"And I was like, ‘That seems totally nuts to me,' " Portman said. It's the way it works, I guess. I'm not particularly proud that in our country I can get a meeting with a representative more easily than the head of a non-profit can."

Well, who is? But it is the way it works. Stars - movie stars, rock stars, sports stars - exercise a ludicrous influence over the public consciousness.

Many are happy to exploit that power; others are wrecked by it. In recent years, stars have learned that their intense presentness in people's daily lives and their access to the uppermost realms of politics, business and the media offer them a peculiar kind of moral position, should they care to use it. And many of those with the most leverage - Bono and Angelina Jolie and Brad Pitt and George Clooney and, yes, Natalie Portman - have increasingly chosen to mount that pedestal. Hollwood celebrities have become central players on issues like development aid, refugees and violence in Darfur.

Activists on these and other issues talk about the political power of stars with a mixture of bewilderment and delight. But a weapon that powerful is bound to do collateral damage. Some stars, like George Clooney, regard the authority thrust upon them with wariness; others, like Sean Penn or Mia Farrow, an activist on Darfur, seize the bully pulpit with both hands. "There is a tendency," says Donald Steinberg, deputy president of the International Crisis Group, which seeks to prevent conflict around the world, "to treat these issues as if it's all good and evil." Sometimes you need the rallying cry, but sometimes you need to accept a complex truth.

Celebrities, and especially Hollywood celebrities, have always engaged in public philanthropy. In An Empire of Their Own, Neal Gabler describes charity dinners of the 1930s where movieindustry moguls would gather at the Hillcrest Country Club and outbid one another with gifts to the United Jewish Welfare Fund and other Jewish causes.

In later years, movie stars and politicians treated Marvin Davis's Carousel Ball, to benefit diabetes research, as a command performance. But the "grip 'n' grin celebrity stuff," as the publicist Howard Bragman calls it, has largely passed into history. Nowadays, says Bragman, "you've got to have something for People magazine to shoot you at. You can't just get $20 million a picture; you've got to serve turkey to the poor too." The old Hollywood philanthropy was passive and dutiful. In those days stars were shaped by the studio system before being delivered to the public. Now, in the era of People and the E! channel and the global swarm of paparazzi, stars shape themselves, and their brands, through their own public acts. And their audience is not just fans but everyone; a star's life is a kind of public movie. You have to do something with all that attention. As Portman says, "If they're going to follow me around, I'd rather talk about Finca than what dress I'm wearing or who I'm dating or whatever nonsense people care about."

Most celebrities no longer have charities; they have causes. Eve Ensler has enlisted Jane Fonda, Salma Hayek, Jessica Alba and others to perform her play The Vagina Monologues to raise money to combat sexual abuse against women. Next month, Ensler's organisation, V-Day, will celebrate its 10th anniversary with a weekend of "Superlove" in the New Orleans Superdome, where Ensler's team of stars will celebrate the resilience of "Katrina warriors" - women in the region who have suffered physical or emotional abuse. Ricki Lake campaigns for natural childbirth; she understands, she says, that "doing something pro-mom and pro-baby and pro-midwife" is her "life's work." Paris Hilton was supposedly planning to go to Rwanda soon after she finished her jail stint, and Playing for Good - an organisation that staged a three-day "international philanthropic summit" on the resort island of Mallorca, with "the acclaimed actress and philanthropist Eva Longoria" as the host - had hoped to use Hilton's redemptive escapade as an episode in a reality show to be titled The Philanthropist. The show appears to be in turnaround.

An entire industry has sprung up around the recruitment of celebrities to good work. Even an old-line philanthropy like the Red Cross employs a "director of celebrity outreach." Oxfam has a celebrity wrangler in Los Angeles, Lyndsay Cruz, on the lookout for stars who can raise the charity's profile with younger people. In addition to established figures like Colin Firth and Helen Mirren, Oxfam is affiliated with Scarlett Johansson, who has visited South Asia (where the organisation promotes girls' education) and is scheduled to go to Mali. Cruz notes that while "trendy young people" are attracted to the star, Johansson had "great credibility with an older audience because she's such a great actress."

The stars themselves have their own retainers to fend off the celebrity recruiters and to screen and sift charitable opportunities; publicists say their major clients get dozens of requests every week. The more deeply committed figures, like Angelina Jolie, retain firms like the Global Philanthropy Group, which, according to a representative, offers "comprehensive philanthropic management." This includes establishing and staffing foundations, bringing in subject-area experts or even helping the novice philanthropist figure out what he or she actually wants to do.

Both the stars and the causes, in turn, depend on corporate sponsorship. It is the sponsors who pay for the galas at which the stars raise money for their causes; sponsors normally pay for the stars' first-class air tickets and hotel suites. Corporations need causes as much as stars do. Like the stars, they understand that they must shape and protect their brand identities; and they understand that those identities will be judged by the broad public, through public acts. As Howard Bragman puts it, "Celebrities, sponsors and a cause: it's the golden troika of branding."

The costs are small compared to the goodwill. Thus Alicia Keys's Keep a Child Alive, which provides antiretrovirals to victims of AIDS in Africa, has 78 "corporate partners," including  CBS, Continental Airlines, Condé Nast and Chanel, to pick a few from the C's. And just as stars have philanthropic managers to help them with causes, corporations with a cause can turn to celebrity recruiters to find just the right star. Thus Rita Tateel, who describes her occupation as recruiting and coordinating celebrities for "causerelated marketing and public relations," recently hooked up Purina, which wanted to support "small animal-rescue organisations," with Emily Procter, a star of  CSI Miami, who, Tateel says, "lives and breathes animal rescue."

The celebrity-philanthropy complex reflects the hierarchy of stardom itself.

Ricki Lake and midwives, or Emily Procter and animal rescue, occupy humble rungs; at the very top stand the global celebrities and the global causes - Angelina Jolie and refugees, George Clooney and Darfur, Bono and foreign aid. Corporate sponsors tend to drop out at these altitudes, both because George Clooney doesn't need anyone to buy him a plane ticket and because few corporations are likely to view Darfur as a good branding opportunity.

These are the moral heights of the celebrity-philanthropy world, and they were first reached by rock musicians.

George Harrison and Ravi Shankar led the way with their Concert for Bangladesh in 1971, but the phenomenon really took off in 1985, when the Irish rocker Bob Geldof helped organise Live Aid to benefit starving Ethiopians. That same year, several British entertainment figures established Comic Relief, a night of performances by Britain's leading comics and other celebrities, broadcast by the BBC, with the goal of raising money and awareness to fight poverty in Africa. Each year, several of the stars would travel to Africa to monitor how the money had been spent and then report back to the national audience.

It was Bono, the lead singer of U2, who expanded this specifically British phenomenon into the US. Bono's own involvement began with Live Aid; in 1997, he agreed to join the campaign to gain debt relief for the world's poorest countries, an effort that involved highlevel lobbying in Washington and elsewhere. In 2002, Bono established DATA, an advocacy group focused on debt, foreign aid, trade reform and AIDS in Africa. DATA demonstrated that the singer was in this for the long haul, and that he cared not just about clarion calls but also about the tedious business of developing expertise and political organisation.

Bono used the leverage of fame in a way that few stars had before. He studied the issues, and he lobbied not just US representatives but their aides.

He approached Bill Gates and George Soros, whose vast wealth has enabled them to become central figures in the world of advocacy, and made them his partners. He shouldered his way into the places where the world's most consequential decisions are made Davos, the G-8, the World Bank, 10 Downing Street and the White House.

Bono offered decision makers an implicit bargain: do the right thing, and I'll say so in public.

Sometimes you have to see this hydraulic action to appreciate its raw power. In late January, George Clooney spoke at the UN after returning from his first trip to peacekeeping sites as the UN's "messenger of peace." Clooney gave an exceptionally judicious and high-minded speech to the press. The first question was "Is this worth more than an Oscar?" Afterwards, Clooney sat for interviews with CBS and NBC, CNN and the BBC, Al Jazeera and Al Arabiya and several European networks. That was instant outreach.

As stars increasingly move into the world of diplomacy, they will run up against the same brick walls that diplomats do. What then? Bono has for years absorbed flak from the left for praising President Bush, and for accepting a half or even a quarter of a loaf on issues like increasing aid to Africa. Clooney now gets the same harsh treatment for finding nuances on Darfur and reaching out to the Chinese. One has to be prepared for this too.