Considering the proclivities of its characters, it's no surprise that The Big Bang Theory saw lots of sci-fi icons make guest appearances on the show. While many of Big Bang Theory's detractors would argue that the show spent more time laughing at nerds and geeks than with them, there's no arguing just how incredibly popular the show became, reigning as TV's top-rated comedy for most of its 12-season run on CBS. A good portion of Big Bang Theory's millions of fans likely are said geeks, considering just how many sci-fi, fantasy, and comic book icons were brought on to guest star.
This, unsurprisingly, included multiple cast members from both the Star Wars and Star Trek franchises, generally considered the two champions of mainstream sci-fi. Sadly, Harrison Ford was never among that lot, as seeing him play off his generally grumpy public persona when dealing with Sheldon being annoying would've been terrific. Still, the sci-fi icons Big Bang Theory did manage to enlist the services of include some of the biggest genre names ever.
Presented below is a round-up of every big sci-fi name to grace The Big Bang Theory soundstage. For anyone wondering where names like Adam West or Stan Lee are, they're icons of the superhero/comic book world, not sci-fi, and thus aren't being included. With that out of the way, it's time to begin.
Summer Glau
Summer Glau, known for playing River Tam on Firefly and Cameron on Terminator: The Sarah Connor Chronicles, was one of the first big-name celebrity guests to appear on The Big Bang Theory. She appears in season 2's "The Terminator Decoupling," referencing the then still airing show. The gang take a train ride to San Francisco, and while onboard, notice Glau. Howard, Leonard, and Raj all make attempts to chat her up, but all fail, although Raj does the best. She does agree to take a photo with Howard, only to break his phone after he suggests they do a picture that looks like they're making out.
Wil Wheaton
Calling Wil Wheaton a sci-fi "icon" might seem like a stretch, but he's certainly associated with the genre, thanks to his years as Wesley Crusher on Star Trek: The Next Generation. He's also the sci-fi-related guest star to have the biggest role on The Big Bang Theory, playing a fictionalized version of himself, and serving as a recurring nemesis and later friend to Sheldon. Wheaton appeared in 17 total episodes of The Big Bang Theory, beginning in season 3.
Katee Sackhoff
Katee Sackhoff, who is perhaps best known for playing the role of Starbuck on the 2004 reboot of Battlestar Galactica, has a rather odd distinction among The Big Bang Theory's celebrity guest stars. She appears twice, as a version of herself, but one that's part of Howard's fantasies when he gets... intimate with himself. Sackhoff first shows up in season 3's "The Vengeance Formulation," then again in season 4's "The Hot Troll Deviation."
George Takei
George Takei, best known for playing Sulu on Star Trek, also appears in "The Hot Troll Deviation," hilariously, also as part of Howard's fantasy. Howard's fantasy is interrupted by the feelings he still has for his eventual wife Bernadette, and Takei and Sackhoff both leave to discuss their respective careers. The two later show up to offer opinions on how Howard's reunion with Bernadette went.
LeVar Burton
Depending on who you ask, LeVar Burton is either best known for playing Geordi La Forge on Star Trek: The Next Generation, or as host of the beloved children's program Reading Rainbow. Burton first appeared in season 4's "The Toast Derivation." Sheldon invited Burton to a party on Twitter, and Burton actually showed up, only to be horrified by what he walked in on. Burton would next show up in season 6's "The Habitation Configuration," appearing as a guest on Sheldon and Amy's Fun with Flags web show. Finally, in season 8's "The Champagne Reflection," Sheldon proceeds to behave in wildly racist ways in front of Burton, albeit not realizing he's doing so, seemingly ending their social relationship.
Brent Spiner
Burton's Star Trek: The Next Generation colleague Brent Spiner - who played Lieutenant Commander Data - also appeared on Big Bang Theory, in season 5's "The Russian Rocket Reaction." Spiner attends a party thrown by another former co-star in Wheaton, and notably, when Sheldon shows up, he and Wheaton finally mend fences. That's followed by Spiner accidentally putting himself on Sheldon's enemies list.
Leonard Nimoy
Unlike the other sci-fi icons discussed here, Leonard Nimoy, famous for playing Mr. Spock on the original Star Trek series, didn't appear in person on The Big Bang Theory. Instead, season 5's "The Transporter Malfunction," saw Nimoy voice Mr. Spock, kind of. Nimoy lent his voice to a Spock action figure that Sheldon imagines convinces him to play with a vintage toy he got as a gift, only for it to break. Sheldon secretly swaps it for Leonard's, but the Leonard Nimoy-voiced Spock figure shames him into doing the right thing in the end.
James Earl Jones
James Earl Jones may not have been in the big famous black suit, but he memorably voiced the iconic and villainous Darth Vader in the Star Wars franchise, among other iconic roles such as Mufasa in The Lion King. Jones appeared in season 7's "The Convention Conundrum," which sees the guys shut out of attending Comic-Con. Not that Sheldon minds, as he ends up on the night of his life with Jones, who he meets at a restaurant. Jones ends up inviting the gang to SDCC as his guests.
Carrie Fisher
Fittingly, Jones' Star Wars co-star Carrie Fisher, who played the iconic Princess Leia in the series, also made her Big Bang Theory guest appearance in "The Convention Conundrum." As part of their wild night out, Jones and Sheldon play a juvenile doorbell prank on Fisher, who comes out of her house brandishing a baseball bat to ward off intruders.
Nathan Fillion
Firefly star Nathan Fillion appeared in season 8's "The Comic Book Store Regeneration." As is common for Big Bang Theory guest stars, he played a version of himself. Leonard and Raj see Fillion eating at a restaurant, and while they initially debate whether to approach him for a picture, they of course eventually do. Needless to say, Fillion isn't pleased.
Christopher Lloyd
Christopher Lloyd became a sci-fi icon due to his legendary portrayal of Dr. Emmett Brown in the Back to the Future trilogy, and will forever be associated with time travel stories. In season 10's "The Property Division Collision," Amy and Sheldon are moving into Penny's apartment, while Penny moves into Leonard and Sheldon's. Predictably, a dispute erupts between Sheldon and Leonard, leading a spiteful Sheldon to rent out his old room to a homeless man named Theodore, played by Lloyd.
Mark Hamill
Perhaps the biggest sci-fi icon guest star to ever appear on Big Bang Theory was Mark Hamill, otherwise known as Star Wars' Luke Skywalker. Hamill took part in the season 11 finale "The Bow Tie Asymmetry," which finally sees Sheldon and Amy get married. After Howard finds Hamill's lost dog, hilariously named "Bark Hamill," Hamill offers him a favor, and that turns out to be officiating Sheldon and Amy's wedding, taking the gig out from under Wil Wheaton.
William Shatner
Surprisingly, it took William Shatner - forever best known as Capt. James T. Kirk in the original Star Trek series and movies - until The Big Bang Theory's final season to show up. Shatner played himself in "The D&D Vortex," in which Wil Wheaton stages a D&D game and invites his celebrity friends. Shatner is one of them, alongside Joe Manganiello, Kareem Abdul-Jabbar, and Kevin Smith.