The Hourglass That Never Runs Out

A tempestuous relationship between an unlikely pair of stars may have created the "Southern Crab Nebula", a stellar hourglass nestled within an hourglass.

Images taken with Earth-based telescopes have shown the outer nebula. But this picture - taken with the HST - reveals a small, bright nebula embedded in the center of the larger one (close-up of nebula in inset).

The possible creators of these shapes are a pair of aging stars buried in the glow of the tiny, central nebula. One 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 is known as a symbiotic system.

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", in 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.