April 20

On April 20, 1902, Marie and Pierre Curie successfully isolate radioactive radium salts from the mineral pitchblende in their laboratory in Paris. In 1898, the Curies discovered the existence of the elements radium and polonium in their research of pitchblende. One year after isolating radium, they would share the 1903 Nobel Prize in physics with French scientist A. Henri Becquerel for their groundbreaking investigations of radioactivity.

Western Electric and Warner Bros. announce Vitaphone, a process to add sound to film. The system logged sound on a record linked electronically to the projector, keeping sound synchronized with image.

Mary Pickford, soon to become the first real movie star, begins her first motion picture job. Biograph pictures hired Pickford, a young stage actress, at less than $200 a week. By 1916, Pickford would be earning $10,000 a week, plus 50 percent of the profits of her films.

Two teenage gunmen kill 13 people in a shooting spree at Columbine High School in Littleton, Colorado. At 11:20 a.m., Dylan Klebold and Eric Harris, dressed in long trench coats, began shooting students outside the school before moving inside to continue their rampage. By the time SWAT team officers finally entered the school at 3:30 p.m., Klebold and Harris had killed 12 fellow students and a teacher, and had wounded another 23 people. Turning their guns on themselves, Klebold and Harris then committed suicide.

On this day in 1945, Allied bombers in Italy begin a three-day attack on the bridges over the rivers Adige and Brenta to cut off German lines of retreat on the peninsula. Meanwhile, Adolf Hitler celebrates his 56th birthday as a Gestapo reign of terror results in the hanging of 20 Russian prisoners of war and 20 Jewish children: Of these, at least nine are under the age of 12. All of the victims had been taken from Auschwitz to Neuengamme, the place of execution, for the purpose of medical experimentation.

1881 - Composer Nicolai Yakovlevich Miaskovsky was born.

1931 - Louis Armstrong recorded "When It’s Sleepy Time Down South."

1935 - "Your Lucky Hit Parade" was first broadcast on radio. It aired for 24 years.

1959 - Dolly Parton's first single, "Puppy Love," was released.

1960 - Elvis Presley's returned to Hollywood to film "G.I. Blues."

1963 - Rick Nelson and Kris Harmon were married.

1964 - The Elvis Presley movie "Viva Las Vegas" premiered.

1968 - The band Deep Purple performed gave their concert debut in Tastrup, Denmark.

1971 - Barbra Streisand recorded "We've Only Just Begun."

1976 - The Rolling Stones' "Black and Blue" album was released.

1986 - Vladimir Horowitz returned to Russia to perform after being away for almost 60 years.

1987 - Ozzy Osbourne released the album "Tribute." It was a live album recorded with Randy Rhoads. Rhoads had died in 1982.

1991 - Steve Marriot of Small Faces was killed in a house fire at the age of 44.

1992 - In London, a memorial concert was held for Freedie Mercury (Queen). Elton John, Guns N' Roses, Roger Daltrey, Liza Minnelli, David Bowie, George Michael, Def Leppard, and Spinal Tap performed.

1992 - Madonna signed a deal with Time Warner to set up a multimedia company. The deal reportedly made Madonna the highest paid woman in pop music.

1993 - Shania Twain released her self-titled debut album.

1994 - In London, Barbra Streisand began her first tour in 28 years.