<P> The series initially centers on five characters: roommates Leonard Hofstadter and Sheldon Cooper, two Caltech physicists; Penny, a waitress and aspiring actress who lives across the hall; and Leonard and Sheldon's friends and co-workers aerospace engineer Howard Wolowitz, and astrophysicist Raj Koothrappali . </P> <P> Over time, several supporting characters have been introduced and promoted to starring roles, including Leslie Winkle, Amy Farrah Fowler, Bernadette Rostenkowski - Wolowitz, Stuart Bloom, and Emily Sweeney . The series also features numerous supporting characters, each of whom plays a prominent role in a small group of episodes . Included among them are parents of the main characters, their dates and their coworkers . Celebrities such as Stephen Hawking occasionally appear in cameo roles as themselves . </P> <Table> <Tr> <Th> Character </Th> <Th> Portrayed by </Th> <Th_colspan="11"> Seasons </Th> </Tr> <Tr> <Th> </Th> <Th> </Th> <Th> </Th> <Th> </Th> <Th> 5 </Th> <Th> 6 </Th> <Th> 7 </Th> <Th> 8 </Th> <Th> 9 </Th> <Th> 10 </Th> <Th> 11 </Th> </Tr> <Tr> <Td_colspan="13"> Main characters </Td> </Tr> <Tr> <Td> Leonard Hofstadter </Td> <Td> Johnny Galecki </Td> <Td_colspan="11"> Main </Td> </Tr> <Tr> <Td> Sheldon Cooper </Td> <Td> Jim Parsons </Td> <Td_colspan="11"> Main </Td> </Tr> <Tr> <Td> Penny </Td> <Td> Kaley Cuoco </Td> <Td_colspan="11"> Main </Td> </Tr> <Tr> <Td> Howard Wolowitz </Td> <Td> Simon Helberg </Td> <Td_colspan="11"> Main </Td> </Tr> <Tr> <Td> Raj Koothrappali </Td> <Td> Kunal Nayyar </Td> <Td_colspan="11"> Main </Td> </Tr> <Tr> <Td> Leslie Winkle </Td> <Td> Sara Gilbert </Td> <Td> Recurring </Td> <Td> Main </Td> <Td> Guest </Td> <Td_colspan="5"> </Td> <Td> Guest </Td> <Td_colspan="2"> </Td> </Tr> <Tr> <Td> Bernadette Rostenkowski - Wolowitz </Td> <Td> Melissa Rauch </Td> <Td_colspan="2"> </Td> <Td> Recurring </Td> <Td_colspan="8"> Main </Td> </Tr> <Tr> <Td> Amy Farrah Fowler </Td> <Td> Mayim Bialik </Td> <Td_colspan="2"> </Td> <Td> Guest </Td> <Td_colspan="8"> Main </Td> </Tr> <Tr> <Td> Stuart Bloom </Td> <Td> Kevin Sussman </Td> <Td> </Td> <Td_colspan="4"> Recurring </Td> <Td> Main </Td> <Td> Recurring </Td> <Td_colspan="4"> Main </Td> </Tr> <Tr> <Td> Emily Sweeney </Td> <Td> Laura Spencer </Td> <Td_colspan="6"> </Td> <Td_colspan="2"> Recurring </Td> <Td> Main </Td> <Td> Guest </Td> <Td> </Td> </Tr> <Tr> <Td_colspan="13"> Recurring characters </Td> </Tr> <Tr> <Td> Kurt </Td> <Td> Brian Patrick Wade </Td> <Td_colspan="2"> Recurring </Td> <Td_colspan="9"> </Td> </Tr> <Tr> <Td> Eric Gablehauser </Td> <Td> Mark Harelik </Td> <Td_colspan="2"> Recurring </Td> <Td_colspan="9"> </Td> </Tr> <Tr> <Td> Mary Cooper </Td> <Td> Laurie Metcalf </Td> <Td> Recurring </Td> <Td> </Td> <Td_colspan="3"> Recurring </Td> <Td> </Td> <Td_colspan="5"> Recurring </Td> </Tr> <Tr> <Td> Mrs. Wolowitz </Td> <Td> Carol Ann Susi </Td> <Td_colspan="8"> Recurring, voice only </Td> <Td_colspan="3"> </Td> </Tr> <Tr> <Td> V.M. Koothrappali </Td> <Td> Brian George </Td> <Td_colspan="5"> Recurring </Td> <Td_colspan="2"> </Td> <Td> Recurring </Td> <Td_colspan="1"> </Td> <Td> Recurring </Td> <Td> </Td> </Tr> <Tr> <Td> Mrs. Koothrappali </Td> <Td> Alice Amter </Td> <Td_colspan="5"> Recurring </Td> <Td_colspan="2"> </Td> <Td> Guest </Td> <Td_colspan="3"> </Td> </Tr> <Tr> <Td> Stephanie Barnett </Td> <Td> Sara Rue </Td> <Td> </Td> <Td> Recurring </Td> <Td_colspan="9"> </Td> </Tr> <Tr> <Td> Barry Kripke </Td> <Td> John Ross Bowie </Td> <Td> </Td> <Td_colspan="9"> Recurring </Td> <Td> </Td> </Tr> <Tr> <Td> Beverly Hofstadter </Td> <Td> Christine Baranski </Td> <Td> </Td> <Td_colspan="2"> Recurring </Td> <Td> </Td> <Td> Guest </Td> <Td> </Td> <Td_colspan="5"> Recurring </Td> </Tr> <Tr> <Td> Wil Wheaton </Td> <Td> Wil Wheaton </Td> <Td_colspan="2"> </Td> <Td_colspan="7"> Recurring </Td> <Td_colspan="1"> </Td> <Td_colspan="1"> Guest </Td> </Tr> <Tr> <Td> Zack Johnson </Td> <Td> Brian Thomas Smith </Td> <Td_colspan="2"> </Td> <Td_colspan="2"> Recurring </Td> <Td_colspan="2"> </Td> <Td> Recurring </Td> <Td> </Td> <Td_colspan="2"> Guest </Td> <Td> </Td> </Tr> <Tr> <Td> Priya Koothrappali </Td> <Td> Aarti Mann </Td> <Td_colspan="3"> </Td> <Td_colspan="2"> Recurring </Td> <Td_colspan="6"> </Td> </Tr> <Tr> <Td> Wyatt </Td> <Td> Keith Carradine </Td> <Td_colspan="3"> </Td> <Td> Guest </Td> <Td_colspan="4"> </Td> <Td_colspan="2"> Recurring </Td> <Td> </Td> </Tr> <Tr> <Td> President Siebert </Td> <Td> Joshua Malina </Td> <Td_colspan="3"> </Td> <Td_colspan="2"> Recurring </Td> <Td_colspan="6"> </Td> </Tr> <Tr> <Td> Mike Rostenkowski </Td> <Td> Casey Sander </Td> <Td_colspan="4"> </Td> <Td_colspan="3"> Recurring </Td> <Td> </Td> <Td> Recurring </Td> <Td_colspan="2"> </Td> </Tr> <Tr> <Td> Alex Jensen </Td> <Td> Margo Harshman </Td> <Td_colspan="5"> </Td> <Td> Recurring </Td> <Td_colspan="5"> </Td> </Tr> <Tr> <Td> Janine Davis </Td> <Td> Regina King </Td> <Td_colspan="5"> </Td> <Td_colspan="3"> Recurring </Td> <Td_colspan="2"> </Td> <Td> Guest </Td> </Tr> <Tr> <Td> Lucy </Td> <Td> Kate Micucci </Td> <Td_colspan="5"> </Td> <Td> Recurring </Td> <Td> Guest </Td> <Td_colspan="2"> </Td> <Td> Guest </Td> <Td> </Td> </Tr> <Tr> <Td> Bert Kibbler </Td> <Td> Brian Posehn </Td> <Td_colspan="5"> </Td> <Td_colspan="2"> Recurring </Td> <Td_colspan="2"> </Td> <Td> Recurring </Td> <Td> </Td> </Tr> <Tr> <Td> Arthur Jeffries </Td> <Td> Bob Newhart </Td> <Td_colspan="5"> </Td> <Td_colspan="2"> Recurring </Td> <Td> </Td> <Td> Guest </Td> <Td_colspan="1"> </Td> <Td_colspan="1"> Guest </Td> </Tr> <Tr> <Td> Dave Gibbs </Td> <Td> Stephen Merchant </Td> <Td_colspan="8"> </Td> <Td> Recurring </Td> <Td_colspan="2"> </Td> </Tr> <Tr> <Td> Claire </Td> <Td> Alessandra Torresani </Td> <Td_colspan="8"> </Td> <Td> Recurring </Td> <Td> Guest </Td> <Td> </Td> </Tr> <Tr> <Td> Alfred Hofstadter </Td> <Td> Judd Hirsch </Td> <Td_colspan="8"> </Td> <Td_colspan="2"> Recurring </Td> <Td> </Td> </Tr> <Tr> <Td> Colonel Richard Williams </Td> <Td> Dean Norris </Td> <Td_colspan="9"> </Td> <Td> Recurring </Td> <Td> </Td> </Tr> </Table> <Tr> <Th> Character </Th> <Th> Portrayed by </Th> <Th_colspan="11"> Seasons </Th> </Tr>

Who are the big bang theory characters named after