What 1604 stellar event did Galileo observe and discuss?

Galileo observed and discussed Kepler’s Supernova, the stellar event now designated SN 1604.

The “new star” appeared in October 1604 in the constellation Ophiuchus. Galileo was teaching at Padua and observed it after its first reports, later discussing its significance in lectures and writings. The event mattered because an apparently new star challenged the Aristotelian belief that the heavens were perfect, eternal, and unchanging.

Johannes Kepler systematically observed the object from Prague beginning on October 17, which is why it became known as Kepler’s Supernova or Kepler’s Star. Modern astronomy classifies it as a Type Ia supernova, a thermonuclear explosion involving a white dwarf. Its remnant is still studied across the electromagnetic spectrum.

The alternatives refer to different phenomena. Tycho’s Supernova was SN 1572, observed by Tycho Brahe, while the Crab Supernova was the much earlier event recorded in 1054. Halley’s Comet is a periodic comet, not a stellar explosion. Galileo’s observations also came before his telescopic discoveries, since his first major telescope work began several years later.