On the track of a young nova.

Nova Cygni 1992, a celebrity among the stellar eruptions known as novas, has been photographed by countless astronomical paparazzi and spacecraft since its debut 2 years ago (SN: 1/16/93, p.43). New images taken by the Hubble Space Telescope continue to track the evolution of this young outburst. A nova occurs in a binary star system. The compact member of this duo, a white dwarf, steals mass from its bloated companion. As the mass accumulates, it sparks a thermonuclear explosion on the surface of the dwarf, hurling a shell of hot gas into space.

A Hubble image taken last May provided the first glimpse of the novas shell of expelled material as well as a mysterious bar-like structure. A new image, taken with a repaired Hubble last month and presented at a NASA press briefing, shows that the shell has elongated and expanded from a diameter of 46 billion to 60 billion kilometers. The bar has all but vanished.

Francesco Paresce of the Space Telescope Science Institute in Baltimore suggests that the bar revealed dense gas in the orbital plane of the double-star system that created the nova. As the explosion punched through the gas, the material grew more tenuous and faded. The shell's oblong shape may indicate that it's easier for expelled gas to expand above and below the orbital plane, where fewer obstacles lie.