WASHINGTON — Flashback to a 1940s jazz club. A saxophonist performs on stage wearing his sunglasses at night. "I'm cool," he says, relaxed in the moment.

Lester Young, the lead saxophonist in Count Basie's orchestra and soul mate to Billie Holiday, was giving voice to what would become an American and global obsession for cool with a phrase rooted in jazz slang. But what does it mean to say someone is cool?

Over the past five years, curators at the National Portrait Gallery set out to examine how that idea of cool has shaped American culture. A new exhibit opening Friday gives the concept an entertaining yet scholarly treatment with 100 photographs of people who defined cool as a word for rebellion, self-expression, charisma, edge and mystery.

"American Cool" traces the idea further back to the "granddaddies of cool," Walt Whitman and Frederick Douglass, and unfolds through the generations, leading to Jay-Z, Johnny Depp and Madonna.

"These are the successful cultural rebels of American culture," said Joel Dinerstein, a professor and jazz scholar at Tulane University who co-curated the show and teaches a college course on the history of being cool.

The exhibit idea was born out of the 2008 presidential election when the buzz was about a new type of political candidate who exuded a cool, relaxed intensity, appealing to a new generation, said co-curator Frank Goodyear III, who is co-director of the Bowdoin College Museum of Art in Maine. No presidents past or present fit the criteria for defining cool, though curators considered Obama, Clinton, Reagan and Kennedy.

What might seem like an arbitrary list of 100 celebrities was actually a much-debated roster whittled down from 500 names, the curators said. They devised a four-point rubric to define cool on a national scale. To be included, a person had to have an original artistic vision with a signature style, represent cultural rebellion or transgression, have instant visual recognition, and have a recognized and lasting cultural legacy.

Jon Stewart was chosen over someone like Johnny Carson because Stewart's signature style for satire and comedy is an original that's been emulated in Egypt and beyond.

The 100 who made the cut include musicians, actors, athletes, comedians, activists and writers. At the roots of cool, there's Fred Astaire, Ernest Hemingway and Georgia O'Keeffe, followed by the birth of cool in the jazz era with Young, Holiday, Muddy Waters and Duke Ellington, as well as Audrey Hepburn, Elvis Presley and Frank Sinatra. Music from 10 jazz, blues and rock-and-roll artists plays throughout the galleries.

In the 1960s and 1970s, cool was tied to the counterculture. To be cool was to be anti-authoritarian and open to new ideas, curators said, with figures like Bob Dylan, Muhammad Ali, Angela Davis and Andy Warhol.

Throughout the show, there are more men than women represented.

"Cool was very much a masculine aesthetic until very recently," Dinerstein said. "This notion of sort of having edge and charisma and mystery and detachment really was not applied to women, except in negative ways, for quite a while. The cross-over period for that is in a lot of ways the '70s."

It's an exhibit about history, race, gender and integration. And it's about photography as the figures each shaped their public images through photographs by Richard Avedon, Annie Leibovitz and other leading fine art photographers and photojournalists. Most of the images are black and white.

The most difficult section to curate was the legacy of cool from the 1980s onward, Goodyear said.

"There are some who thought that cool died in the '80s," Goodyear said. "It was Reagan's America and increasing materialism and the growth of sort of corporate power and its influence."

Being cool was more important than being rich in the decades before. But more recently being cool has been validated by being rich. Someone like Jay-Z is "selling in" to have more power in the culture, rather than selling out, Dinerstein said. "Hip-hop totally crosses that over in which bling becomes positive," he said.

It was so difficult to choose just 100 people, the exhibit also includes an alternate list of 100 names at the end who the curators argued about the longest.

"We want nothing more than for this to cause inter-generational debates," Dinerstein said. "People in the greatest generation can tell their grandkids who Gene Krupa was and why he's important. People who are young can tell their grandparents who Jay-Z is and why he is important."

"American Cool" will be on view through Sept. 7 in Washington.