European astronomers see relics of ancient explosions in symbiotic stars
3 September 1999One thousand years ago an explosion in the vicinity of a star created a huge bubble of gas, one of those objects that astronomers call a nebula. Some hundred years afterwards a second outburst followed and another nebula was born. Today, European astronomers have pictured the relics of both events with the NASA/ESA Hubble Space Telescope: two hourglass-shaped nebulae, one nestled inside the other like Russian dolls. The system, called 'Southern Crab Nebula' (He2-104) because it looks like the body and legs of a crab, is located a few thousand light years from Earth in the Southern Hemisphere.
The true story behind such a beautiful object has to do with stellar cannibalism - or companionship - if you prefer. The star where the explosions took place is a dying 'white dwarf' star, very dense, hot and small. It no longer has any fuel of its own to keep on shining, but it does have a companion: a red giant star that is also very old, but still massive enough to feed the white dwarf with its material. Both stars are very close to each other - at about ten times the Earth-Sun distance - and form a so-called 'symbiotic system'.
"Strong stellar winds are making the red giant lose its outer layers, and that's the material the white dwarf is living on", says Italian researcher Romano Corradi, at the Instituto de Astrofmsica de Canarias (Spain). "These observations will help us to learn how symbiotic systems work. We still don't fully understand the evolution of stars which are so close together".
Astronomers think that the explosions leading to the hourglass-shaped nebulae were triggered by the accumulation of gas from the red giant star - the 'donor' - on the surface of the white dwarf star - the 'recipient'. The material from the donor star formed an accretion disk around the recipient star, and spiralled onto its hot surface; gas continued to build up on the surface until it sparked the eruptions, blowing material into space and creating the nebulae.
Nearly 150 symbiotic stars like these are known in our galaxy, but only a few are surrounded by nebulae like the Crab Nebula. Nebulae of this kind are very important for astronomers because they provide the only clue to reconstructing the past history of the symbiotic system.
"Without the nebulae we can't guess whether there have been ancient explosions or not, and when. This is what makes this object so special", says Corradi.