The 1604 Supernova

On the evening of October 9, 1604, near that place in the sky of a close conjunction between Mars, Jupiter, and Saturn, a new light, brighter than all the planets except for Venus, suddenly appeared. It remained there for a year and a half and then disappeared after slowly fading. That stella nova (new star), as it was called, was unexpected: many believed at that time that stars were fixed and immutable, but obviously this was not true. Today, we know that the new star was a supernova, the last of the seven supernovae observed with the naked eye in the Milky Way of which documentation exists. So, not a new star, but a star that dies and explodes. When the new star appeared, Galileo Galilei, then professor of astronomy in Padua, was the reference figure to address the doubts and questions that such an apparition brought with it. Between November and December 1604, Galilei devoted in Padua three public lectures, which had a huge attendance, to the new object. He demonstrated that it was indeed a star, since it was farther away than the planets. Galilei’s lecture notes are translated in this chapter.