The nebula, long known as the Southern Crab because of its overall resemblance to that marine crustacean, contains an hourglass-shaped region that astronomers wanted to scrutinize more closely. The resulting image shown here, made with the Hubble Space Telescope, reveals yet another, smaller hourglass shaped nebula nested within the larger one.

Somewhere at the heart of this oddly shaped complex are two stars: one is a red giant and the other is a white dwarf. The red giant, like all red giants, is a swollen, dying star that is shedding its outer layers of gas via powerful stellar winds. The white dwarf companion, also typical, is the collapsed core of a once normal star. It is very hot and, because it is very dense (twice Earth's diameter, but containing the mass of the Sun), it exerts a powerful gravitational field.

Together, however, these stars form an unusual symbiotic system in which some of the expelled gas from the red giant is captured by the white dwarf. This gas collects into a swirling hot disk around the white dwarf and from there cascades onto its hot surface. Eventually, over periods of thousands of years, the accumulated gas is touched off by the white dwarf in a tremendous thermonuclear flash that blows gas into space in shells, or gaseous bubbles.

Judging from the image, astronomers think the hourglass-shaped nebulae may represent two separate outbursts that occurred several thousand years apart. They further speculate that 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 Southern Crab nebula is located in the southern constellation of Centaurus, and lies a few thousand light-years from Earth.