Another bites the dust. 'Heroes' stars Hayden Panettiere and Milo Ventimiglia have called it quits!

The co-stars, who began dating in September 2007, reportedly broke up "within the last week."

The reason for the split? Lifestyle conflict. Basically Hayden, 19, realized she's too young and isn't finished hitting up the party circuit and Milo, 31, is all about work work work.

There were reports that Hayden was spotted getting cozy with Jesse McCartney on February 9 at Crown Bar in Los Angeles.

According to eyewitnesses, Hayden was caught sitting on McCartney's lap while he kissed her neck.

So could Jesse have been the straw that broke the camel's back? Quite possibly. But sources close to the couple said they saw the breakup coming a while ago. Are you surprised by the split?