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Michael Trucco (as an experimental physicist), Dec. 15, 2008
The Battlestar Galactica sports-jock-turned-space-jockey plays a famed and impossibly hunky experimental physicist who begins dating Penny (Kaley Cuoco), much to the chagrin of his new research partner, Leonard (Johnny Galecki). Turns out the cad is married, and things ended up working out pretty well for Leonard and Penny after all, so, you know, the universe is fair.
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Summer Glau, March 9, 2009
The Firefly and Terminator: The Sarah Connor Chronicles star wound up on the same train as the Big Bang bunch, en route to a conference in San Francisco. Leonard, Raj (Kunal Nayyar), and Howard (Simon Helberg) all try to talk with her, and all strike out. No shocker: Sheldon (Jim Parsons) was not interested. Wah wah.
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Wil Wheaton, first appeared on Oct. 19, 2009
The recurring Big Bang guest star first became Sheldon Cooper's sworn enemy when the Star Trek: The Next Generation star neglected to show up at a sci-fi convention when Sheldon was a child. To date, ''Evil Wil Wheaton'' has appeared on four episodes of Big Bang, appearing to resolve his dispute with Sheldon in his most recent guest spot on the Oct. 13, 2011 episode ''The Russian Rocket Reaction.'' But it looked like another Trek alum took his place...
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Katee Sackhoff, first appeared on Nov. 23, 2009
When Howard was desperate for romantic advice about his girlfriend Bernadette (Melissa Rauch), Battlestar Galactica star Katee Sackhoff appeared to him in the bathtub — first as a fantasy, then as his conscience. The second time Sackhoff popped into Howard's head, though, she needed backup...
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Danica McKellar (as a university student), Jan. 11, 2010
A real-life advocate of mathematic education, the Wonder Years star plays a university co-ed who takes a liking to Raj, while her friend Martha (Jen Drohan) is stuck futilely attempting to hit on Sheldon.
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Stan Lee, March 1, 2010
Sheldon stalks the ne plus ultra of comic book geekdom after missing Lee's comic book signing due to a dust-up at traffic court.
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Steve Wozniak, Sept. 30, 2010
Terrified of all the world's perils, Sheldon creates a ''Shelbot'' to interact with the world while he stays in his bedroom. It's all going dandy until the Big Bang gang run into the famed Apple Computer co-founder (pictured here with exec producers Chuck Lorre and Bill Prady). ''Woz'' tells Sheldon via his ''Shelbot'' that he would've happily signed Sheldon's cherished Apple II if he had been there in person.
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George Takei, Oct. 10, 2010
After a long breakup — Bernadette caught Howard having cybersex with a troll on World of Warcraft, naturally — the Star Trek helmsman appears along with BSG's Katee Sackhoff to help Howard realize he still has feelings for his future fiancée.
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Neil deGrasse Tyson, Nov. 4, 2010
Sheldon gives the famed astrophysicist and Hayden Planetarium director a piece of his mind for Dr. Tyson's perceived role in demoting Pluto from planetary status. The jerk.
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Eliza Dushku (as an F.B.I. agent), Nov. 4, 2010
When Howard gets a chance to work on a top secret government surveillance satellite, the Buffy the Vampire Slayer and Dollhouse star grills all his friends as part of his background check. Because she looks like Eliza Dushku, things do not turn out too well for Howard.
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Keith Carradine (as Penny's father), Nov. 18, 2010
The pop-culture icon played the intimidating father of Penny, who convinces Leonard to act as if they were still dating, since her father doesn't know they broke up.
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LeVar Burton, Feb. 24, 2011
The Star Trek: The Next Generation chief engineer shows up at Sheldon's party after Sheldon sends him an invite via Twitter... and then bails just as quickly.
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Brent Spiner, Oct. 13, 2011
Wheaton's ST:TNG compatriot becomes Sheldon's new mortal enemy when he opens the mint-in-box Wesley Crusher doll Wheaton gives Sheldon to make amends. Data can be such a tool.
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Leonard Nimoy (as the voice of Spock), March 29, 2012
When Penny gives both Sheldon and Leonard a rare Star Trek toy as a gift, it plunges Sheldon into an existential crisis: Should he actually open its mint-condition box and play with the toy? Who better to help Sheldon through this dilemma than his trusty Spock action figure, brought to life by the voice of Spock himself. Nimoy has largely retired from on-camera acting, which is just as well, since in the world of Big Bang, he's had a restraining order out on Sheldon for quite some time.
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Stephen Hawking, April 5, 2012
There is arguably no scientist on the planet more famous, respected, or brilliant than Stephen Hawking. Watching him interact with Sheldon, who is stewing over the fact that Hawking is working with Howard — who doesn't even have a Ph.D! — promises to be a high water mark in Big Bang history.