Kurt Cobain at 40

He would have hit one of life's milestones this week. But, 13 years after his death, what is his true legacy?

In the space of two years, Kurt Cobain soared to global stardom then plummeted into a pit of drugs and despair so deep he ended his life in the most violent manner. After a hit of heroin he blasted his brains out with a shotgun.

Yet as what would have been his 40th birthday approaches on Tuesday, his name lives on. More importantly, his legacy generates huge sales. Despite releasing only three proper albums with his band Nirvana in his lifetime, almost 13 years after his death the Kurt industry is stronger than ever.

To hammer home the point, last year he became the first to eclipse Elvis as the biggest earning dead celebrity, generating in excess of £26m according to Forbes magazine. Though most of that money came from his widow Courtney Love's sale of just a quarter stake in his back catalogue, it illustrates the worth of his material: thrashy, wild, angry but lucrative. Even the rights to his diaries, published four years ago, earned a £2m advance.

His musical legacy and social significance are perhaps even more profound. Pundits suggest Cobain, and Nirvana, changed the music business for ever.

"I think he single-handedly changed the way major record labels and TV companies thought about how rock music is sold to a mainstream audience," said Paul Rees, editor of Q magazine. "This was angry punk rock, not something that had been put together by a committee of marketing people. Here was a guy who was so scruffy he wore a cardigan his grandad might have worn, who sang about rage and anger yet sold millions. He changed what was acceptable. His influence is everywhere. Any band who picks up a guitar, from the Arctic Monkeys to My Chemical Romance, is aware of what Kurt Cobain did."

Devotees will mark Tuesday's anniversary by gathering at his unofficial shrine, a graffiti-strewn spot under the Young Street bridge in his hometown of Aberdeen, Washington, where he claimed to have slept rough as a homeless teenager. The town is just 100 miles from Seattle, famous for only two things - Microsoft and being the birthplace of grunge, of which Cobain was king.

Few predicted Nirvana would make it big, certainly on the strength of their first album Bleach - championed by DJ John Peel but loved only by the cognoscenti. The major label Geffen released second album, Nevermind, pressing no more than 50,000 copies. But the slacker generation adopted the single "Smells Like Teen Spirit" as its anthem, chanting en masse Cobain's anguished scream: "Here we are now, entertain us." Within a month of its release in late 1991, Nevermind had sold half a million; global sales now stand at 24 million.

The success gave a massive boost to rock music. Every record company wanted its own Nirvana, and every band wanted to sound like them. But success was no comfort to Cobain. A troubled childhood of divorced parents and alienation left him deeply depressed. Crippling, undiagnosed stomach pains led to self-medication with heroin - a drug he despised - and an uneasy marriage to fellow junkie Courtney Love worsened his mental state. After one failed suicide attempt in a Rome hotel room in March 1994, he succeeded a month later. Quitting the Exodus Recovery Center rehab clinic in Los Angeles, he flew back to Seattle and lay low for a week before heading to his Lake Washington home and blasting himself in the head with a shotgun.

The Radio 2 DJ Mark Radcliffe, who frequently played the band's songs, said: "Here is this guy who was clinically depressed for most of his adolescence and adult life and the only thing that got him through that was his dream of making it in a rock'n'roll band: that idea that all the problems will melt away if I become incredibly successful.

"And when he makes it, everything is the same. So when he gets there he is staring into the abyss. His dreams have come true, but the mists haven't cleared. So where do you go from there?"

Since Cobain's death, there has been a slew of Nirvana releases. Within months there were two live albums and more recently there have been a box-set of out-takes and a best-of collection. His life and death have inspired numerous films, including Nick Broomfield's documentary Kurt and Courtney and Gus Van Sant's Last Days.

Paul Smith, the singer with the Mercury Prize-nominated group Maximo Park, was only in his early teens when Nirvana were at the peak of their powers but said Cobain's music had a lasting influence. "The whole of the band has gone and bought the reissues of their vinyl," he said. "At the time I was a bit sceptical about the noisier stuff, but now we've come to appreciate it with that bit of distance."

The Radio 1 DJ Zane Lowe added: "To me he was an amazing songwriter who was both blessed and cursed with this voice that cut through. I've read the books, like everybody else, but analysing his life and death has never really taken me any further than the music. Those albums are the real testament."