– It doesn’t take a nuclear physicist to figure out why “Young Sheldon” flew out of the gate as the hottest new sitcom of the season.

As a prequel to “The Big Bang Theory,” viewers were bound to sample the series, if only to see how a 9-year-old version of the megahit’s breakout character would drive his folks batty in small-town Texas. A Thursday time slot immediately following its big brother didn’t hurt, either.

But 13 episodes in, “Sheldon” has remained in the Nielsen Top 10 for total viewership by forging its own identity, a strategy that helped “Cheers” spinoff “Frasier” develop into the most Emmy-winning sitcom ever.

“We never wanted to lean on ‘Big Bang,’ ” said Chuck Lorre, co-creator of the two shows. According to Lorre, they told themselves: “Let’s build a show that lives and dies by its own merits.”

Not that “Young Sheldon” ignores its roots. The title character’s bedroom wall includes a poster of Stephen Hawking, a future friend of grown-up Sheldon, who sought the celebrity scientist’s blessing before proposing to his longtime girlfriend. In a recent episode, we watched the little Einstein soothed for the first time by “Soft Kitty,” a song he will continue to depend on whenever he catches a dreaded cold.

But in other ways, the two Sheldons seem more like ­distant cousins.

The young version, played by Iain Armitage, prefers bow ties to superhero T-shirts, doesn’t completely shun human contact and comes across as more fascinated than fed up with mere mortals. You never want to sock him in the pocket protector, something that can’t be said about Jim Parsons’ adult version.

Last month, Parsons and Armitage sat together on the front lawn of the “Young Sheldon” set, surrounded by bargain-basement patio chairs, a Radio Flyer wagon and patches of fake grass. “You’ll get to be more irritating before you know it,” Parsons told his young doppelgänger. “Be patient. You’ll get there.”

It was Parsons who originally pitched the idea of a prequel. He provides the show’s narration, a first for a Lorre-produced sitcom. Also new for the wildly successful producer is a single-camera approach, one without a studio audience or laugh track.

The result: a sweeter, subtler sitcom without the broad, double-entendre jokes that fueled such Lorre hits as “Two and a Half Men” and “Mom.” It comes across as a 1980s “Andy Griffith Show,” in which a “good boy” struggling with detention is considered a major crisis.

“It’s an entirely different way to tell a story and the comedy plays differently,” Lorre said. “The moments are more intimate because the camera can be more intimate. On any show performed in front of an audience, the actors hold for laughs and it changes the rhythm of a scene. It’s just the nature of a theatrical performance. This is a film. It’s been quite a learning curve, but I love the opportunity to work with it.”

One element that “Big Bang” and “Sheldon” do have in common: perfect casting.

“Big Bang” famously sat on the shelf for two years after a pilot didn’t impress the network. The lead female character was originally conceived as a bit of a con artist. Given a second chance — and the casting of Kaley Cuoco — the character became a less manipulative, more openhearted foil for the two nerds played by Parsons and Johnny Galecki. It has now been on the air for 11 seasons, the past five as a consistent member of the Nielsen Top 5.

The kids are just right

“Young Sheldon” especially gets the kids right.

As the big brother, Montana Jordan could just as easily be Matthew McConaughey’s son, charming his female classmates with good ol’ boy charm. Raegan Revord, as Sheldon’s twin sister, is mature beyond her years, at least when it comes to pressing the rest of her family’s pressure points. And Armitage, who turns 10 in July, could wind up being the second youngest performer to get an Emmy nomination. (“Cosby” cutie Keshia Knight Pulliam got a nod when she was 6.)

“It’s kind of like a maze,” said Armitage, dissecting his approach to the character. “You know how to get to the end, but you still have to navigate.”

Armitage may get some of his smarts from real-life granddad Richard Lee Armitage, who served as deputy secretary of state under President George W. Bush.

But the cast member with the most intriguing family connections is Zoe Perry, who plays the super-religious, super-protective mother, Mary. She’s the daughter of “Scandal” actor Jeff Perry and Oscar nominee Laurie Metcalf — who originated the role of Mary on “Big Bang.”

“Genetics are strong,” said Perry, who previously played a younger version of her mom’s character, Jackie, on “Roseanne.”

“I’ve had these funny little revelatory moments, on stage or in shows, when I hear them both coming out of my own voice. I don’t have to be scared of it in the way that maybe sometimes we are when we channel our parents. I hold them both in such high regard that when I just trip on something that sounds familiar, I know it’s a good thing.”

Touted as the sitcom’s breakout star, Perry is getting used to being recognized in public, even when popping by a Big Boy restaurant in workout sweats.

For the kids, it’s a more challenging adjustment.

“I mean, if I’m with my friends, like on the street, and somebody’s like, ‘Oh, can I have a picture of you?’ it’s like, OK,” said 10-year-old Revord, holding tight to a stuffed animal between takes. “But if we’re playing or something in the park and we’re in the middle of something, it’s kind of annoying.”

Annie Potts, who plays the show’s hip grandmother, had her fair share of fans during her time on “Designing Women.” She’s confident that her castmates can handle the adulation to come.

“I think it’s going to be like a tsunami soon and that will be something new,” she said. “But they’ve all got their feet on the ground.”




Twitter: @nealjustin