Apple Inc began a new era on Thursday without Steve Jobs as chief executive, a momentous shift that surprised investors, but barely dented confidence in the near-term outlook for the stock.

In announcing that he could no longer fulfill his duties, Jobs stepped away from his role as CEO and cleared the way for Tim Cook to take over leadership of one of the world's best known and valuable companies.

The resignation of Jobs -- often mentioned in the same breath as Henry Ford, Thomas Edison or Walt Disney -- made news around the world and prompted tributes from business leaders and fans alike. One headline, in Brazil's O Globo newspaper, proclaimed: Steve Jobs created our world.

Now Cook, 50, must convince investors that Jobs' vision and spirit have been institutionalized within Apple, a company that revolutionized entertainment and communication with its iPod, iPad and iPhone devices.

His first big test could come as early as next month, when, according to a source, Apple is planning to introduce the iPhone 5 -- the sort of occasion that Jobs the showman could turn into a spectacle. Cook is typically more low-key.

Yet he promised in a memo to staffers on Thursday that Apple would stay on the course set by Jobs. Steve built a company and culture that is unlike any other in the world and we are going to stay true to that -- it is in our DNA, Cook wrote.

Investors, at least for now, appear convinced that Apple can keep churning out blockbuster products and oversized profits with Cook in charge. Jobs, who has been on medical leave since January, will stay on as chairman.

Apple shares were down about 1 percent on Thursday, performing better than either the Dow Jones industrial average or the Standard & Poor's 500 index.

Over the course of last year, investors have become more comfortable with the idea of life after Jobs, said Bill Kreher, an analyst with Edward Jones. I think it is encouraging that he will remain with the company as chairman but the real story is that Tim Cook has emerged as a capable successor.

Jobs, who has fought a rare form of pancreatic cancer, is deemed the heart and soul of a company that became the most valuable in the world for a brief period this year.

While it is unlikely that his departure as CEO will derail Apple's ambitious product-launch roadmap in the near term, there are concerns about whether the company will be as creative without its founder and visionary at the helm.

Since Jobs returned to Apple in 1996 -- after an absence of more than a decade -- the company's stock price has climbed roughly 9,000 percent.

In recent years, Jobs' battle with pancreatic cancer has been of deep concern to Apple fans and investors. Even board members have confided to friends their concern that Jobs, in his quest for privacy, was not being forthcoming with directors about the true condition of his health.

His privacy, at times, has also frustrated Wall Street.

I think a lack of clarity of its succession plan in the past has been a distraction so we appreciate that this plan represents a smooth and orderly transition, Kreher said.

Jobs, 56, has been on medical leave since January 17, with his duties being filled by Cook, who was chief operating officer.

Jobs briefly emerged from his leave in March to unveil the latest version of the iPad and later to attend a dinner hosted by President Barack Obama for technology leaders.

His gaunt appearance, however, sparked questions about how bad his illness was, and his ability to continue at Apple.

Cook, a former Compaq executive and an acknowledged master of supply-chain management, has taken over the helm in each of Jobs' three health-related absences.

I will say to investors: 'Don't panic and remain calm -- it's the right thing to do. Steve will be chairman and Cook is CEO, said BGC Financial analyst Colin Gillis.

Jobs did not give details on the state of his health. But oncologists who have not treated the Apple founder said he could be facing several problems tied to his rare form of pancreatic cancer and subsequent liver transplant.

Such problems include possible hormone imbalances or a recurrence of cancer that is harder to fight once the body has already been weakened.

The Apple chieftain has earned a reputation for commanding every aspect of operations -- from day-to-day running to broad strategic decisions -- suggesting he would not give up the job if he had a choice. Even the front yard of his home in Palo Alto is dotted with Apple trees.

(Additional reporting by Bill Rigby, Alexei Oreskovic, Sarah McBride and Jim Christie in San Francisco, Edwin Chan, Lisa Richwine and Nichola Groom in Los Angeles, Peter Lauria, Liana B. Baker and Tiffany Wu in New York, Brian Winter in Tarmo Virki in Helsinki and Georgina Prodhan in London)

(Editing by Maureen Bavdek and Derek Caney)

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