Rupert Cornwell: Fifty years on, we're still in love with Marilyn

Out of America: She created the modern idea of celebrity, and now her stock is on the rise again

Ahead of the curve: Rather than seeing her as a ditzy blonde, we now view her as a proto-feminist (Getty Images)

Celebrities ain't what they used to be. Oh yes, we've got more of them than ever: Brangelina and Clooney, Jagger and Lady Gaga, Oprah, Sarah Palin, not to mention the King of Cool currently resident in the White House, and tens of thousands more. If your taste runs to the recently deceased, throw in Michael Jackson, Whitney Houston, even Princess Di. But quantity doesn't match quality. Which brings me to Marilyn Monroe.

Were she still alive, she would be 86, the same age as the Queen (arguably the biggest living celebrity of them all). But the film actress died exactly 50 years ago today at her home in Los Angeles, from an overdose of barbiturates, in circumstances that remain unclear. The coroner's verdict was probable suicide, but some speak of murder, conceivably involving the Kennedys. Another version is that the overdose was simply an accident.

Over the decades, the unresolved mystery has only added to our fascination at what might have been of a person, talented, beautiful and flawed, who never lived to grow old. Much the same may be said of Diana, who like Marilyn was just 36 when she died in the underpass in Paris. The circumstances, however, could not have been more tragically banal: a car being driven too fast by a drunk driver, in which she was not wearing a seat belt.

Back in August 1962, Marilyn's death struck no small chord: according to The New York Times, shortly afterwards a dozen people in the city committed suicide on a single day, the most ever recorded: "If she had nothing left to live for, then nor have I," said one of them. But it was nothing compared to the emotional earthquake that followed Diana's passing in 1997, when the very institution of the monarchy trembled on its pedestal.

Now, memories of "the people's princess" are already fading. The Queen, fresh from a hugely successful Diamond Jubilee, is more popular than ever, while the headlines that once hounded Diana have switched to the glamorous young duchess who has assumed Diana's most important duty, of extending the royal bloodline. By contrast, half a century on, Marilyn Monroe glows more luminous than ever. In some ways, she is as famous as any celebrity alive today.

This year alone has seen four more books and a memoir devoted to various aspects of her life to add to the hundreds already published. And, all the while, the borderline between reality and representation becomes more blurred. 2011 saw the movie My Week with Marilyn, dealing with her tumultuous stay in London in 1956 to make the romantic comedy The Prince and the Showgirl alongside Laurence Olivier – while right now The Smash, a musical drama not about the actress herself but the backstage story of a fictional show recreating her life, is running on network television.

Or take the billowing white dress she wore in the famous scene on the subway grating in Billy Wilder's 1955 film The Seven Year Itch. Last year it fetched $4.6m (£2.9m) at auction, making it the costliest article of clothing in history.

At one level, the reasons for our fixation are obvious. If any single person stood at the arterial intersections of the American century, it was her. Marilyn was the American Dream made flesh, the girl who overcame humble and disadvantaged origins to attain global fame. In the golden age of movies, she was one of Hollywood's greatest stars, certainly its greatest sex symbol. Among her husbands were perhaps America's most romanticised sporting figure of the 20th century, the baseball player Joe DiMaggio, and its greatest playwright, Arthur Miller. Her many lovers included a president, John Kennedy, and perhaps his brother Robert, members of the family that was America's equivalent of royalty.

To this day, the Marilyn look may be seen in stars like Madonna, Scarlett Johansson and Lady Gaga. But the original, that irresistible blend of luscious sexuality and wrenching vulnerability – the way she made men feel that, if only given the chance, they could make her live happily ever after – is inimitable. Attempts to match it result only in parody. Miller once described her as "a whirling light, all paradox and enticing mystery... Street-tough one moment, then lifted by a lyrical and poetic sensitivity that few retain past early adolescence."

Now, 50 years after she died, Marilyn Monroe is moving upmarket. At one level, this is visible in the effort by the Authentic Brands Group, which acquired her estate in 2010 and is now linking the Monroe name to cosmetics and clothing. No more vulgar trinkets and trashy souvenirs. The company's CEO, Jamie Salter, is aiming at what he calls the "mid-tier luxury business": cosmetics, clothing and the like.

With history's perspective, her image is changing too. Once we thought of her just as a ditzy blonde with a breathy voice and a gift for comedy, emblem of the unliberated 1950s and early 1960s, when women were either sex kittens or housewives. But much of Marilyn's insecurity lay in her desire to be taken seriously, to escape the mould in which the movie industry imprisoned her.

That, she declared in her posthumously published memoir My Story, explained her unlikely union with the fiercely possessive and old-fashioned DiMaggio. Although the marriage lasted only a few months, he loved her to the end of his life.

"In truth we were very much alike," she wrote. "My publicity, like Joe's greatness, is something on the outside. It has nothing to do with what we actually are."

Marilyn was, as anyone who has seen Some Like It Hot can attest, a heck of a good actress. She was also ahead of her times. She had a terror of rejection, yet also the guts to stand up to the mighty studios and set up her own production company. In an age of stifling discretion, she told of how she was molested as a teenager, and of the search for a father figure that may have led to her promiscuity.

She worried about her divided sexuality, the attraction she could feel for women. All thoroughly modern concerns. Ditzy blonde? More a proto-feminist, who'll be just as fascinating 50 years from now.