Microsoft has offered to pay nearly $45 billion to buy Yahoo, the Internet search engine.  Mike O'Sullivan reports, the software giant's bid is a challenge to Google, which dominates the online advertising market.

Microsoft made a surprise offer of $31 a share for Yahoo stock late Thursday, and Microsoft Chief Executive Steve Ballmer said during a conference call with reporters Friday the potential alliance would help web users, advertisers, and the owners of online websites.

"When you combine the strengths of our two companies, the result will be an incredibly efficient and competitive offering for consumers, for advertisers, and for publishers," he said.

Yahoo said in a statement it will study Microsoft's bid carefully and promptly.

Microsoft says the online advertising market has annual revenues of $40 billion, and it expects the market to nearly double in three years.  Industry monitor Comscore says Google has more than 60 percent of the Internet search market, and nearly five times the share of second-ranked search site Yahoo.

Yahoo's stock reached a four-year low this week, and the Microsoft offer is 60 percent higher than Yahoo's closing price on Thursday. 

An earlier offer by Microsoft was rejected by former Yahoo chief executive Terry Semel, who left last year under shareholder pressure.  Yahoo co-founder Jerry Yang stepped in as CEO in June.

Jon Fisher, a portfolio manager at Fifth Third Asset Management, questions the wisdom of the offer, saying Microsoft is paying full price for a broken business.  He notes that this is Microsoft's biggest acquisition, and says the company usually buys innovative small firms that serve a specialized market. 

"This is not typical of the types of acquisitions that Microsoft has done over the last 15 or 20 years," he explained.  "So I question their ability to integrate such a large business.  Two, they do not typically buy broken businesses.

Microsoft chief Ballmer says his company is determined to bring the two tech giants together.

"We've thought, and I've personally thought, long and hard about, and we're very, very confident is a right path for Microsoft and for Yahoo," he added.

The deal will cost Microsoft $44.6 billion in cash and stock.   

A Justice Department spokeswoman told the Associated Press the department's anti-trust division will be interested in looking at the competitive effects of the proposed acquisition.