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93_21.txt
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1996-01-12
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CONTACT: Ray Villard, STScI FOR RELEASE: September 30, 1993
(410) 338-4514
PRESS RELEASE NO.: STScI-PR93-21
Dr. Francesco Paresce, ESA, STScI
(410) 338-4823
HUBBLE SEES GAS SHELL AROUND NOVA CYGNI 1992
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 dwarf's surface until
pressures and temperatures increase to the stage where thermonuclear
reactions take place.
The entire white dwarf's 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 the 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.
********
The Space Telescope Science Institute is operated by the Association
of Universities for Research in Astronomy, Inc. (AURA) for NASA,
under contract with the Goddard Space Flight Center, Greenbelt, MD.
The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency (ESA).