She was "America's sweetheart." For 10 years in a row she appeared on the list of the top-10 Hollywood stars, and in 1943 she was the most popular star in the world, her pinup a keepsake accompanying American soldiers to war. But when was the last time you saw a Betty Grable movie? Can you, in fact, think of the name of a Betty Grable movie?

Stardom is as unstable as an atom. Exposed to time, it mutates. If your name is Grable, it can be as ephemeral as a passing fashion. If your name is Gable, it's as permanent as marble. After the headlines and the gossip fade, there are only the movies you've left behind to argue your case. But which ones will last is not so easy to predict. Could anyone have guessed, when a transplanted working-class Brit started his career in light '30s comedies such as "Topper," that Cary Grant would leave more lasting movies behind than any other actor in history? His films weren't the prestige items that won best picture or earned him any acting prizes (those went to "serious" stars like Paul Muni). Still, they seem miraculously immune to aging: "The Awful Truth," "Bringing Up Baby," "The Philadelphia Story," "Notorious," "An Affair to Remember," "North by Northwest," to name just a few.

So 50 years from now, when his couch-jumping days on "Oprah" are long forgotten, what, if anything, will we make of Tom Cruise? Could Julia Roberts--America's Sweetheart at the end of the 20th century--fall into Grable-like obscurity? How many of Roberts's films will stand the test of time, the way Katharine Hepburn's or Audrey Hepburn's have become part of the basic syntax of Hollywood film? It's safe to predict longevity for many in the generation that preceded Tom and Julia: Jack Nicholson, Clint Eastwood, Al Pacino, Meryl Streep, Harrison Ford, etc. But let's take a hard look at the output of today's A-list stars, particularly the ones born after 1960. I'm going to be ruthless here, even banishing some movies that deserve to survive--but won't.

Julia Roberts is the most incontestably starlike actress of her generation. She's a natural, her appeal as easy to parse as that radiant smile and startled laugh in "Pretty Woman." With that movie "Julia Roberts" became a Platonic concept that's larger than the roles she plays, no matter how hard she tries ("Mary Reilly," "Michael Collins," "Closer") to make us think otherwise. That's what makes her a bona fide star--and what puts her at risk. For no one of her stature has made so many instantly forgettable clunkers: "Dying Young," "I Love Trouble," "The Mexican," "America's Sweethearts," "Mona Lisa Smile." The only films that look like surefire survivors are "Pretty Woman," which for its fans was the "Breakfast at Tiffany's" of the early '90s, and the deliciously passionate "Erin Brockovich," her best film. The rest is mighty lean pickings. America's love affair with Roberts is almost in spite of her movies.

Cruise is even more of a paradox. He's the biggest male star of his era, to be sure, but he has the least definable personality. Until recently, the one role he consistently failed to portray convincingly was... himself. The earnest, enthusiastic person who appeared on talk shows as "Tom Cruise" was painful to watch. He seemed not quite human, wearing his affability like armor. Now that everyone is making fun of him for acting like a Scientology-programmed Martian, he seems more human: he's gained the courage of his obsessions.

It's precisely that obsessive quality, a doggedness verging on mania, that's made him a much more interesting actor than you might think from his All-American public image. He's not afraid to play a jerk. In Steven Spielberg's spectacular nightmare, "War of the Worlds," which is three quarters of a great sci-fi thriller, Cruise is a selfish, working-class divorced dad. He was a coldblooded killer in "Collateral," a preening, nasty, macho guru in "Magnolia," a workaholic sports agent in Cameron Crowe's "Jerry Maguire." Crowe, Barry Levinson ("Rain Man"), Martin Scorsese ("The Color of Money"), Tony Scott (the crass but popular "Top Gun") and Paul Brickman ("Risky Business") all saw in him a cocky, callow hustler in need of a last-reel redemption.

Cruise is dull as a conventional action hero in the second-rate "Mission: Impossible" movies, and seems out of his league putting on great-warrior airs in "The Last Samurai." He's never been a "natural"--you're always aware of his working hard at a character, a diligent student. But he's been smart about choosing directors and material. His real gift, seldom acknowledged, is as a team player. The movies of his most likely to last are rarely the ones he must carry alone as Tom Terrific. He thrives with a strong partner: Jamie Foxx, Dustin Hoffman, Renee Zellweger, Paul Newman. Tom Cruise the heartthrob movie star may be forgotten by 2055, but Cruise the actor, with his odd mix of the dashing and the dogged, will be remembered.

There's a fundamental difference between the big American male stars of Gen X and their predecessors. The icons of the past were men. Paul Newman, Robert Redford and Warren Beatty were young and beautiful at the start of their careers, but they were never "boys." Brad Pitt, Johnny Depp, Will Smith and Cruise, not to mention Matt Damon and Leonardo DiCaprio, are defined by their boy-ishness. They began their careers as kids and, even as they move into their 30s and 40s, have never fully lost their dew.

This was possible only because their parents--the baby boomers--had redefined the culture's perception of maturity, masculinity and aging. The boomers were conscientious objectors to adulthood. (How could maturity be a good thing if you didn't trust anyone over 30?) And as the boomers entered their 40s and 50s, and became the men and women running the Hollywood studios, they still clung to their right to dress like 20-year-olds. How could the Gen-X stars not be boys? That was virtually the only model available. "Youth" had become a lifestyle, a commodity, an ideal, a fetish. Indeed, when a romantic leading man came along who wasn't cut from the American Boy cloth--George Clooney, say--he would be explained as an anomaly, a throwback, a reincarnation of a Cary Grant or William Holden.

How well will the movies of boyish movie stars age? Depp has made the most miraculous escape from the ghetto of teen idoldom, shedding the stigma of a TV pretty boy ("21 Jump Street") with his quirky, uncompromising choices. Depp is an original. His pale, androgynous beauty and playful sexuality make him a seductive romantic figure, but on screen he's the most solitary and ambiguous of figures: his characters spin on their own private orbits. He's the ultimate boy-man, refusing to sit at the grown-ups' table.

Right now, his best bid for immortality rests with two Tim Burton quirk classics, "Edward Scissorhands" and "Ed Wood, " in which he manages to be both as stylized as a kabuki player and touchingly human. He singlehandedly turned "Pirates of the Caribbean" from a routine summer adventure into a fitfully inspired absurdist comedy--and guaranteed its shelf life. The question is whether two of his best movies will get the recognition in the future they failed to get when they opened: Mike Newell's smart, taut gangster drama "Donnie Brasco" will, but Jim Jarmusch's haunting, black-and-white Western "Dead Man," a pristine example of genuinely independent American moviemaking, is too demanding for mass acceptance. More than most, Depp's longevity may depend on the mood of the future. Will our culture still make room for sly, sensitive whimsy?

Both Depp's and Cruise's careers are well into their second acts. Both had made their mark by the time Brad Pitt unveiled his abs in 1991's "Thelma and Louise." Pitt has made 21 movies since then, but it feels as if he's only now, with "Mr. and Mrs. Smith," acquired a full head of steam. His filmography is a reflection of his own internal tug of war with his golden-boy image. The more facial hair he grows, the more seriously he wants to be taken as an actor. Pitt's best films may be in the future. "Fight Club" has the makings of a perennial cult movie, fanatically revered by a select band. But the shock appeal of the dank, clammy "Seven" will wear off. "Ocean's Eleven" is just the kind of clever, stylish genre movie that holds up well--and Pitt's in his best charming-devil mode, though he's just one of its many flavors. The outdoor epic "Legends of the Fall" has its ardent fans, but it's too stolid to stand the test of time. His sexy turn as Mr. Smith opposite Angelina Jolie confirms his movie-star status, but doesn't answer the question: will he be this generation's Robert Redford or its Van Johnson? I'm placing a wager on the former. A small one.

When the studios need a young star with straight-ahead, old-fashioned masculinity, they import him. It was a Brit, Daniel Day-Lewis, who ran around the forest in that most American of tales, "The Last of the Mohicans." A Spaniard, Antonio Banderas, who swashbuckled in "Zorro." And it's the Australian Russell Crowe who steps into "Gladiator's" sandals and assumes heavyweight champ Jim Braddock's fighting stance in "Cinderella Man." Not that the versatile Crowe's in any danger of being typecast as Mr. Macho. He was gay in the Aussie movie "The Sum of Us," and a skittish corporate whistle-blower in Michael Mann's brilliant "The Insider" (Crowe's best performance and his best film, so I'm calling it a keeper in spite of the public's kiss-off). Crowe seems built to last. There's little doubt that Curtis Hanson's twisty film noir "L.A. Confidential" will be watched over and over. Ridley Scott's Roman epic and Ron Howard's crowd-pleasing Oscar winner "A Beautiful Mind" are likely backups, though the fact that both won best-picture Oscars may be a bad sign. Is anybody watching "Gandhi" and "The Greatest Show on Earth"?

Crowe's fellow Aussie Nicole Kidman is a curious case: her stardom is as undeniable as her talent and her beauty, but it's almost unheard-of for such a big star to have so few big hits in her portfolio. Kidman seeks out bold directors and ambitious, challenging projects that tend to look better on paper than on screen. "The Portrait of a Lady," "Eyes Wide Shut," "Dogville" and "The Human Stain" were all movies I wanted to love but couldn't. "The Hours" moved me, and putting on Virginia Woolf's schnoz made her the darling of the Academy, but I have a suspicion that it's a movie for the moment, not for all time. Gus Van Sant's wicked satire "To Die For" was the film that sold me on Kidman: it deserves a long life, but it's already fading from the public's memory. Strangely, there's only one Nicole Kidman movie to date that I'm convinced will live on for decades to come--the exhilaratingly hyper "Moulin Rouge."

There are many more names we could consider. Sean Penn's depth and range assure him a berth in the actors' hall of fame. And Cate Blanchett may get one, too, if she can find some movies that live up to her talents. Clooney? As of now, his legacy will be "Three Kings," "Out of Sight" and "Ocean's Eleven." He, Hugh Grant and Will Smith are the urbane charmers of their age, and if they can add the depth charges and curveballs that Cary Grant slipped into his repertoire, immortality is theirs. Aside from "Ali" and "Six Degrees of Separation," Smith has tended to stay within his (and his fans') comfort zone. "Independence Day" will vaporize quickly--helped on its way to oblivion by the more terrifying "War of the Worlds" --but "Men in Black" has the kind of zany wit that's good for the long haul. The youngest of all these actors, Reese Witherspoon, already has two surefire claims to fame: "Election" and "Legally Blonde." If she were a stock, I'd invest.

Who, in the end, promises to leave the most lasting legacy? When I toted up my speculative lists, the answer came as a surprise. He hadn't even come to mind when I started playing this game. Nor does he comfortably fit into any of the standard categories, being the most unconventional of romantic leads, uncategorizable as either boy or man, and enough of a chameleon to morph from the most wigged-out roles in indie movies to the most mainstream Hollywood action fodder. In fact, when I first encountered Nicolas Cage in "Peggy Sue Got Married," he seemed such a peculiar choice for a leading man I wondered if he'd ever work again. But then came "Raising Arizona," a Coen brothers classic. And "Moonstruck," an almost-perfect Hollywood romance. And David Lynch's Palme d'Or-winning grotesquerie "Wild at Heart." The amiably lunatic comedy "Honeymoon in Vegas." The ravishingly bleak "Leaving Las Vegas," for which he won a well-deserved Oscar. Then came huge, noisy blockbusters, the best of which, John Woo's "Face/Off," was exuberantly demented. Not to mention Cage's inspired double role as twins in the one-of-a-kind "Adaptation."

Cage has certainly made his fair share of junk. But there's a fearlessness in his acting that quickens the pulse of almost every movie he's in. I'd like to believe there's a lesson in this: that the future belongs to those who break the mold, that the most lasting young star of risk-averse Hollywood is the one who's taken the most flying leaps.