There are many roles that define a diva: Aida, Isolde, Violetta in “La Traviata.” But the title part in Bellini’s “Norma,” which opens the Metropolitan Opera’s season on Monday, Sept. 25, in a new production starring Sondra Radvanovsky, may be the greatest part of all — the one Maria Callas performed more than any other.

“This is the Everest of opera,” the soprano Renata Scotto, a brilliant, controversial Norma in the 1980s, said in a recent interview. “You want to climb the mountain. You know you are supposed to climb the mountain. But it is so difficult.”

But what is it, exactly, that makes this perhaps the most demanding part in all of opera? What about it stretches a singer to the limits of her abilities?

Training for Perfection

Before anything else, an aspiring Norma has to master “bel canto” technique. This school of singing, which dominated Italian opera in the early 19th century, is comparable to modern sports training, with its specialized drills to develop strength, speed and so forth. The goal of bel canto is to perfect the elaborate vocal gestures all over the role: trills, scales, arpeggios and leaps. (These are the roller-coaster fireworks you might hear when you think of opera. They’re also the way Norma, a druid high priestess, demonstrates her power.)