The first six years of Galileo’s professorship at Padua had passed away, but the senate were eager to retain so bright a light for their University, and prolonged the appointment of the professor, whose renown was now great, for another six years, with a considerable increase of salary.[35] As we have seen, he had for a long time renounced the prevailing views about the universe; but up to this time he had discussed only physical mathematical questions with the Peripatetic school, the subject of astronomy had not been mooted. But the sudden appearance of a new star in the constellation of Serpentarius, in October, 1604, which, after exhibiting various colours for a year and a half, as suddenly disappeared, induced him openly to attack one of the Aristotelian doctrines hitherto held most sacred, that of the unchangeableness of the heavens. Galileo demonstrated, in three lectures to a numerous audience, that this star was neither a mere meteor, nor yet a heavenly body which had before existed but had only now been observed, but a body which had recently appeared and had again vanished.[36] The subject, though not immediately connected with the Copernican question, was an important step taken on the dangerous and rarely trodden path of knowledge of nature, uninfluenced by dogmatism or petrified professorial wisdom. This inviolability of the vault of heaven was also conditioned by the prevailing views of the universe. What wonder then that most of the professors who had grown grey in the Aristotelian doctrine (Cremonio for instance, Coressio, Lodovico delle Colombo, and Balthasar Capra) were incensed at these opinions of Galileo, so opposed to all their scientific prepossessions, and vehemently controverted them. The spark, however, which was to set fire to the abundant inflammable material, and to turn the scientific and religious world, in which doubt had before been glimmering, into a veritable volcano, the spark which kindled Galileo’s genius and made him for a long time the centre of that period of storm and stress, was the discovery of the telescope.