I'm afraid I may remember too few details about a not-very-good book, but it's stuck in my mind all these years.
Science fiction novel, English language, I read it from a U.S. public library, late '70s or early '80s. Hardcover. Probably young adult.
Some future with humanity widespread in space, I don't remember any aliens. An adventure novel, with at least one protagonist an officer in the police/military of the overall interstellar government. And three specific details:
In case of serious threat or rebellion, the government (a positively portrayed, government, as I recall) can maintain control with the threat of the ultimate weapon, the nova bomb: as advertised, it can turn a system's sun nova. Only the most important of the military spaceships carry a nova bomb, in a specially secured chamber.
The troublemakers in the book are people from a planet that orbits a unique violet sun, which gives the planet's inhabitants strange powers.
In one of the climactic scenes, the (a?) protagonist opens the nova-bomb chamber, to discover that it is empty. The ultimate threat has been a bluff all along.
