The story
On October 9, 1604, observers in northern Italy and elsewhere noticed a brilliant new star in the constellation Ophiuchus, near Mars, Jupiter and Saturn that were then close together. Johannes Kepler, the imperial mathematician in Prague, began observing it a few days later after cloud had hidden the sky, and wrote a book on it in 1606. At peak it was brighter than any other star and brighter than Jupiter, and it stayed visible to the naked eye for about 18 months.
Kepler and other astronomers, including Galileo in Padua, showed the star had no measurable parallax and so lay among the fixed stars, not near Earth. That repeated the lesson of Tycho's supernova of 1572: the heavens were not unchanging. Modern astronomers know it was a Type Ia supernova, the explosion of a white dwarf star, about 20,000 light years away. Its expanding remnant is still observed with telescopes today. No supernova in the Milky Way has been widely seen by the naked eye since.
Why it mattered
- Confirmed that the stars could change, strengthening the case against Aristotle's unchanging heavens.
- Gave Kepler a public astronomical subject and results that he published in De Stella Nova in 1606.
- Left a well-dated remnant that modern astronomers use to study Type Ia supernovae.
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