On the first day of the Western civilization class that he taught at Shaw University in Wilmington, NC, Larry Thomas introduced himself in his usual manner. It was 1990 or ’91. He can’t quite remember.

The prof handed out a bio that included details on a jazz radio show he hosted. A reserved-looking, 60-something woman in the front row, Helen Morgan, perked up. She expressed a love of jazz and told the class that her husband used to play.

“I asked her husband’s name and she said, ‘Lee,’ ” Thomas tells The Post. “I said, ‘Lee Morgan? The trumpet player?’ She said, ‘Yeah.’ I nearly fell on the floor and told her that I’d like to speak with her sometime. She said, ‘I’ll think about it.’”

Thomas’ sense of shock stemmed from the fact that the humble looking lady sitting in his classroom was a murderer.

Among jazz enthusiasts, Helen was notorious for having gunned down her husband Lee, a brilliant 33-year-old horn player, between sets one cold February night in 1972 at an East Village club called Slug’s Saloon, with a sawdust-covered floor.

She kept putting off talking to Thomas until one month before her death from a heart ailment in 1996. His resulting 90-minute cassette tape interview with Helen forms the backbone of a stunning documentary, “I Called Him Morgan,” directed by Kasper Collin and opening Friday.

“What struck me was the complexity of their relationship,” says Collin, explaining that Helen helped Lee battle heroin addiction, yet killed him. “Had he not met Helen, we can safely assume that Lee would have died sooner than he did,” Collin says, referring to the drugs.

Collin’s documentary tells of the star-crossed love affair between Lee and Helen. He was a lauded but troubled trumpeter who ranked among bebop’s premier horn players. Helen was a street-smart jazz aficionado whose apartment on West 53rd Street served as a gathering spot for musicians after gigs at the nearby clubs. She cooked for them and was known as the hip square.

“Helen was cool and didn’t do heroin,” says Thomas, explaining that she was tough, had her first child at age 13, and may have stabbed her first husband, a bootlegger she married at 17, in the back.

Had he not met Helen, we can safely assume that Lee would have died sooner than he did.

Helen saved Lee, got him into a methadone program, managed his career. Then, he took up with another woman. That fateful night, he pretty much pushed her out onto the snowy street after she showed up at his Slug’s gig.

Musician Billy Harper was there as shots were fired and Lee went down. He recalls Helen getting arrested — she barely served any time in prison — and an ambulance being slow to arrive.

“We were stunned and nobody knew who to hate,” says Harper. “People wanted to hate Helen and some guys did. I didn’t know what to think. They were so close, together all the time. I was just confused. It shouldn’t have happened. He did not deserve to be killed.”

Today, there is nothing to mark the tragic death of Lee Morgan at the East Third Street site that now houses Rossy’s Bakery.