At least on paper, Sheldon Cooper doesn’t sound like the kind of television character you would root for. The Galveston-born theoretical physicist of CBS’s The Big Bang Theory, played by Houston-born Jim Parsons, is imperious and smug, and he relishes belittling those he regards as less intelligent, which is basically everyone. By the same measure, The Big Bang Theory, created by Bill Prady (Gilmore Girls) and Chuck Lorre (Two and a Half Men), doesn’t seem like it should have lasted more than a few episodes. A show about two adult nerds and their gorgeous across-the-hall neighbor, it harks back to the dippy, dopey era of Three’s Company or Full House, sitcoms that featured unlikely characters, contrived living circumstances, and laugh tracks so insistent they threatened to bruise your ribs. Didn’t the working-class realism of Roseanne and the resplendent meta-irony of Seinfeld put an end to this disposable froth once and for all?

Yet from its first episode, in September 2007, The Big Bang Theory has vastly exceeded expectations. Witty, fast, and uncommonly humane, it brings to mind those old Moss Hart–George S. Kaufman stage comedies, like Merrily We Roll Along or You Can’t Take It With You, except with frequent references to string theory. After only so-so ratings during the first season, the audience grew and then exploded; the show is now the highest-rated comedy on TV. And much of that success can be traced to Parsons, who instead of allowing Sheldon to turn insufferable has fashioned a character as mysterious, original, and—yes—lovable as any in television history. As much as you want to smack this Caltech physicist, you mostly just want to shield him from the slings and arrows of a world that so often holds intelligent people in contempt. Last fall, Parsons bested more-famous competition—including Alec Baldwin, Steve Carell, and Tony Shalhoub—to win the Emmy for Outstanding Lead Actor in a Comedy Series. Now, on September 18, he will compete in the same category, this time against his terrific co-star Johnny Galecki. Four days later, The Big Bang Theory kicks off its fifth season, on a network still reeling from the highly public flameout of Charlie Sheen and his firing from Two and a Half Men. Bizarre as it sounds and impossible as it may seem, Parsons—who four years ago was best known for a Quiznos commercial—has emerged as arguably the biggest comedy star on TV. As Sheldon might say, in that strange, sui generis style of his: Bazinga!

Part of what makes Parsons’s triumph so sweet is that he looks nothing like a network leading man should: neither aloof and model-pretty (see Simon Baker on The Mentalist) nor ripped and smarmy (see Scott Caan on Hawaii Five-O). Tall, with no discernible musculature, he wears his hair cut high on his forehead; though he’s 38 years old, he could pass for a high school student. Educated at the University of Houston and the University of San Diego, Parsons moved to New York in his late twenties and landed a string of small parts in movies (Garden State) and on television (Judging Amy). Maybe you’d recognize the face, but nothing about his résumé suggested you’d know his name. But then came the role of Sheldon Cooper, and Parsons found that once-in-a-lifetime fit between actor and character on a show that celebrates the odd man out. Sheldon lives with his best friend and Caltech colleague Leonard (Galecki), who pines for their neighbor, the Cheesecake Factory waitress Penny (Kaley Cuoco). Frequently they are visited by two friends, Howard (Simon Helberg), a would-be ladies’ man who dresses in vintage sixties couture, and Raj (Kunal Nayyar), who literally can’t speak in the presence of women. The Big Bang Theory gently satirizes these guys’ social ineptitude without tipping into mockery, and the relationship between Sheldon and Leonard—who is short and bespectacled and, again, nobody’s idea of a leading man—provides the show’s beating heart. Two superbright guys, picked on as kids, still hopelessly alienated from a world that doesn’t quite get them, find security and respect in each other (even if Sheldon’s eccentricities sometimes drive Leonard to fantasies of violence).

Of course, a sitcom can get only so far on heart—it also needs to deliver the laughs. And without slighting his gifted co-stars, the comedy here is fueled almost entirely by Parsons. The first thing you notice is his voice, haughty and patrician but also slow and faintly Southern—almost as if  Katharine Hepburn had morphed into the Church Lady. In the first three seasons, Parsons and the writers worked wonders to deepen the biography of Sheldon, who was raised by an evangelical mother (Laurie Metcalf) and a now deceased drunken father. More recently, they’ve shown Sheldon—who many viewers assumed had Asperger’s syndrome due to his inability to grasp basic emotional exchanges—genuinely and often hilariously struggling to connect to those around him. (“I’m not adept at reading facial cues, but I’m going to take a stab here,” he tells Raj, who is facing deportation to India, in one episode. “You’re either sad or nauseated.”) Last season, he hit it off with neurobiologist Amy Farrah Fowler (Mayim Bialik), a “girl” and a “friend” but not, Sheldon insists, his “girlfriend.” As for the reason Sheldon has never crossed that fine line from endearingly annoying to unbearably smug, well, Parsons deserves credit for that too. Even when the character is at his most gratingly superior, the actor never loses grip on his core of decency and sweetness. Witness Parsons’s note-perfect work on the season-two episode titled “The Financial Permeability,” when Sheldon extends a loan to Penny—and then couldn’t care less if he gets repaid.

A performer as talented as Parsons faces only one real challenge: He might become so deeply associated with Sheldon that it will be difficult for audiences to accept him in other roles. But he already seems to be making smart choices (see “His Expanding Universe”), and personally I’d rather linger on his present moment than brood about what comes next. Indeed, as much as Parsons looks nothing like most big stars on television, he looks even less like the male actors we usually see representing Texas: the good-time guys (Matthew McConaughey) and Marlboro men (Tommy Lee Jones) and earnest boys next door (Chace Crawford). He looks instead like the emo you might see at a Death Cab for Cutie concert at the Granada Theater, in Dallas; or the neo-slacker you might see perusing the racks at Waterloo Records, in Austin; or the nerd-chic club kid you might see dancing at South Beach, in Houston. Which is to say, he looks like the face of twenty-first-century Texas, a place where cowboys and nuclear physicists, manly men and overgrown boys comfortably coexist. And for 24 episodes a year, four years and counting, he’s done his home state proud.

Let’s say it one more time: Bazinga!

His Expanding Universe : Parsons after the big bang

In 2010 Jim Parsons re-upped his Big Bang Theory contract for four more seasons, meaning that we’ll likely be seeing him as Sheldon Cooper at least through mid-2014. But he also seems to be laying the groundwork for a long, diverse career. Earlier this year, he made his Broadway debut in the Tony-winning revival of The Normal Heart, playing a Southern AIDS activist named Tommy Boatwright. Time Out New York singled Parsons out of the ensemble cast, noting he provided “exemplary comic relief and unexpected depth.” Next month we’ll see the actor in his first major film role, alongside Jack Black, Owen Wilson, and Steve Martin, in The Big Year, a comedy about a birdwatching competition. Parsons’s part sounds like the sort of supporting role for which character actors were born, but don’t be surprised if this deft scene-stealer finds a way to make himself the star.