It turns unknowns into icons overnight — but "American Idol" isn't just for aspiring singers. With its unparalleled exposure — the weekly audience is usually somewhere north of 30 million viewers — "Idol" is now launching record sales of the established artists who appear as guests.

As such, it has become a hot destination for music stars. Stevie Wonder and Shakira have appeared on recent programs, and veteran rocker Rod Stewart coached contestants on Tuesday's show and was set to sing on Wednesday's episode.

"It's a great opportunity for artists to be on the show in front of tens of millions of people," said Fletcher Foster, senior vice president of marketing for Capitol Records. "There's not a venue like that for music acts except when you come to the Grammy Awards and CMAs. But here's one program a week when you can be in front of that many eyeballs."

And those eyeballs are connected to wallets.

Barry Manilow's "Greatest Songs of the Fifties" soared from No. 24 to No. 4 on the pop charts — a sales increase of 140 percent — following his March appearance on the program. Shakira's most recent album, "Oral Fixation Vol. 2," which was reissued with a new single shortly before her hip-shaking "Idol" performance, jumped from No. 98 to No. 6 on the Billboard charts.

Artists have enjoyed sales boosts even when they didn't sing on the show, which began in June 2002, said Geoff Mayfield, senior analyst for Billboard magazine. Lionel Richie appeared as a guest judge during "Idol's" early days and saw sales of his greatest-hits collections shoot up, Mayfield said.

In fact, stars don't even have to appear on "Idol" to enjoy a sales spike. Sales of two Keith Urban singles increased more than 20 percent after contestants performed the songs on the show, Foster said.

"It's just those songs reaching a new audience," he said.

Music stars started appearing on "Idol" during its second edition. LL Cool J, Brandy, Smokey Robinson, Gladys Knight, Gloria Estefan and record mogul Clive Davis are among the big names who've lent their star power to the show.

No other weekly program promises such a massive audience of music fans, Mayfield said.

"In this era, you don't really have a variety show in prime time," he said. "So even though it's a newfangled contraption, and more 'Amateur Hour' than 'Ed Sullivan,' 'American Idol' is today's version of a variety show. It's a weekly vehicle, not a once-a-year shot like the Grammys."

An "Idol" appearance can create its own media frenzy. After Kenny Rogers sang on the show two weeks ago, People magazine published a story about the country star, and radio stations added his new single to their playlists, Foster said.

Shakira's appearance was pegged to the release of a new single, said Lois Najarian, senior vice president of publicity for Epic Records: "It's an ideal place for a music star to be."

"American Idol" airs on FOX, which is owned by the parent company of FOXNews.com.