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On the night of October 9, 1604, sky watchers looking at a rare clustering of Mars, Jupiter and Saturn, were amazed by the appearance of a "new star" as bright as the planets. The famous astronomer, Johannes Kepler, heard about it, but had to wait a week for the skies to clear over Prague before he had a chance to see the phenomenon. From then on he observed the new star regularly for a year until, in October 1605, it became too faint to see with the naked eye. Telescopes were first used in astronomy only four years after that. It was not until the mid-20th century that astronomers, using large telescopes, searched for and found a cloud of glowing gas around the location of the new star of 1604. Today this nebula is known as Kepler's supernova remnant: the remnants of a stellar explosion that was the last supernova seen in our Milky Way galaxy.

Four hundred years after its discovery, astronomers are using the combined power of NASA's three Great Observatories (Hubble, Spitzer and Chandra), the Very Large Array radio telescope, and ground-based telescopes with modern spectrographs to unravel intricate features of Kepler's supernova remnant. Multiwavelength images show a bubble-shaped shroud of gas and dust, 14 light-years wide. There is a fast-moving shell of iron-rich material surrounded by the primary shock wave from the supernova, expanding at 4 million miles per hour (2000 kilometers per second) that is sweeping up gas and dust from the surrounding medium.

The Hubble Telescope image, obtained with the Advanced Camera for Surveys in August 2003, shows regions that have been lighted up by the passage of the supernova shock wave. Knots and filamentary sheets of emission viewed edge-on are revealed in glorious detail. The bright knots are dense clumps that form behind the outward moving shock wave. The shock plows into material lost from the progenitor in a stellar wind prior to the supernova explosion, and instabilities cause the swept up gas to fragment into clumps. The thin filaments trace regions where the shock front is encountering more uniform, lower density material.

Filters onboard Hubble have been used to isolate light emitted by hydrogen atoms, and nitrogen and oxygen ions present in the gas. These filters also transmit starlight from foreground and background stars. Regions in which there is only glowing hydrogen are red, the yellow regions are strong in nitrogen, and regions where oxygen emission is present are pink or white.

The multi-wavelength study, for which the Hubble observations provide a crucial component, will help astronomers identify the type of star that produced the explosion. There are two different types of supernovas: one formed by the thermonuclear explosion of an accreting white dwarf star, and the other formed by the rebound explosion following the collapse of the core of a massive star. Of the six known supernovas in our Milky Way in the last 1000 years, SN 1006, and SN 1572 (Tycho's supernova) are of the former type, while SN 1054 (Crab Nebula), SN 1181 and SN 1680? (Cassiopeia A) are of the latter type, and SN 1604 (Kepler's supernova) is the only one for which the type is as yet unknown.