Cassiopeia A lies about 11,000 light-years away, close in direction to the familiar "W" of the northern constellation Cassiopeia. It is the remnant of the most recent supernova explosion in the Milky Way Galaxy. Cassiopeia A forced itself on the attention of astronomers in the mid-20th Century as the strongest source of cosmic radio waves detectable on the Earth. Since then its radio waves, visible light and X-rays have been studied repeatedly, with each waveband giving different impressions of the expanding shell of debris from the exploded star.

The ISO image gives the first detailed picture of Cassiopeia A at infrared wavelengths of 11-12 microns, which are clearly observable only from a satellite above the Earth's atmosphere. The dusty shell makes a hollow sphere, about 5.5 light-years in diameter. The ring-like appearance is due to the greater density of dust seen tangentially from the Earth's vicinity. Also visible towards the top left corner of the ISO image is a streak of dust formed by a high-speed jet of material flung out in the stellar explosion.