Gaspare Spontini (1774-1851) was an Italian opera composer who spent many years in Paris and later in Berlin. His best-known opera, La Vestale, was first performed in Paris in 1807.
In this opera Licinius, a victorious Roman general, returns to Rome in triumph only to discover that his fiancée, Julia, had given hope of ever seeing him again and was now a Vestal Virgin, a priestess of the goddess Vesta who had taken a vow of chastity for thirty years.
When Licinius visits Julia at the temple one night, they are so busy singing love duets that she lets the sacred fire go out. For this she is sentenced to death, but the rule is that she has to take off her white shawl and lay it on the altar. If it spontaneously catches fire, that is a sign that the goddess has pardoned her, otherwise she has to die. Of course it does catch fire — one of her colleagues puts a torch to it while no one is looking — so Julia and Licinius can get married and live happily ever after.

Lobby of the Théâtre des Champs-Élysées
In October 2013 I saw a marvelous production of La Vestale at the Théâtre des Champs-Élysées in Paris, with the American tenor Andrew Richards as Licinius and the Albanian soprano Ermonela Jaho as Julia. The singing, acting and staging were all superb, and the happy ending really did look happy, with the chorus chasing the loving couple around the stage to the strains of Spontini’s ballet music.

La Vestale posters in a Métro station in Paris
The white smoke on these opera posters is the smoke from the burning shawl which proves that the goddess has pardoned Julia.
Location and aerial view of the theater on monumentum.fr.
My photos in this post are from 2013. I revised the text in 2018.