The Oprah Winfrey Channel

She may be leaving her show, but the queen of daytime TV has grand plans to expand her broadcasting empire. David Usborne reports

"Big day ...tune in my tweet friends." So read the short message on Twitter from a certain Oprah Winfrey yesterday morning. As if they, and everybody else, weren't tuned already. Word had been leaked late Thursday that this was going to be the fateful date on which America's Great Aunt of Talk would finally hand in her notice.

"After much prayer and careful thought," she began, pausing to scan the studio audience in Chicago, her eyes glistening with emotion. Then she said it, and the entertainment industry was changed for good: her eponymous show that began in 1986, is still watched by 42 million viewers every week and is broadcast in 145 countries, will go dark in September 2011. "Twenty-five years feels right in my bones and feels right in my spirit," she offered humbly.

If the world did not stop turning it is because not everyone is terribly surprised. Speculation has raged for weeks that the sell-by date for The Oprah Winfrey Show was, for better or worse, approaching. Ms Winfrey, who over the years has boosted presidents, poets, authors and even ailing car manufacturers (and infuriated a few other people too), is 55 now, having started her broadcast career, on Tennessee radio, as a teenager. And, according to Forbes, she is worth $2.7bn (£1.6bn). She may want a rest. She has places to be other than your front room and mine.

But vanish from the dial she will not. Chicago, it seems, will have to relinquish her – a blow bigger even than missing out on the 2016 Olympic Games – but its loss will be Los Angeles's gain. Ms Winfrey will be going to the West Coast to focus her prolific energies on the Oprah Winfrey Network, or OWN, a new cable channel that she half owns with Discovery Communications and which, after serial delays, is due to open for business in January 2011.

Whether her talk show will simply resume on OWN is not something anyone seems to have decided yet – or if they have, it's a secret. But regardless, the shuttering of the show as it now exists will be a seismic event on the American cultural landscape and a sad one for her legions of fans. In this country, at least, everyone knows someone for whom Oprah is a daily fix that they cannot live without. That's why they are being given 22 months for weaning.

It will take a while to sink in. "It's impossible to comprehend the show's reach," noted the co-host of ABC's Good Morning America, Robin Roberts. "She has been one of the family for Americans for 25 years," chimed in Tina Brown, editor of The Daily Beast website. And as professional pundits and ponderers turned to the question of who might fill her shoes, Howard Kurtz, media editor of The Washington Post, voiced the general sentiment: "I'm not sure that Oprah can be replaced."

Many things have made Oprah the broadcasting powerhouse that she is. Much just comes back to her and her story. Born in Mississippi to teenage parents, she was raised in extreme poverty, sometimes to the point of wearing clothes made from potato sacks. She was raped at the age of nine and at 14 had a son who died shortly after birth.

"I came from nothing," she wrote in her 1998 book Journey to Beloved. "No power. No money. Not even my thoughts were my own. I had no free will. No voice. Now, I have the freedom, power, and will to speak to millions every day – having come from nowhere."

But her life trajectory became the quintessential American story. From radio, she went to a Nashville TV station to read the news, where she drew the attention of another television station in Baltimore. She read the news there, too, before being moved to start a local talk show, People are Talking. Then Chicago came calling. She was hired first to co-host a breakfast TV show but quickly moved on to create a daily programme, called The Oprah Winfrey Show. It was deciding to put the programme out for syndication that gave the Ms Winfrey the platform on which to build her phenomenal media empire, at the centre of which is her company, Harpo Productions. She is much more than her daily show. Aside from her 50 per cent interest in OWN, she is a magazine publisher and a producer of musical theatre, films and numerous other successful television shows including Rachael Ray and Dr Phil.

She is also the nation's arbiter-in-chief. Political analysts reckon that when Barack Obama was frantically challenging Hillary Clinton in last year's primary races, Ms Winfrey shifted one million votes into his column by taking his side. When she said in 1996 that mad cow was a reason she would never eat another burger, the beef industry sued her for $11m because they feared her influence so deeply. (The industry lost.)

Most famously, Ms Winfrey has made America read more and, through her book club, more or less told it what to read. "If it is the end of her daily talk show, we probably won't see something else to match its overall potential impact on book sales in the broadcast arena any time soon," said Stuart Applebaum, a spokesman for Random House, the New York-based publisher.

"Oprah Winfrey is in a category of her own," concedes Robert Thompson, professor of television and popular culture at Syracuse University. "This is a great American story and like any great American story it's supersized." Assessing the scope of her impact, he went on: "We have come to use the term 'Oprahfication' in almost same way we use a term like 'Hellenisation'. And it's not completely inappropriate. She was able to colonise cultural territory the way Alexander the Great was able to colonise physical territory."

The hyperbole of yesterday reflects the hyperbole that came so often from her show, not least when she was surprised at the beginning of the current season in September when 20,000 people gathered on the Magnificent Mile in Chicago and, flash mob-style, dipped and dived in unison to the Black Eyed Peas performing Good Day as a gift to her. She seemed flabbergasted.

Other big moments for the O-scrapbook include landmark celebrity interviews: Liberace months before he died; Michael Jackson in 1993 revealing his skin pigmentation disease; Tom Cruise going bonkers for Katie Holmes (jumping on her couch); and this week snagging the Republicans' Sarah Palin before her book tour. There was the day in 1988 when she came on set after losing 67 pounds. Nor will her fans forget the installment in 2007 when nearly 300 audience members were given boxes to open, each with a key inside. Presto, a new Pontiac for everyone! For free! That's big.

The phenomenon that is Oprah, however, may also have to do with her ability to show vulnerability, to talk about the pain she has had in her life and, thus, to persuade others to open up as if in the confessional. She does it, however, with a sense of the spiritual rather than the tabloid, and the formula has been enduringly successful. "She has kind of created the culture of 'sharing is caring'. People feel they can go on her show and say things they don't want to say anywhere else," Tina Brown said. Added Russell Simmons, the Hip-Hop impresario: "I call her 'Queen of the New Consciousness' because she did so many things to change lives."

It also means that when she has committed gaffes, she is forgiven. Among them was the embarrassment of giving her all to promote a so-called memoir by writer James Frey. Called A Million Little Pieces, it shot to the top of the best-sellers lists because of her endorsement. But it turned out that large sections were actually fiction, and on a subsequent much watched edition, entitled "The Man Who Kept Oprah Awake at Night", the doyenne of daytime TV took Frey to task for his lapse. He went home with his tail between his legs, and the Winfrey name was restored.

The weeks and months ahead will reveal what OWN will look like and what roles Ms Winfrey will have there, both on and off the air. The industry gossip on OWN is that it has largely been at sea since its creation was announced in early 2008. The launch date was delayed twice amidst management turnover with the chill winds of declines in advertising. There were those who started to question if Ms Winfrey was really serious about her commitment to it.

Those doubts should now evaporate. "I think the indication here is that she is serious about this," said Andy Friendly, former chief of King World Productions, who worked with Ms Winfrey. "This will be the next chapter in her life and her career, and she's giving up the richest oil field in the business to do it."

But for the television industry something very important has happened already: she has placed her bets on cable and turned her back on the broadcasting networks. While it is mostly ABC-affiliate stations in cities across the US that air her show, the distribution rights belong to CBS. The loss of Ms Winfrey is a serious blow to both networks, whose places on the cultural landscape are eroding, thanks largely to the rise of cable channels like HBO, ESPN and Discovery. In its tug-of-war with traditional network broadcasting, cable yesterday gained a mile.

"This show has been my life, and I love it enough to know when it's time to say goodbye," Ms Winfrey told her fans yesterday. Then she walked into the mostly female audience for a frenzy of hugs. On Monday, though, it will be back to work, to finish the rest of this season, plan the next and then plot the future.

After Oprah Who will be the new doyenne of daytime?

*Ellen DeGeneres

Oprah shared the cover of her O magazine with Ellen last month, and a fortnight ago invited the rival host on to her afternoon chat show. Soothsayers now see that as a symbolic gesture which makes DeGeneres her heir apparent.

*The hosts of The View

The five wise women shot to prominence during the 2008 Presidential election, when left-leaning Whoopi Goldberg and Barbara Walters clashed vigorously with the panel's "token Republican," Elisabeth Hasselbeck. They've been essential viewing ever since.

*Tyra Banks

Young and breezy, Oprah's departure will leave Banks as America's most prominent black daytime talk show host. However her stock as an opinion-former isn't helped by the show's tabloid news values, which have sometimes seen it dubbed lightweight.

*Judge Judy

There's not much chat on her show – but Judy Sheindlin, who presides over a small claims court in which guests resolve their personal grievances, boasts 10 million daily viewers, and is perhaps the perfect everywoman for the world's most litigious nation.

*Martha Stewart

A spin in jail for tax evasion hasn't dented Martha's popular appeal, or her ability to separate the middle classes from their hard-earned cash. But she'll be 70 by the time Oprah leaves the airwaves. Does that put her over the hill?

*Michelle Obama

On Good Morning America, the former magazine editor Tina Brown suggested that Michelle could hit the chat show circuit once her husband is out of the White House, saying she shares Winfrey's warmth and empathy. But Michelle will hope to be unavailable for seven more years.

Oprah in numbers

1986

The year in which the show was first broadcast nationally

42 million

The number of viewers The Oprah Winfrey Show averages each week

145

The number of countries in which The Oprah Winfrey Show is broadcast

$2.3 bn

Oprah's estimated fortune, according to Forbes

3.5 minutes

The length of Oprah's speech yesterday, announcing the show would end in 2011 after its 25th season