WASHINGTON — James Dean is leaning back, drawing on a cigarette, looking away from the camera as a mirror behind him in the black-and-white photograph creates a shadow image of a good-looking young man who, even in a quiet moment, seems remote, resistant to what’s expected of him.He’s cool.But who else is? What does it take? And what is it about America that’s defined cool to the world?The National Portrait Gallery has decided it knows, and on Friday it opened the exhibit “American Cool,” with 100 photographs of American men and women who define cool.Portraits of three Texans made the cut: iconic entertainers Willie Nelson, from Abbott, and Selena, from Lake Jackson, and boxer Jack Johnson, from Galveston, who became the first African-American to win the world heavyweight championship.“ ‘American Cool’ is about America’s greatest cultural export — cool — and who embodies it,” Kim Sajet, the director of the National Portrait Gallery, said at the media preview for the exhibit.During his brief life, Dean created a new American icon — the rebellious teen — in the stifling atmosphere of the 1950s. He defined himself in the film Rebel Without a Cause, his most-celebrated role, before dying in 1955 at age 24 when his Porsche crashed.And being cool, according to this exhibit, is very much tied to being a rebel.Most of the personalities photographed here are in the arts: actors Marlon Brando, Paul Newman and Audrey Hepburn; jazz musicians Miles Davis, Thelonious Monk and Charlie Parker; singers Frank Sinatra, Elvis Presley and Chrissie Hynde; painters Georgia O’Keeffe and Jackson Pollock.A few sports figures make the grade, such as boxer Muhammad Ali and basketball legend Michael Jordan.But there are no elected politicians or anyone from the business world beyond Apple’s co-founder, the late Steve Jobs.The show’s curators, Joel Dinerstein and Frank Goodyear III, who described themselves in an interview as cultural historians, are quick to say they aren’t making subjective judgments. They aren’t deciding who’s in with the “in” crowd.They’ve laid out four criteria of cool, and each of the subjects picked for the show had to have at least three.“An original artistic vision carried off with a signature style; the embodiment of cultural rebellion or transgression for a given generation; iconic power or instant visual recognition; and a recognized cultural legacy.”“They are the successful rebels of American culture,” said Goodyear, who added that the criteria came down to being “edgy, dark, mysterious.” They were also successful at what they did, not necessarily making a lot of money but excelling at singing, writing, painting or performing.Goodyear, a former curator of photographs at the gallery, is now a co-director of the Bowdoin College Museum of Art in Maine.“ Cool is an American concept,” said Dinerstein, the director of American studies at Tulane University in New Orleans, who teaches the class “The History of Being Cool in America.” “It comes out of our culture, being middle-class and creating a new persona. It is a singular American self-identification.”Texas connectionThe curators met in graduate school at the University of Texas at Austin and are so engaged in the subject of cool that they finish each other’s sentences.They’ve even pinpointed the birth of cool: The father of cool was Lester Young, the Mississippi-born jazz tenor saxophonist who honed his craft in Kansas City. He was the soloist for the Count Basie Orchestra and created a new smooth sound, often accompanying blues singer Billie Holiday, who’s also on the cool list.“ Cool was a 1940s jazz slang term,” Dinerstein said. “Cool was born in New York City and became a national sensation and a global obsession.”Young would say “I’m cool” — launching the use of the term — and wear a porkpie hat and sunglasses in the darkened clubs, creating a style for generations of musicians.“He created an aesthetic of detachment in music, style and public deportment,” Dinerstein said.Texas has bragging rights on three of the coolest people ever — singers Nelson and Selena and boxer Johnson. Nelson gets the nod for an outsider image that has not mellowed with age; Selena, who died at 23, for what Dinerstein calls “a fierce dignity in her Mexican-American heritage”; and Johnson for standing up to racism as the first black heavyweight champion.Texas itself is a cool state, Dinerstein and Goodyear said. “One of the spinoff exhibits that I’d love to see is a ‘Texas Cool’ exhibit,” Dinerstein said.Periods of coolThe exhibit divides cool into four periods: “The Roots of Cool,” before 1940; “The Birth of Cool,” 1940-1959; “Cool and Counterculture,” 1960-1979; and “The Legacy of Cool,” 1980-present.There are also grandfathers of cool going all the way back to the 19th century, with poet Walt Whitman, who celebrated rugged individualism; and Frederick Douglass, the author, abolitionist and newspaper publisher who escaped slavery and came to symbolize the dignity of African-Americans.The pictures in the exhibit are striking, taken by some of the world’s most-noted photographers, including Diane Arbus, Richard Avedon, Henri Cartier-Bresson, Annie Leibovitz and Herman Leonard. Most are from the gallery’s permanent collection.The museum, part of the Smithsonian Institution, is not on the National Mall but in downtown Washington in what was once the Patent Office Building.The exhibit includes kiosks that enable visitors to play music and video clips of the people featured. The music of 10 of the artists plays throughout the exhibition space.Quite a few of the photos contain a dated element that’s no longer considered cool: smoking. Before the public health outcry over smoking, cigarettes were an almost essential prop to be cool.So there is, to some degree, a changing aesthetic of cool.But the exhibit puts a frame around what had seemed indefinable: the incredible appeal of everyone from a rugged Clint Eastwood to a porcelainlike yet steely Faye Dunaway.