Halo, the leading North American sire in 1983 and 1989, died Tuesday morning at Arthur Hancock's Stone Farm near Paris, Ky. The horse, pensioned since 1997, was 31 years old. Among the 62 stakes winners sired by Halo were 1989 Horse of the Year Sunday Silence and 1983 Kentucky Derby winner Sunny's Halo.Hancock said the horse was in his paddock when he collapsed as a result of an apparent heart attack. Hancock said Halo died immediately. "He went quickly and did not suffer," Hancock said. "He had a great life."Halo led all sires by earnings in 1983 when Sunny's Halo became his first Derby winner, and again in 1989 when Sunday Silence won the Derby and Breeders' Cup Classic. Halo sired four other champions, including full siblings Devil's Bag and Glorious Song, dam of Rahy and Singspiel.In addition to his progeny's success on the racetrack, Halo has gained a reputation as a top sire of sires. Among the sons of Halo at stud are leading Japanese sire Sunday Silence, as well as Saint Ballado, Devil's Bag, and Southern Halo. Also standing at Stone Farm are two horses from the Halo male line -- Strodes Creek, a son of Halo, and Menifee, by Halo's grandson Harlan – who placed in the Kentucky Derby.