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updated 3/29/2012 12:53:14 PM ET 2012-03-29T16:53:14

The popular CBS sitcom "The Big Bang Theory" is set to welcome two guest stars: legendary "Star Trek" actor Leonard Nimoy and famed astrophysicist Stephen Hawking, according to news reports.

Nimoy returns to television tonight (March 29), voicing his most well-known character Spock, in a guest role on the hit show. The veteran actor will speak as Spock to one of the show's main characters, Sheldon Cooper, in a dream sequence, according to The Hollywood Reporter.

On the show, Cooper idolizes Nimoy, and the character often talks about his obsession with the actor and "Star Trek." But having Nimoy on the show has also been a longtime goal for the show's producers.

"For years, we thought it would be fantastic to have Leonard Nimoy appear on the show and we think we've found a really different and fun way to make that happen," executive producer Steve Molaro told The Hollywood Reporter. "As thrilled as we are he's doing it, we're even more thrilled just to get out pictures taken with him." [ 10 Best Sci-Fi TV Shows Ever ]

Executive producer Bill Prady echoed these sentiments, saying the team hopes to welcome the sci-fi hero onto the set one day.

"Leonard Nimoy's first appearance on Big Bang was as DNA on a napkin," Prady said in a statement, according to The Hollywood Reporter. "For this episode, we are delighted to hear his voice. We look forward to introducing Mr. Nimoy, body part by body part, over the course of many seasons to come."

Nimoy, who turned 81 on March 26, originally announced his retirement from show business in 2010. The actor, who is best known for his portrayal of the half-Vulcan, half-human Spock, from the original "Star Trek" TV series, films and spinoffs, also retired from the show's convention circuit last year.

Before then, Nimoy made guest appearances on the television show "Fringe," which is produced by J.J. Abrams, director of the 2009 "Star Trek" film. Nimoy also made an on-screen cameo in Abrams' film reboot of the classic series.

But Nimoy isn't the only famous guest star in upcoming lineup for "The Big Bang Theory."

Astrophysicist Stephen Hawking will make a guest appearance in the April 5 episode of the show, reported The Wrap.

In the episode, Hawking will visit Cooper at work "to share his beautiful mind with his most ardent admirer," the network said, according to The Wrap.

Like Nimoy, the show's producers said they had always hoped that Hawking would agree to guest-star in an episode.

"When people ask us who a 'dream guest star' for the show would be, we would always joke and say Stephen Hawking — knowing that it was a long shot of astronomical proportions," Prady said, according to The Wrap. "In fact, we're not exactly sure how we got him. It's the kind of mystery that could only be understood by, say, a Stephen Hawking."

Nimoy's episode "The Big Bang Theory" airs Thursday (March 29) at 8/7 p.m. Central on CBS. Hawking's episode of the show will air April 5. Check local listings.

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