This is not the end of Oprah Winfrey.

Maybe not even the end of her TV show.

True, in a tearful announcement yesterday, the self-made media mogul - host of the highest-rated talk show for 23 years, seen in 145 countries, worth a breathtaking $2.7 billion - confirmed for her audience what had been rumored for weeks: that the show would be ending.

"I love this show," said the 55-year-old Winfrey. "This show has been my life, and I love it enough to know when it's time to say goodbye."

But when she ends her run at ABC on Sept. 9, 2011 - 25 years to the day since she began - she'll be taking her stuff to cable. To the Oprah Winfrey Network (OWN), to be exact, which will debut in 80 million homes. At OWN, she'll try to extend the Oprah brand as far as she can.

But it may well be the end of broadcast Oprah. And that says much about her and the state of conventional TV.

Betting on the future. This move amounts to a big wager on the future of TV. Winfrey is betting that the migration of viewers from broadcast (free and open to all) to cable (not and not) will continue.

Even Oprah's stalwart audience - an estimated 42 million U.S. viewers a week - is eroding. It's still the most popular talker on midday TV, but its viewership is half of what it was 10 years ago. Doubtless Winfrey is pondering ways to keep growing her empire.

So what will she do in 20 months? She and her company promise big, vague things. The original announcement Thursday by Tim Bennett, president of Harpo Productions Inc., Winfrey's company, said, "If you think the last quarter century has been something, then 'don't touch that dial' as together we plan to make history in the next 20 months . . . and beyond."

Cable TV presents many tantalizing options. She could do a daily or weekly show. As Robert Thompson, director of the Bleier Center for Television and Popular Culture at Syracuse University, puts it: "It's hard to imagine that you'd launch an Oprah Winfrey Network without an Oprah Winfrey Show."

But Winfrey doesn't need to have a show; she just needs to have a presence. On OWN, she can be like Macy's at the mall: the anchor that draws viewers to all the other content. If OWN is the only place viewers can see her, and they want to see her, OWN may attract the subscriptions and ad revenue that spell cable success.

Broadcast loses. Broadcast TV loses one of the biggest shows in its history. ABC loses the revenue from Oprah - estimated at hundreds of millions annually - and CBS may lose the syndication rights. (CBS says it's negotiatimg to keep working with Winfrey.) Local stations take a major hit, too, losing the audience and revenue that carry over from Oprah to local shows.

The sofa that changed the culture. What has Winfrey accomplished? She remade the daily TV talk show, that's all, and created a phenomenonally influential force in American culture.

The show concentrated at first on sensationalistic topics. Winfrey is often credited with (or blamed for) inventing "tabloid TV," which gave the world Jerry Springer and his spawn.

But in the 1990s, she found the power of the sofa. Sitting with Winfrey, her guests discuss their private lives, problems, and emotions, frankly, openly. Much as if they were sitting in America's living room.

The rich and famous dish, diss, and vent, too.

Michael Jackson discussed his skin disease with Winfrey as if with a trusted aunt. Tom Cruise told her he loved Katie Holmes. Over two entire shows, Whitney Houston recounted her addiction and marital problems.

If that were all, Oprah might have remained a midday sobfest aimed at women. But the show's huge, loyal audience gave it unprecedented impact.

Winfrey took that audience into culture. Beginning with The Book of Ruth by Jane Hamilton in 1996, her book club, in which Winfrey selected a book and often interviewed the author, created instant best-sellers. Viewers that might otherwise not have heard of hundreds of deserving books and authors now did - and went to buy them, even in an era of the decline of publishing.

With spectacular judgment, Winfrey embraced celebs, issues, and figures in the news. Viewers saw both Chesley Sullenberger (who landed US Airways Flight 1549 on the Hudson River) and Beyoncé, both George W. Bush (talking about his past drinking) and Mike Tyson. She led her audience where most shows feared to tread, and millions followed.

Michael Delli Carpini, dean of the Annenberg School for Communication, said: "Oprah began to discuss race, family issues, public policy, bringing experts and people involved, getting them to speak knowledgeably about them. Her show became a way many people learned about the issues and the news."

Newsmakers clamored to be on her show. Now, not only did Tom Cruise come leaping over the couch - but also power-brokers and presidents, self-improvement gurus and generals, supermodels and spiritual guides sat with her. And Nobelists (Nelson Mandela). She launched the television careers of doctors - Phil and Oz. She gathered clout. And her support of Barack Obama was a signal event in the election of 2008. The recent appearance of Sarah Palin on Oprah drew the show its highest ratings in two years.

"Politicians tried to get on her show, just as people now clamor to be on Comedy Central's The Daily Show or on MTV," says Delli Carpini. "Such shows have now become crucial venues on which public figures can get issues out. And they get longer face time, more time to explain themselves on these shows than you can on nightly news sound bites."

Her viewers trust Winfrey as guide, says Delli Carpini, "so a lot of information and culture reaches people who would normally not get that information."

Oprah created an alternative community, its own social world. Its host has parlayed that into a media empire, including O magazine (2.37 million circulation as of 2008), Oprah.com (70 million page views a week), books, and a 24-hour satellite radio channel, Oprah & Friends, on XM Satellite Radio. Her move to cable is just that - one more move.

A gamble that's a gamble. "Oprah is taking a risk," Thompson says. "She was on a network you could find anywhere, and now she's going to be on one of those numbers on your cable box that sometimes can be hard to find." Oprah is the foundation of her empire because millions see her five days a week. "It's an important part of people's daily ritual," says Thompson. "I wouldn't want to bet the farm that she can replicate that."

Thompson also points out that 20 months makes for a long goodbye - "longer than most talk shows run."

Contact staff writer John Timpane at 215-854-4406, jt@phillynews.com, or twitter.com/jtimpane.