What do you get when you put Mariah Carey, Fergie, Rihanna, Beyoncé, Miley Cyrus and Carrie Underwood together on one stage? No, not a giant diva standoff, but a history-making music moment. These chart-topping songbirds are all uniting for the Stand Up to Cancer benefit. They're recording a single together, "Just Stand Up," produced by Antonio "L.A." Reid and Kenny "Babyface" Edmonds and during the live telethon on September 5, they will all perform the song live. Other big-hearted A-listers scheduled to appear include Jennifer Aniston, Charlize Theron and newly cancer-free Christina Applegate.