He2-104

A tempestuous relationship between an unlikely pair of stars may have
created an oddly shaped, gaseous nebula that resembles an hourglass
nestled within an hourglass.

Images taken with Earth-based telescopes have shown the larger,
hourglass-shaped nebula. But this picture, taken with NASA's Hubble
Space Telescope, reveals a small, bright nebula embedded in the center
of the larger one (close-up of nebula in inset). Astronomers have
dubbed the entire nebula the "Southern Crab Nebula" (He2-104),
because, from ground-based telescopes, it looks like the body and legs
of a crab. The nebula is several light-years long.

The possible creators of these shapes cannot be seen at all in this
Wide Field and Planetary Camera 2 image. It's a pair of aging stars
buried in the glow of the tiny, central nebula. One of them is a red
giant, a bloated star that is exhausting its nuclear fuel and is
shedding its outer layers in a powerful stellar wind. Its companion is
a hot, white dwarf, a stellar zombie of a burned-out star. This odd
duo of a red giant and a white dwarf is called a symbiotic system. The
red giant is also a Mira Variable, a pulsating red giant, that is far
away from its partner. It could take as much as 100 years for the two
to orbit around each other.

Astronomers speculate that the interaction between these two stars may
have sparked episodic outbursts of material, creating the gaseous
bubbles that form the nebula. They interact by playing a celestial
game of "catch": as the red giant throws off its bulk in a powerful
stellar wind, the white dwarf catches some of it. As a result, an
accretion disk of material forms around the white dwarf and spirals
onto its hot surface. Gas continues to build up on the surface until
it sparks an eruption, blowing material into space.

This explosive event may have happened twice in the "Southern Crab."
Astronomers speculate that the hourglass-shaped nebulae represent two
separate outbursts that occurred several thousand years apart. The
jets of material in the lower left and upper right corners may have
been accelerated by the white dwarf's accretion disk and probably are
part of the older eruption.

The nebula, located in the Southern Hemisphere constellation of
Centaurus, is a few thousand light-years from Earth.

This image, taken in May 1999, captures the glow of nitrogen gas
energized by the white dwarf's intense radiation.

These results were presented at the "Asymmetrical Planetary Nebulae
II: From Origins to Microstructures" conference, which took place at
the Massachusetts Institute of Technology, August 3-6, 1999.

Credits: Romano Corradi, Instituto de Astrofisica de Canarias,
Tenerife, Spain; Mario Livio, Space Telescope Science Institute,
Baltimore, Md.; Ulisse Munari, Osservatorio Astronomico di
Padova-Asiago, Italy; Hugo Schwarz, Nordic Optical Telescope,
Canarias, Spain; and NASA