Ben S. Bernanke was confirmed by the U.S. Senate today for a second term as chairman of the Federal Reserve. The Senate voted 70 to 30 to confirm the 56-year-old former Princeton University professor. Bernanke overcame opposition from lawmakers who said he failed to head off the worst financial crisis since the Great Depression and then overstepped his authority by participating in rescues of firms including New York-based insurer American International Group Inc. and Citigroup Inc.