It's time to say goodbye to Penny and Leonard — but probably not Sheldon — because America's most popular comedy The Big Bang Theory is coming to an end after 12 seasons.

The season will premiere next month, while the finale, episode 279, will air early next year. According to the New York Times, it's going to pass Cheers (275 episodes) as the longest-running multicamera comedy.

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The sitcom about the interactions of brilliant but socially challenged scientists was still a massive ratings success too — 14 million people watched each episode live last season in America, with an episode also reaching 1 million viewers in Australia according to Mumbrella.

But while just weeks ago the head of CBS Entertainment had said "we don't believe it's the final year", there were signs the cast were ready to move on from the show, which began in September 2007.

"I think at this point everyone is very comfortable with 12 seasons being a good time to go home and see our families," Johnny Galecki, who plays Leonard, told reporters in January.

Shortly afterwards, Mayim Bialik, who plays Amy, also spoke positively about her experiences on the show, but added: "I think everyone will be really happy to do other things in their life."

Jim Parsons with his Hollywood Walk of Fame star alongside his co-stars. ( Reuters: Mario Anzuoni )

Last year Jim Parsons, who has won four Emmys and a Golden Globe for his portrayal of Sheldon, seemed OK with season 12 possibly being the end.

"There's no way, as an actor, I'm going to feel anything other than: 'We left that all on the table.' It's been a wonderful ride. I'm fulfilled in so many ways," he told Entertainment Weekly.

The show has proven quite divisive and not everyone is upset about the idea of it ending:

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But others say those who hate it just don't understand its appeal:

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Whatever the critical reception, Big Bang Theory is incredibly popular and its stars receive pay packets to match.

At one point the original five stars were earning a massive $US1 million per episode, but according to The Hollywood Reporter they took $US100,000 pay cuts ahead of the 11th and 12th seasons so more recent stars Bialik and Melissa Rauch (Bernadette) could get their own pay rises (to over $US425,000 per episode).

Don't worry if you count yourself as a fan, you'll probably still be able to watch new episodes of Young Sheldon, the spin-off prequel which debuted last year, after Big Bang Theory ends.

The show stars Iain Armitage — who was born the year after Big Bang Theory started— as a younger version of the character made famous by Parsons (who still features as the narrator).

It, too, has proven to be a hit. Episodes of the first season regularly reached above 12 million viewers in the United States, while the Australian premiere had over 1 million viewers.