Robert Stack, best known as the star of the TV classic The Untouchables and, more recently, as host of Unsolved Mysteries, died Wednesday of heart failure. He was 84. The baritone-pitched actor had undergone radiation treatment for prostate cancer in October. "He was feeling so good," his wife Rosemarie told The Associated Press. "He had a bout with a tumor but that was gone. It wasn't that, it was his heart. He was too weak. He wouldn't have lived through a bypass."