The Starry Universe of Johannes Kepler

Johannes Kepler described the Copernican universe as consisting of a central, small, brilliant sun with its planetary system, all surrounded by giant stars. These stars were far larger than, and much dimmer than, the sun – his De Stella Nova shows that every visible star must exceed the size of the Earth’s orbit, and the most prominent stars may exceed the size of the entire planetary system. His other writings, including his response to Ingoli, his Dissertatio cum Nuncio Sidereo, and his Epitome Astronomiae Copernicanae, also reflect this Copernican universe. To Kepler, such a universe was an illustration of divine power – and solid evidence against the stars being suns, against the universe of Giordano Bruno. Kepler’s starry universe was in fact the Copernican universe supported by observations of the stars, which showed them to have measurable apparent sizes. Not until the later seventeenth century were those apparent sizes shown to be spurious, allowing for a universe in which the stars were suns.