HAYDEN PANETTIERE AND MILO VENTIMIGLIA SPLIT

Heroes costars Hayden Panettiere and Milo Ventimiglia have called it quits, Usmagazine.com has learned. "It was a lifestyle conflict," a source close to the couple tells Us, adding that the split happened within the last week. "They were in very different places." Adds the insider, "They are still on friendly terms. There are no hard feelings."

The source tells Us that Panettiere, 19, "is young. She likes to go out in the Hollywood scene and that's not his style." Ventimiglia, 31, "has been concentrating on work on his production company [Divide Pictures]," the source adds. The two, who began dating in 2008, had been an open secret for months – in part because of their 12-year age gap.