The supernova proves the Vermeer painting is fake in “The Great Game.”
Sherlock examines a supposedly lost Johannes Vermeer worth £30 million after security guard Alex Woodbridge is murdered. Woodbridge had discovered that the painting was a forgery, but Sherlock initially has difficulty explaining exactly how he knows. During the episode’s countdown, he recognizes a supernova depicted in the painting’s sky.
The clue is the fictional Van Buren Supernova. It appeared roughly two centuries after the period in which Vermeer supposedly painted the work, so the astronomical event could not have been visible to the artist. The impossible detail gives Sherlock the proof he needs and saves the hostage.
The astronomical clue also connects two apparently unrelated story threads. Woodbridge was interested in astronomy rather than art, and Professor Cairns was studying astronomy when the killer attacked her. The episode plants references to the supernova before Sherlock identifies it.
Viewers sometimes answer “star” or “constellation,” but the episode specifically identifies the decisive object as a supernova. The Van Buren event is fictional, created for the mystery rather than a real astronomical discovery.