In “RBG,” Gloria Steinem calls her “the closest thing to a superhero that I know,” explaining why diminutive Ruth Bader Ginsburg has become an icon.

As the second woman appointed to the U.S. Supreme Court — after Sandra Day O’Connor — Justice Ginsburg has become an unlikely celebrity. With rapper Dessa’s “The Bullpen” playing over glimpses of her exercising, along with disparaging quotes from her right-wing critics, this documentary illuminates why Ginsburg is one of our best and brightest legal eagles.

When Ginsburg entered Cornell University, there were four men for every woman undergraduate. She met her future husband, Marty, there; they married when they graduated. He became a New York tax lawyer and was her staunchest supporter; he died in 2010.

At Harvard Law School in the mid-1950s, she was one of nine women in a class of more than 500 men. At a dinner for the first-year women, the dean asked each one why they were taking a seat that could be occupied by a man.

When Ginsburg argued her first case at the Supreme Court — 1973’s Frontiero v. Richardson — she was speaking to men who didn’t acknowledge or understand gender-based discrimination. In 1980, President Jimmy Carter put her on the federal bench; President Bill Clinton appointed her as the 107th Supreme Court Justice in 1993

Becoming “notorious,” Ginsburg firmly believed that the equal protection clause of the 14th Amendment should apply to women and could be used to rectify discrepancies in hiring, business practices and public policy.

“Marching and demonstrating wasn’t Ruth’s thing,” biographer Wendy W. Williams notes. Instead, Ginsburg favored incremental change through litigation, which is how the civil rights movement achieved its greatest successes.

Directors Betsy West and Julie Cohen intercut archival material with informative interviews and public appearances. A two-time cancer survivor, Ginsburg forged a firm friendship with her right-wing foil, the late Antonin Scalia, and laughed at Kate McKinnon’s parody on TV’s “Saturday Night Live.”

On the Granger Movie Gauge of 1 to 10, “RBG” is an inspiring 7, highlighting this trendsetter’s dissenting opinions.