Four hundred years ago this past weekend, skywatchers witnessed the appearance of a new object in the western sky. Later identified as a supernova, the entity was dubbed Kepler's remnant after the legendary astronomer Johannes Kepler, who studied it during its early years. Now astronomers are using space telescopes to better understand our galaxy's most recent supernova.

The glow from young remnants, such as Kepler's supernova remnant, comes from several components, explains Ravi Sankrit of Johns Hopkins University. Each component shows up best at different wavelengths. Sankrit and William Blair, also at Hopkins, are thus employing three NASA observatories--the Spitzer Space Telescope, the Hubble Space Telescope and the Chandra X-ray Observatory--to study the supernova remains in detail. The combined image (above) reveals a cloud of dust and gas 14 light-years across, which is expanding at a rate of four million miles an hour. Infrared observations highlight the hot pieces of interstellar dust, whereas X-ray data from Chandra garner information on gas within the cloud. Hubble's visible light camera, meanwhile, captures the supernova's advancing shock wave butting up against dense surrounding gas.

Astronomers are still trying to pinpoint the source of Kepler's remnant. Supernova typically result from the demise of one of two types of stars, white dwarfs and massive stars more than eight times the size of our sun. Of the six known Milky Way supernovae, Kepler's remnant remains the only one for which scientists do not yet know the cause. When the analysis is complete, Blair says, we will be able to answer several questions about this enigmatic object.