Supernova 1604, also known as Kepler’s Supernova, Kepler’s Nova or Kepler’s Star, was a supernova that occurred in the Milky Way, in the constellation Ophiuchus. Appearing in 1604, it is the most recent supernova to have been unquestionably observed by the naked eye in our own galaxy, occurring no farther than 6 kiloparsecs or about 20,000 light-years from Earth.
Visible to the naked eye, Kepler’s Star was brighter at its peak than any other star in the night sky, and brighter than all the planets other than Venus, with an apparent magnitude of −2.5. It was visible during the day for over three weeks.
The first recorded observation was in northern Italy on October 9, 1604. Johannes Kepler began observing the luminous display while working at the imperial court in Prague for Emperor Rudolf II on October 17. It was subsequently named after him because his observations tracked the object for an entire year and because of his book on the subject, entitled De Stella nova in pede Serpentarii (“On the new star in Ophiuchus’s foot”, Prague 1606).
It was the second supernova to be observed in a generation (after SN 1572 seen by Tycho Brahe in Cassiopeia). No further supernovae have since been observed with certainty in the Milky Way, though many others outside our galaxy have been seen since S Andromedae and SN 1987A in the Large Magellanic Cloud was visible to the naked eye.
Present day astronomical evidence exists for a Milky Way supernova whose signal would have reached Earth ca. 1680 (Cassiopeia A), and another object whose light should have arrived ca. 1870. However there is no historical record of either having been detected at the time, by the unaided human eye.
The supernova remnant resulting from this supernova is considered to be one of the prototypical objects of its kind, and is still an object of much study in astronomy.