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Deep Ground-Based Images of the Necklace Nebula...

"Image based on data obtained as part of the INT Photometric Hα Survey of the Northern Galactic Plane (IPHAS): prepared by Nick Wright, University College London; the additional narrowband data was collected by Romano Corradi and collaborators .

"This beautiful image of the Necklace Nebula is a composite built from follow-up narrow band imaging of this IPHAS discovery. The equatorial ring, broken up into head-tail cometary structures was picked up first in Hα in IPHAS exposures. The more diffuse structure bathing the central equatorial ring is seen in [OIII], while the detached polar knots ('ansae', shown in blue) were detected in [NII]. From ansa to ansa, the aky angle is a few arcminutes. (credits: Nick Wright, University College London; the additional narrowband data was collected by Romano Corradi and collaborators). Further information on the science of the Necklace Nebula can be found here.
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A Closer Look at Cometary Knots in Planetary Nebula...

Necklace Nebula Cometary Knots
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Helix Nebula Cometary Knots
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The Necklace Nebula (at left) consists of a bright equatorial ring, measuring 12 trillion miles across, dotted with dense, bright knots of gas. The knots glow brightly due to absorption of ultraviolet light from the central stars. The knots are made up of glowing hydrogen and oxygen gas. Each knot also dons a small tail pointing away from the central star.

The Helix Nebula (at right) was the first planetary nebula discovered to contain knots. Its main ring contains knots of nebulosity, which have now been detected in many nearby planetaries. These knots are highly radially symmetric and are described as "cometary", each containing bright cusps and fainter tails. All extend away from the PNN in a radial direction. There are more than 20,000 cometary knots estimated to be in the Helix Nebula.