Stern means “star” and Stunde “hour.” A Sternstunde (plural Sternstunden) is a “dramatically compressed, fateful” event in which a “lasting development is being condensed into a single day, a single hour, or even a single minute” as it occurs only “rarely in the life of an individual or in the course of history.” These are the words of Stefan Zweig, who published between 1927 and 1943 fourteen historical “miniatures” under the title Sternstunden der Menschheit (Sternstunden of Humankind). He called the events he described Sternstunden because they “outshine the night of transience brilliantly and lastingly like stars.” [Source]
What Zweig had in mind becomes clear when we look at some of the Sternstunden he chose to describe: Händel composes the Messiah in a state of creative intoxication after a near-fatal illness (1741); the Janissaries enter Constantinople through a secret gate during the Turkish siege and conquer the city for the Turks (1453); Lenin returns to Russia in a sealed train to lead the Bolshevik revolution (1917). What unifies these events is their momentousness—in Zweig’s depiction, they changed the course of political or cultural history almost over night. If it was for the better or worse is not a concern of his.*
In present-day usage, however, the term Sternstunde has a distinctly positive connotation—it designates a highpoint or a pivotal moment that turns things around in the course of history. For example, Einstein’s publication of his paper on special relativity could be called a Sternstunde in the history of physics, and Germany’s unexpected victory in the 1954 World Cup a Sternstunde for German soccer.
*Historians have also questioned the accuracy of Zweig’s narratives or the importance he assigns to certain events; for example, his account of the creation of the Messiah appears to be entirely fictitious. But this does not diminish the usefulness of the term he coined.