LOS ANGELES (AP) – Apple Inc., the world’s most valuable company, is now worth $500 billion – and counting.

The company’s market value passed the rarefied half-trillion-dollar mark in trading Wednesday, joining just a handful of firms to have reached those heights, though none managed to stay there.

For much of last year, Apple was running neck and neck with Exxon Mobil Corp. for the title of most valuable firm. But over the last month Apple has catapulted nearly $100 billion past Exxon, which is now worth more than $400 billion. Market value is a measure of the total combined value of all of its outstanding shares of stock.

Apple’s stock jumped $7.03, or 1.3 percent, to $542.44 on Wednesday, putting the Cupertino, Calif., company’s value at $506 billion when regular trading ended. The lift came a day after Apple sent invitations to members of the media for an event next Wednesday, at which it is expected to release its third-generation iPad.

Only a few companies have reached the $500 billion mark, the financial equivalent of scaling Mount Everest. But the footing at that altitude is tricky, and companies that reached the summit soon slid down again.

Exxon stayed at about $500 billion for two brief periods toward the end of 2007, according to an analysis by the Associated Press. Microsoft Corp., Apple’s longtime computing rival, passed the $500 billion line in 1999 and then again in 2000, and even reached $600 billion for one day. But Microsoft’s stock has remained static for most of the last decade, and the company is now worth about $267 billion.

Also passing $500 billion a decade ago were Cisco Systems Inc., Intel Corp. and General Electric Co. Those firms are worth far less now, with Cisco and Intel closer to $100 billion and GE at $200 billion.

Apple’s stock price has risen 500 percent over the last five years and more than 4,500 percent since 2002, soon after Steve Jobs introduced the iPod, the first of his groundbreaking trio of mobile devices that later included the iPhone and the iPad. The company posted revenue of $46.3 billion in the quarter ended Dec. 31, one of the largest quarterly hauls ever for a technology firm.

Analysts have said that considering its current size, it will be difficult for Apple to continue growing at such a fast clip. They noted that even 10 percent growth in annual sales would mean the company would have to do an additional $15 billion in business.

Still, with skyrocketing iPhone sales and the likelihood that Apple will begin selling many more millions of devices in huge untapped markets such as China and Brazil, Wall Street observers remain optimistic.

Apple “continues to be one of our top fundamental favorites,” Sterne Agee analyst Shaw Wu wrote in a note to investors Wednesday. Wu also said he believed that Apple’s stock price would rise an additional 14 percent by the end of the year, to $620 a share.

That would put Apple’s market value at $577 billion, tantalizingly close to the $600 billion mark.