New near-infrared image of the Helix Nebula, showing comet-shaped knots within.

The Helix Nebula, NGC 7293, is not only one of the most interesting and beautiful planetary nebulae; it is also one of the closest nebulae to Earth, at a distance of only 710 light years away. A new image, taken with an infrared camera on the Subaru Telescope in Hawaii, shows tens of thousands of previously unseen comet-shaped knots inside the nebula. The sheer number of knots — more than have ever been seen before — looks like a massive fireworks display in space.

The Helix Nebula was the first planetary nebula in which knots were seen, and their presence may provide clues to what planetary material may survive at the end of a star’s life. Planetary nebulae are the final stages in the lives of low-mass stars, such as our Sun. As they reach the ends of their lives they throw off large amounts of material into space. Although the nebula looks like a fireworks display, the process of developing a nebula is neither explosive nor instantaneous; it takes place slowly, over a period of about 10,000 to 1,000,000 years. This gradual process creates these nebulae by exposing their inner cores, where nuclear burning once took place and from which bright ultraviolet radiation illuminates the ejected material.

Astronomers from the National Astronomical Observatory of Japan (NAOJ), from London, Manchester and Kent universities in the UK and from a university in Missouri in the USA studied the emissions from hydrogen molecules in the infrared and found that knots are found throughout the entire nebula. Although these molecules are often destroyed by ultraviolet radiation in space, they have survived in these knots, shielded by dust and gas that can be seen in optical images. The comet-like shape of these knots results from the steady evaporation of gas from the knots, produced by the strong winds and ultraviolet radiation from the dying star in the center of the nebula.

Unlike previous optical images of the Helix Nebula knots, the infrared image shows thousands of clearly resolved knots, extending out from the central star at greater distances than previously observed. The extent of the cometary tails varies with the distance from the central star, just as Solar System comets have larger tails when they are closer to the Sun and when wind and radiation are stronger. “This research shows how the central star slowly destroys the knots and highlights the places where molecular and atomic material can be found in space,”says lead astronomer Dr. Mikako Matsuura, previously at NAOJ and now from University College London.

These images enable astronomers to estimate that there may be as many as 40,000 knots in the entire nebula, each of which are billions of kilometers/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 are they material ejected from the star at some stage in its life? Either answer will help astronomers answer important questions about the lives of stars and planetary systems.

The innovative technology of the Subaru Telescope with its near-infrared camera, MOIRCS, enabled researchers to produce such impressive images. Mounted on one of the largest infrared optical telescopes in the world, MOIRCS (Multi-object Infrared Camera and Spectrograph) has a large (4 arcmin by 7 arcmin) field of view, allowing it to capture, with a single shot, such detailed features in a large PN.

This paper will be published in the Astrophysical Journal in August 2009.

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