LOS ANGELES: Australian singer Olivia Newton-John, who gained worldwide fame as high-school sweetheart Sandy in the hit musical movie “Grease,” died Monday, her family said. She was 73.The entertainer, whose career spanned more than five decades, devoted much of her time and celebrity to charities after being diagnosed with breast cancer in 1992.Newton-John “passed away peacefully at her ranch in Southern California this morning, surrounded by family and friends,” said a statement from her husband John Easterling posted on her official social media accounts.