IN the late afternoon chilly dark, the pop star Justin Bieber, 15, emerged from the radio station in Providence, R.I., where he had just been interviewed. As if on cue, a large pack of tween girls screamed and pounced.

So did their mothers.

“Justin, my daughter Elizabeth is going to your show tonight!” shouted one woman, shoving girls out of the way to push her cellphone camera in Mr. Bieber’s face. “Want a play date with her, Justin?”

A baby-faced fawn, Justin has become ridiculously successful at an age so tender that his preferred mode of greeting is a hug.

A scrappy athlete and chess player, Justin kept his musical passions to himself. At 12, he entered a local talent contest, coming in second. For family and friends who missed the competition, he uploaded videos on YouTube.

That summer, Justin opened his guitar case and started busking in front of a Stratford theatre. He earned nearly $3,000.

Meanwhile, the YouTu­be videos — Justin singing covers of Usher and Chris Brown — took on a life of their own. Scooter Braun, searching for another singer’s videos, mistakenly clicked on Justin’s.

Braun, 28, is a hip-hop industry marketer, known most recently for discovering the rapper Asher Roth.

She had not envisioned a future in mainstream pop music for him. Braun had a strategy: “I wanted to build him up more on YouTube first,” he said. “We supplied more content. I said: ‘Justin, sing like there’s no one in the room. But let’s not use expensive cameras.’ We’ll give it to kids, let them do the work, so that they feel like it’s theirs.”

Then he flew Justin to Atlanta to record demos.

In the studio parking lot, Justin bumped into his idol, Usher. He offered to sing for him on the spot. Usher was amused, but he took a pass. Word of Justin spread. Usher saw the videos. “His voice was magical and his personality was so keen,” he recalled. “He just knew how to be.” The singer called Scooter Braun to meet Justin.

But Usher had to get in line. “Scooter said, ‘Justin is interested in this kid,’ ” he recalled. “I said, ‘Justin? You mean Justin Timberlake?’ ”

Exactly. Justin Timberlake was flying Justin Bieber out to Los Angeles.