At 10:15 on Tuesday night Nancy Ringham was a star. The audience for "My Fair Lady's" opening night at Broadway's Uris Theatre was quavering and breathing as a single tissue. It rose to roar and greet the morning glory whose saga had been set up in that day's papers:

Nancy Ringham, the understudy, had stepped in just as Ruby Keeler and a million girls in their dreams had. The original star, an english actress named Cheryl Kennedy, was out as of Saturday because of illness, and so Nancy Ringham had three days of madness, grueling rehearsals and terrible, terrible pressure.

But Tuesday night, the 26-year-old, Minnesota-born Ringham was brought forward to the stage lights by Rex Harrison, having just played Eiza Doolittle to his Henry Higgins. She held his hand and he bowed to her, and the audience stood to greet the newest conqueror. Almost giving off light in her peach gown, she grinned and bowed once, bowed twice, in the great traditions of newborn stars. In the front row, her parents, Clark and Ruth Ringham, stood and watched her August coronation.

Rex Harrison stepped forward once more and kissed her hand. Then the houselights went on as the curtain went down. Harrison had been touring this production of "My Fair Lady" for almost six months -- taking it to New Orleans, Chicago, Los Angeles and Boston. Audiences had turned out just as expected to see the legendary performance by the 73-year-old actor as Henry Higgins -- the Shavia phonetician whose philosophy of class and language embodied progressive Victorian ideas of civilization and its decline -- and he disappointed only those who knew the performance well enough to expect new life in it. Harrison, customarily brilliant and comfortable, walked through his evening with 25 years of Higgins-honing showing in every move.

Nancy Ringham, however, had a harder night. Her singing was equisite, with notes carrying through the huge theater like lines of color. Her acting was not the stuff of instant legend. There was nothing pathetic about it, but she showed Harrison's original instincts to have been right when he demanded an English actress play Eliza. Her accent was strictly a tale of two cities: Minneapolis and St. Paul.

Ethel Merman and Steve Rubell and Jean Kerr and Otto Preminger all stood among the opening night noise, and when the curtain went down for the last time Nancy Ringham was ushered to her dressing room to await the verdict. A publicity man yelled through the closed door of her dressing room, "Are you in there, darling? Stay dressed, The photographers are here." Rex Harrison was nowhere in sight. A bunch of Ringhams were pushed up a ramp into Nancy's dressing room as an explosion of flashbulbs recorded their first post-stardom reunion. Clark Ringham, a tall, bulky insurance executive, grabbed his daughter and hugged her. Ruth Ringham, the star's mother, cried.

Three TV crews thrust themselves into Nancy Ringham's dressing room as Ruth Ringham cried some more. "Well, Nance," said a New York TV reporter, "I've got a question for you.

"How," the TV reporter asked momentously, "does . . ."

"It feel?" piped up one of Nancy Ringham's kid brothers.

"That's right," the TV reporter said. "How does it feel?"

Nancy Ringham looked blank for a moment. "Great I guess," she said. She giggled, and the TV reporter pushed closer. "Tonight you're the talk of Broadway," the reporter said.

"I am?" asked Nancy Ringham.

"You're the only game in town," she was told.

Nancy Ringham looked at the reporter for a moment in silence. "Well," she said, "tomorrow there'll be someone else." Her family laughed and a new crush of Minnesota visitors fell into the dressing room as young Bill Ringham opened a bottle of champagne.

"Okay if I drink, Pop"? he asked his father. Clark Ringham fixed a look on his son, and turned to another reporter's question: "How does it feel, Mr. Ringham, to have a daughter as young as yours going out with a man as old as Rex Harrison in the show?"

Clark Ringham looked at his son walking off with a glass of champagne, then fixed an even harder look on the reporter. "Just what do you mean," he said, "by going out?" Ron Deal, Nancy's college boyfriend who now teaches law at Temple University, crossed in front of Clark Ringham and went to hug Nancy. "C'mon," Clark Ringham said to his family, "let's go and get something to eat."

Twenty minutes later, Nancy and Ron, after long, quiet consultation, emerged from her dressing room into the now-quiet theater. They walked across the stage arm in arm, and got into the scenery elevator that would take them to the ground floor. Ringham was calm. The doors opened and a stagehand and a security policeman both hugged her.

Outside, eight people waited for her to autograph their programs. "Let's go to the cast party," she said to Deal, "but not for long." She walked through the small crowd. No one recognized her. She walked about a hundred feet into the plaza under the Uris when suddenly one of the fans perked up and said, "That's her!"

Ringham turned around and stood while the eight got the signature of the star they didn't know. The moment she could, she put herself and Deal into a cab that mired itself into a standstill on Seventh Avenue.

At the Piccadilly Hotel in the Cafe Ziegfeld, the cast waited for the reviews of the 25-year-old show. Legendary New York police reporter Gabe Pressman stood in a white cream jacket with a microphone waiting for Nancy. "My Fair Lady's" legendary grande dame, Cathleen Nesbitt -- playing Henry Higgins' mother this time, as she had in 1956 -- waited for Nancy. The New York Post's legendary gossip columnist, Earl Wilson, sat at a table with his legendary B. W. (Beautiful Wife), waiting for Nancy. An entire flight deck of slightly watery catered entrees sat waiting for Nancy.

A television above the bar had been turned on for the 11 o'clock TV reviews. And several PR assistants were on the street trying to scrounge up early editions of the morning papers. Jim Wooten of ABC national news had been called off a tennis court to come in and ask Nancy Ringham how it felt. He stood and waited next to his cameraman and electrician, who wore a giant television light on his head.

Nancy swept into the room with the timidity of a star who doesn't know the measure of her imperial rights. The Cafe Ziegfeld stood and applauded her. She was led up to Wilson, who brought her into an all-but-inaudible huddle and asked, "How does it feel?" She was led to a far corner of the dining room where Wooten's ABC crew clustered around her as Wooten asked, "How does it feel?" A blaring tape played "What I Did for Love? as she tried to answer. The song ended and the first words to surface above the silence were her saying, "Flash in the pan." A PR man rushed over and grabbed her wrist. He dragged her to the bar where there was an NBC camera waiting for a live hookup. Gabe Pressman asked her, "How does it feel?" She smiled and giggled --that it was great. When the hookup was over the PR man came around to get her, but she was gone. The TV reviews were good, the newspaper reviews were tough. Nancy Ringham was nowhere in the room. A man at the bar ordered a double martini and asked the PR man where Nancy was. "I don't know," said the PR man. "She's gone."

"To bad," said the man with the drink. "I wanted to ask her how it felt."