancer,” musician Nathan Stanley announced on his website on Thursday.