The 25-year-old Leonard Bernstein had spent the morning scrambling to prepare for what he believed should not have happened for years. It was November 14, 1943, and Bernstein had gotten the call informing him he was to step in that night for the famous conductor Bruno Walter at Carnegie Hall, who was ailing. Bernstein, who at the time was the New York Philharmonic’s assistant conductor, rushed to visit the famous conductor. With Walter wrapped in blankets, the pair reviewed the musical scores.

Later that night, Bernstein's performance was met with an explosion of applause, though the audience—initially disappointed—had no idea the man who would come to shape classical music for a generation had just made his debut. In a trifecta of great, good fortune, the performance was broadcast nationally on the radio, made the front page of the New York Times and was seen by Bernstein’s parents, who happened to be in town.

“My first reaction was one of shock,” Bernstein told the New York Times following the show. “I then became very excited over my unexpected debut and, I may add, not a little frightened.” By the end of that season, Bernstein would conduct ten more times, by the end of his life, thousands.

The late American composer, conductor, educator, pianist and humanitarian would have turned 100 on August 25 of this year. The Smithsonian’s National Portrait Gallery joins the estimated 3,000 celebrations in honoring Bernstein by displaying a portrait of the renowned conductor in rehearsal at Carnegie hall. Taken in 1960 by the influential French photographer Henri Cartier-Bresson, author of The Decisive Moment , Bernstein conducts with arms whirring as if in flight—a style all his own.

“I am most intrigued by the access that Cartier-Bresson offers us, in this behind-the-scenes moment, away from the audience’s discerning eyes,” says Leslie Ureña, associate curator of photographs at the Portrait Gallery. “While Bernstein would have been aware of Cartier-Bresson’s camera, this is a more intimate moment of the master working with the Philharmonic to perfect a performance.”

Cartier-Bresson captured Bernstein’s spirit and larger-than-life personality. He conducted the orchestra with the exertion of his entire body, beads of sweat rolling down his face and exaggerated expressions to translate the emotion of the music. His energy unified the orchestra and audiences raved, experiencing the music on an elevated, unforgettable level.

By 1960 Bernstein had graduated from Harvard and the Curtis Institute of Music and attended Tanglewood Music Center. He had composed a ballet, five musicals, two operas, two major symphonies and various other orchestral, choral and theatre pieces. Bernstein had conducted in Europe and Israel. He developed educational programming for adults and children. He had gotten married. He had been appointed musical director of the New York Philharmonic. Ultimately, he had become the face of classical music all in under 20 years after his debut.

“No one is as famous of a musician as Bernstein,” says Rob Kapilow, composer, conductor, author and musical commentator. “The music world today really began with Bernstein. During the time that he was alive, he was absolutely blasted with critics for focusing on so many projects. The idea that you could actually go back and go between the classical and popular world was inconceivable. They couldn’t believe that someone who wrote a Broadway show or jazz could possibly be a serious conductor.”

Low and high brow music did not exist for him—it was all just music. Divisions were unnecessary and restricting and Bernstein found himself drawn to all different categories of music.

“Bernstein was uniquely able to blend multiple types of music into one piece, but in that piece he allowed those different kinds of music to stand right up against one another,” says Ivy Weingram, the associate curator of the National Museum of American Jewish History and curator of special exhibition Leonard Bernstein: The Power of Music. “He was more of a salad bowl guy than a melting pot guy.”

Crossing and juxtaposing musical concepts was not the only effect of Bernstein’s work, he also radically redefined the job of a conductor. By tradition, conductors were foreign born and trained, distinguished by age, and largely focused on orchestral work. Yet, Bernstein was young, American and Jewish.

“He was validly Jewish and he was validly American,” Kapilow says. “And in a world of elitist white-tie and tails classical music, he was not maestro but Lenny.”

One of his mentors, Serge Koussevitzky, even suggested Bernstein change his name to “Burns,” warning that he would “never see the name ‘Leonard Bernstein’ on the marquee outside of Carnegie Hall.’”

“Bernstein considered changing his name, but said, ‘No, I’ll do it as Bernstein or not at all.’ So, at a very young age we see him planting his flag with his Jewish identity and being willing to see where that would lead,” Weingram says.

As a Jewish American, he begin a search for what he called the “crisis of faith.” Through music, and his life, Bernstein explored his faith—or his lack thereof—in religion, society and government.

“Bernstein lived through a time in the 20th century that would challenge a lot of parts of his identity and faith,” Weingram says. “From World War II and the Holocaust to the Cold War and Vietnam, he thought a lot about what faith meant to him. All these different aspects of his life and his work contribute to why I think he said that the thing he was struggling with his whole life was a solution to the 20th century crisis of faith.”

Works like Kaddish or Mass readily come to mind, but Bernstein also lived out this search in his aim to educate the public about music or support a social movement, such as the Black Panthers or the AIDS crisis. Bernstein used his position on the podium and in society to inspire a generation and music lovers around the world.

“I think he is the proto-artist activist, he set a tone, no pun intended, for the power of the arts to heal communities in times of crisis,” Weingram says.

In the spirit of Bernstein, his portrait at the National Portrait Gallery expands the definition of portraiture. Though he is not posed, his movement in the photo emphasizes his persona.

“No one has ever been more of the definition of extravagant-, hyper-, over-the-top emotional than Bernstein,” Kapilow says. “There were times he would jump three feet off the podium, in ecstasy, looking to God. He brought a level of enthusiasm, emotionalism, openness. Bernstein was really about connection, about a direct, visceral, hyper-emotional kind, in the way he worked in the way he lived, in the causes he supported, Bernstein was just all in. Every measure, every note, and every second of his life.”

Leonard Bernstein’s portrait by Henri Cartier-Bresson is on display at the Smithsonian’s National Portrait Gallery August 23 through September 23. “Leonard Bernstein: The Power of Music,” at Philadelphia’s The National Museum of American Jewish History, a Smithsonian affiliate, closes September 2, 2018.