NASA's Hubble Space Telescope (HST) has given astronomers their earliest look at a rapidly ballooning bubble of gas blasted off a star. The shell surrounds Nova Cygni 1992, which erupted February 19, 1992.
A nova is a thermonuclear explosion that occurs on the surface of a white dwarf star in a double star system. Nova Cygni 1992 was one of the brightest novae in 20 years, reaching naked-eye visibility for a brief period.
Hubble Space Telescope's high resolution provides a unique opportunity to understand the nova mechanism, according to Dr. Francesco Paresce of the European Space Agency and the Space Telescope Science Institute. "This is the first time we've been able to separate the white dwarf star from the ejecta so early in the nova event," he says.
Hubble, because of its high resolution, has "arrived at the scene of the crime" much earlier than ground based telescopes could have, according to Paresce. (This shell will not be resolved from the ground for at least another five years.) The shell is so young it still contains a record of the initial conditions of the explosion, he explains. This will allow astronomers to construct the history of the nova explosion.
By the time the shell can be resolved with ground based telescopes, it will have been deformed and chemically contaminated by passing through other material around the star. "The HST observations provide a unique opportunity to study the 'pristine' envelope of gas ejected by the explosion," Paresce explains.
The HST image, taken with the European Space Agency's Faint Object Camera (FOC), reveals a remarkably circular yet slightly lumpy ring-like structure. The ring is the edge of a bubble of hot gas. The shell is only 37 billion miles across, or 400 times the diameter of the Solar System. A beam of light could cross the shell in less than 2-1/2 days. By knowing the shell's diameter, as calculated from a comparison between its angular size and it expansion velocity (as measured from ground-based observations), astronomers can measure precisely the distance to Nova Cygni, which turns out to be 10,430 light-years.
A striking relic of the explosion is an unusual bar-like structure across the middle of the ring. It might mark the edge-on plane of the orbits of the two members of the binary system, astronomers think. A large amount of gas stirred-up along the plane would make the shell denser in the plane of the orbit of the double star. An alternative possibility is that the bar is produced by twin jets of gas ejected from the star and spanning the distance between the shell and the star.
A nova occurs in a double star system where one member is a normal star and the other is a white dwarf, a compact corpse of the core of a Sun-like star that has dimensions the size of Earth. The compact and gravitationally powerful white dwarf pulls material from its stellar companion. This material accumulates on the white dwarfs surface until pressures and temperatures increase to the stage where thermonuclear reactions take place.
The entire white dwarfs surface explodes as a gigantic hydrogen bomb. A nova releases as much energy as our Sun produces in 1,000 years. As the expanding shell of hot gas envelopes both stars, they continue to orbit inside it. This should produce a thick disk of gas created through the "eggbeater" motion of the two stars, according to astronomers. The bar in the Nova Cygni 1992 image may be the relic of this event.
Shells around older novae, seen by ground-based telescopes, are deformed by passage through interstellar space, and are faint and tenuous. HST observations will be able to trace the expansion of the shell to a much earlier time in the history of a nova.
Watching the evolution of the shell also will reveal how heavy elements, processed in the star's envelope, are ejected back into space. Such explosions enrich space with elements such as oxygen, carbon, and silicon that are the fundamental building blocks for new generations of planets and presumably life.
Ray Villard, STScI
Dr. Francesco Paresce, ESA, STScI