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The Helix Nebula, NGC 7293, is one of the most interesting and beautiful planetary nebula, and is also the closest nebula to Earth, at a distance of only 700 light years away. This new image, taken with an infrared camera on the Subaru telescope in Hawaii, shows hundreds of previously unseen comet-shaped knots inside the nebula. The sheer number of knots, more than has ever been seen before appears like a massive firework display in space.
Planetary nebulae are the final stages in the lives of most low-mass stars, such as our Sun. As they reach the ends of their lives they throw off large amounts of material into space. This exposes their inner cores where nuclear burning once took place, and from which bright ultraviolet radiation illuminates the ejected material, creating these nebulae. The Helix Nebula was the first planetary nebula in which knots were seen, and they may hold clues as to what planetary material may survive to the end of a star's life.
Astronomers from the National Observatory of Japan and Universities in the UK and USA made this discovery when studying the emission from molecules of hydrogen in the infrared. These molecules are often destroyed by ultraviolet radiation in space, but have survived in these knots, shielded by dust and gas that can be seen in optical images. The comet-like shape of these knots is due to steady evaporation of gas from the knots due to the strong winds and ultraviolet radiation from the dying star in the centre of the nebula.
Unlike previous optical images of the Helix Nebula knots, the image shows hundreds of knots, clearly resolved and extending out to greater distances from the central star than previously seen. The extent of the cometary tails varies with the distance from the central star, in the same way that Solar System comets have larger tails when they are closer to the Sun and the wind and radiation are stronger. This shows how the knots are slowly being destroyed by the central star and highlights the places where molecular and atomic material can be found in space.
From these images astronomers have estimated that there may be as many as 7000 knots in the entire nebula, each of which are billions of miles across. Their total mass may be as much as 30,000 Earths, or one tenth the mass of our Sun. The origin of the knots is currently unknown. Are they remnants of the star's planetary system or material ejected from the star at some stage in its life? Either answer would help astronomers answer important questions about the lives of stars and planetary systems.
The full original image is available here, and available with the inset marking the image's position available here. Image credit: M. Matsuura (UCL / NAOJ) & N.J. Wright (CfA).
This work has been published in the Astronomical Journal and the press release from the Subaru Telescope at the National Astronomical Observatory of Japan can be found here.
Updated: 23rd August 2009