Madonna has a lot more to celebrate Saturday than 50 candles on her cake - assuming her ascetic fitness regimen allows for a carb splurge on milestone birthdays.

The latest from the celebrity rumor mill is that she'll be passing the half-century mark quietly, possibly with a kabbalist ceremony to renew her vows with hubby Guy Ritchie (and to put all those A-Rod rumors to rest, maybe). But Madonna being Madonna, she won't keep quiet for long.

One week after her birthday, she launches a world tour in Cardiff, Wales, in support of her 11th studio album, Hard Candy. Released in April, it debuted on top of the Billboard chart, proof that her recent induction into the Rock and Roll Hall of Fame was not an exclamation point at the end of her career, but merely a semicolon. If anyone doubts that, she need only point to her new 10-year, $120 million recording contract with Live Nation Artists.

"Don't stop me now, don't need to catch my breath," she declares on her current single, Give It 2 Me. "I can go on and on and on."

"She didn't invent celebrity, but she came onto the scene with the ability to constantly repackage herself, not only as a human being but as a look and a phenomenon, which was definitely a harbinger of things to come," says pop-culture guru Robert Thompson, a professor at Syracuse University in New York.

Yes, 25 years after her self-titled debut, Madonna's icon status is beyond question. But what, exactly, is she an icon of?

Not anything musical, certainly. She's had a long string of hits, but few that would merit even an honorable mention as a timeless pop classic. She has reinvented her sound more times than David Bowie, but she's never been a trendsetter; at best, she's merely the first to hop on the bandwagon.

Her acting career is far less impressive (two words: Swept Away). As for fashion, there's no doubt her many looks have been widely imitated, starting with her fishnet stockings and baby-doll hair ribbons in the '80s, but, contrary to popular belief, she didn't invent the cone bra. She merely, um, extended its reach.

She's really good at . . . ?

It would be fair to say that Madonna's true artistic creation is her celebrity itself. But even in this arena she has been surpassed by inexplicable paparazzi magnets with far less talent and substance. In the famous-for-being-famous category, Paris Hilton is the new queen. So what makes Madonna Madonna?

Conveniently enough, her new single offers as complete an answer to that question as any song in her catalog.

Like so many of her hits, Give It 2 Me is an infectious dance track that sounds up-to-the-minute without actually challenging the listener with anything new. (Guest artist Pharrell Williams was cutting-edge 10 years ago when he made his mark as half of the production team known as the Neptunes.)

Despite the contemporary beats, however, there's a certain cheesy quality to the track that makes it feel instantly dated. It's hard not to notice that one of the rhymes - "Give me the bass line and I'll shake it / Give me a record and I'll break it" - unintentionally echoes the theme song to Laverne & Shirley.

The lyrics might not be poetry, but they work perfectly as a Madonna manifesto:

What are you waiting for?
Nobody's gonna show you how
Why work for someone else
To do what you can do right now?

"She's one of the first female artists who was actually in control of her career - the way she looked, the way she sounded," says New York music agent Mona Scott-Young, whose top clients include rapper Missy Elliott. "No one thought she was the product of someone else's marketing meeting."

Madonna's take-charge attitude has inspired generations of women, both in and outside the music industry. Phoenix native Suzi Figueroa, who also turned 50 this year, has been a fan from the beginning.

"I remember Madonna hitting the MTV scene in all her tulle and over-the-top jewelry and then being amazed that she was my age," she says. "I was so entranced by her individual style and drive to make it to the top."

Figueroa taught elementary and junior-high school in the Valley for 22 years and now mentors other teachers. And she still considers Madonna a role model.

"She is a great example of what a 50-year-old woman is and can be in today's society," she says. "She's beautiful, fit, intelligent and involved.

"This is a woman who can turn on the sex appeal like no other, but at the same time you know that she knows her stuff. She has a brain. She is a shrewd businesswoman. She knows what she wants, and she knows how to go about getting it. And a lot of people have a problem with that kind of behavior, because it's more masculine. . . .

"I identify with her because, in my life, I do know what I want, and I go out and get it. And I teach my daughters the same way."

Imperfect idol

While Figueroa admires the way Madonna embodies both personal power and sex appeal, she wonders if her idol might have crossed the line in some of her artistic expressions, such as her infamous 1992 book of "art photography," Sex.

"All of that kind of took me aback," she says. "I didn't really quite agree with it or feel it was something that needed to be done, but she did for some reason, and all of that put together has created who she is today."

Got no boundaries and no limits
If there's excitement, put me in it
If it's against the law, arrest me
If you can handle it, undress me

Madonna's iconoclastic sexuality has been inseparable from her music, her style and her celebrity. Her earliest hits, Borderline and Lucky Star, seemed innocent enough, but she followed them up with the scandalous Like a Virgin and Papa Don't Preach, which was condemned as a paean to teen pregnancy. After the controversy over her 1989 video for Like a Prayer, in which the scantily clad singer fantasizes about sucking face with a Black Jesus, the '90s era of cone bras and Sex almost seemed ho-hum.

"There's a difference between being objectified as a sex symbol and owning your sexuality and using it as part of your image," agent Scott-Young says. "She wanted to show that strong and sexy went hand in hand. It was all part of who she was."

Or, as pop-culture professor Thompson puts it: "She was the master of her own exploitation.

"If you were to just isolate some of her looks, it would be any early feminist's worst nightmare. But it was done on her own terms," he says. "There was a sense that she was highly sexualized, put out there for the male gaze. The difference was her sexuality and her presentation of self were being done on her own terms, which made her highly upsetting for some feminists but a model for others."

Give it to me, yeah
No one's gonna stop me now

More than anything else, Madonna is the icon of the postfeminist generation.

That's a term that was coined - like the vast majority of words beginning in "post" - in the academic world, right around the time that 23-year-old Madonna Louise Ciccone was inking her first record deal.

Postfeminism can mean different things, but in general it's a reaction against the perceived stridency and ideological rigidity of '70s-style, ERA-era feminism. With her girlie clothes and garish makeup, Madonna was the perfect symbol for a generation of women who wanted to be empowered without having to give up their femininity - or their (hetero)sexuality.

Even further, Madonna throughout her career has served up sexuality as empowerment, and it's an idea that has taken root and spawned waves of "girl power" sex symbols - for better or for worse.

She makes the choices

"For Madonna, it's empowerment to some extent because that's exactly how she defined it," Thompson says. "She made the formerly offensive sexualizing and objectification of women suddenly respectable again.

"But in its new respectability, lots of people started using it in the old unrespectable way."

Music agent Scott-Young points to Beyoncé Knowles and her client Missy Elliott as examples of next-generation pop stars who combine Madonna's sex appeal with her independence and business savvy. Yet such examples remain the exception in an endless parade of sex objects, from Britney Spears to - sad but true - Miley Cyrus, who hardly seem to be in control of their own exploitation.

"When you look at the Spice Girls and some of the other groups, that's a product of a marketing meeting," Scott-Young says. "They've taken certain superficial qualities, maybe the look, but they don't represent" what Madonna represents. That is: "headstrong, smart, in control."

But maybe that's because they just don't have what Madonna has. Maybe . . .

They'd do it too
If they were you.