Billy Joel fans got a double dose of the veteran rocker Saturday night in the Hamptons during a benefit for the S.T.A.R. (Standing for Truth About Radiation) organization. Joel, asked to play by his ex-wife, Christie Brinkley, a member of S.T.A.R.’s Board of Directors, gave a concert and answered audience questions at Southampton High School for three hours. The sold-out event attracted celebrities such as Alec Baldwin, Kristin Davis, Andrew Cuomo, Jerry Seinfeld and Michael J. Fox and Tracy Pollan. “We’re so glad to have Billy raise lots of money to help us shut down the Millstone Plant,” Brinkley said of her organization’s effort to close the nuclear plant located 15 miles across the Long Island Sound in Waterford, Conn. “There’s a big rise in childhood cancers on Long Island, most likely from this plant and Brookhaven Laboratory. I, like all mothers, want to protect my children from cancer.” After the concert, about 100 guests paying $100 a ticket attended a private cocktail and dinner party at their oceanfront Southampton estate. Kristin Davis stayed by Alec Baldwin’s side all night and even sat on his lap during dinner while Heidi Klum and her husband boogied on the dance floor.