So what is this thing called cool? A major photography exhibition that opens Friday at the National Portrait Gallery in Washington dares to tackle the question.
From Elvis Presley and James Dean to Jay-Z and Johnny Depp, "American Cool" namechecks 100 actors, actresses, artists, musicians and writers in the United States whose creativity and style have shaped the concept of cool.
"Cool is America's greatest cultural export," said Aussie Kim Sajet, who took over last year as director of the National Portrait Gallery, part of the Smithsonian network of museums.
"It's interesting for me, coming from another country, that I know all these people in this exhibition -- they were very much cool in Australia where I grew up."
To pull the show together, jazz professor Joel Dinerstein and photography scholar Frank Goodyear spent five years going through a draft list of 500 names of charismatic Americans who might be regarded as cool.
To make their selection, the curators came up with four defining factors of cool: an original artistic vision and signature style, the embodiment of rebellion, instant visual recognition and "a recognized cultural legacy."
If someone hit at least three of those markers, they made the grade.
"What we're examining are people who had an impact," said Dinerstein, who hopes "American Cool" will provoke "an inter-generational debate" over who's hot and who's not.
Their final top-100 list opens with two 19th century figures -- humanist poet Walt Whitman and African-American writer and abolitionist Frederick Douglass -- who the curators call the grandfathers of cool.
Stars of the golden age of Hollywood illuminate the "roots of cool" section -- screen legends like Fred Astaire, Louise Brooks, Greta Garbo, Buster Keaton and Mae West -- along with writers Ernest Hemingway and Dorothy Parker.
"Cool has been central to American self-expression since at least the 1930s," said Dinerstein, a Brooklyn native who teaches a course called "The History of Being Cool in America" at Tulane University in New Orleans.
But it was in the 1940s and 1950s that cool was truly born, with jazz legends like Miles Davis, Duke Ellington, Dizzy Gillespie, Billie Holiday, Thelonious Monk, Charlie Parker and Lester Young coming to the fore.
Sharing the limelight were James ("Rebel without a Cause") Dean, Jack ("On the Road") Kerouac, Frank Sinatra, John Wayne, and, of course, Elvis Presley.
With the 1960s and 1970s, rock and R&B stars arose in force: Bob Dylan, Marvin Gaye, Blondie's Debbie Harry, Lou Reed, Carlos Santana, Patti Smith, Frank Zappa, Madonna, Prince, Bruce Springsteen and -- the show's token Canadian -- Neil Young.
Fittingly, the predominantly black and white images -- many by top photographers such as Diane Arbus, Richard Avedon, Robert Mapplethorpe and Edward Steichen -- are hung on walls painted a cool shade of blue.
The curators view the exhibition as "very small-D democratic," but to ward off potential complainers, they whipped up an "Alt 100" list of also-rans that includes Sam Cooke, Janis Joplin and George Clooney.
"These were the men and women who lingered the longest during the selection process," the exhibit organizers explained. "American Cool" runs through September 7, and the National Portrait Gallery's website is www.npg.si.edu