A supernova remnant's shocking trail.

Plowing through its surroundings, the racing shock wave from a supernova explosion sets interstellar gas aglow. Gas recently struck by the wave generally radiates at a higher temperature, while gas farther behind the wave glows cooler. Images taken by the unrepaired Hubble Space Telescope had revealed such a pattern for the Cygnus Loop, a supernova remnant 2,500 light-years from Earth (SN: 3/13/93, p.168).

Now, the sharp-eyed, repaired Hubble has captured this remnant in finer detail. The new picture shows a narrow zone directly behind the shock wave, in which hydrogen atoms --rather than ions -- radiate most of the light. John J. Hester of Arizona State University in Tempe discussed the image this week in Atlanta at the annual meeting of the American Association for the Advancement of Science.

Hester notes that before the shock hurtled past, the gas consisted mainly of neutral atoms, electrons, and protons. When the wave blasted through, it energized the electrons and protons, which began to collide with the atoms. Even after a few collisions, some atoms remained un-ionized, holding onto their outermost electrons. But the collisions did cause the atoms to radiate. This accounts for the zone of atomic emissions, which spans a width a few times the Earth-sun distance, Hester says.

As the charged particles continued to bombard the atoms, they succeeded in ionizing them, generating the high-temperature emissions directly behind the thin region of atomic hydrogen radiation. As the ions radiated, they began to cool, creating the low-temperature region behind the hotter layer.