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Olivia Wilde's husband, handsome Italian prince Tao Ruspoli, wasn't by her side when she glittered in a dazzling Marchesa gown at last month's Golden Globe awards -- or when she hit the red carpet for December's premiere of box office smash Tron: Legacy.

And now the world knows why: The couple have separated after eight years of marriage, a rep for Wilde, 26, told Tuesday.

"They have been separated for a while," one insider explains of the pair.

What went wrong?

"It was Olivia's decision...for the typical Hollywood reasons," another source close to Wilde tells Us. "Her career has exploded and she saw being married is not as much fun."

Indeed, six months after meeting, the couple eloped (marrying in a school bus in rural Virginia) when Wilde was just 18. The son of late Italian Prince Dado Ruspoli, filmmaker/musician Tao lives in California and, at 35, is nine years older than the sexy House M.D. star.

Continues the pal of Wilde, who has about half a dozen film roles on her plate (including this year's Cowboys and Aliens with Daniel Craig, Harrison Ford and Sam Rockwell): "She is a hot property now ... it was weighing her down being married."

Her estranged husband, meanwhile, "is heartbroken and really wanted to stay together."

Adds a third source: "Olivia got married really young. She's done a lot of growing since then, just like anyone would...they've just grown in different directions."