For 19-year-old Stefani Germanotta, a little-known but scarily ambitious New York University dropout turned pop singer desperate to make a name for herself in the hipster club circuit of Manhattan’s Lower East Side, it was a pivotal moment. There she was, ready to sing some of her hot new material in a dingy dive, dressed to the max — a carefully calibrated mix of American Apparel promiscuity and Bride of Frankenstein weirdness. She downed a few drinks. She was ready. But the blasé crowd didn’t seem to care: they never stopped talking, barely mustering a glance at the stage. Enough, she thought. Without missing a beat, she peeled off her clothes, sat back down at the piano in a skimpy bra and panties, and started her set.

Everyone shut up and listened. And they haven’t stopped listening since.

Years later, Stefani, now better known as Lady Gaga — the biggest, wildest, least predictable pop star on the planet — told a reporter that her alter ego sprang to life that night, fully formed. “That’s when I made a real decision about what kind of pop star I wanted to be,” she said. “Because it was a performance-art moment then and there…. Unless you were in the audience in that very spontaneous moment, it doesn’t mean anything. It’s like, she took her clothes off, so sex sells, right? But in the context of that moment, in that neighborhood, in front of that audience, I was doing something radical.”

Doing radical stuff has become a big business for Lady Gaga. Meat dresses. Bubble dresses. Meat bikinis. Naked thanksgivings. Flaming pianos. Music videos strewn with corpses, unicorns, and hunky, oiled-up Judases. It all works for her, brilliantly. She’s parlayed an unerring sense for crazy, high concept fashion, insanely catchy dance beats, larger-than-life oddness — and, yes, amazing talent as a singer, songwriter, and musician — into one of the quickest and quirkiest rides to superstardom ever. The numbers have become legendary: 12 straight No. 1 singles on the Billboard dance chart; 32 million Twitter followers; more than 23 million albums sold; more than one billion views of her videos on YouTube.

Her fans — little Monsters, as she calls them — are fanatical, and her concerts are huge, meticulously produced extravaganzas. She’s the pop star we needed, the first true superstar of her era. She’s weirdly famous and famously weird. But still, everyone wonders: Who is Lady Gaga, really? And, most intriguing, how on earth did she get that way?

“The goal has never been to be famous,” she says. “My goal has been to be a star.” She succeeded beyond anyone’s wildest imagination. Not surprisingly, trying to separate myth from reality in the astonishing tale of Lady Gaga’s meteoric rise from a nice Upper West Side girl into the biggest star in a generation is a bit like trying to separate a vapor trail from a streaking comet.

Stefani Joanne Angelina Germanotta, born on March 2