A Swedish astronomer, of noble birth; spent his life in the study of the stars; discovered a new star in Cassiopeia; had an observatory provided for him on an island in the Sound by the king, where he made observations for 20 years; he was, on the king's death, compelled to retire under persecution at the hand of the nobles; accepted an invitation of the Kaiser Rudolf II. to Prague, where he continued his work and had Kepler for assistant and pupil (1546-1601).