Seven Guest Stars Who Blew Our Minds on The Big Bang Theory

Charlie Sheen! Stan Lee! Bill Nye! Watch our favorite surprise celeb moments from the series

STEPHEN HAWKING

There is no name more synonymous with physics than that of Stephen Hawking. And Sheldon gets the chance to meet his hero in episode 21 of season 5, when pal Howard (Simon Helberg) passes along a paper he wrote. "He was really impressed. He wants to meet you," Howard tells Sheldon, who appears to play it cool before screaming with excitement. The meeting, however, doesn't go exactly as he hoped …