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Ring Nebula
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===Nebula structure=== M57 is of the class of such starburst nebulae known as [[Bipolar nebula|bipolar]], whose thick equatorial rings visibly extend the structure through its main axis of symmetry. It appears to be a [[prolate spheroid]] with strong concentrations of material along its [[equator]]. From Earth, the symmetrical axis is viewed at about 30°. Overall, the observed nebulosity has been currently estimated to be expanding for approximately 1,610 ± 240 years. Structural studies find this planetary nebula exhibits knots characterized by well-developed symmetry. However, these are only silhouettes visible against the background emission of the nebula's equatorial ring. M57 may include internal N II emission lines located at the knots' tips that face the central star; however, most of these knots are neutral and appear only in extinction lines. Their existence shows they are probably only located closer to the ionization front than those found in the [[Lupus (constellation)|Lupus]] planetary [[IC 4406]]. Some of the knots do exhibit well-developed tails which are often detectable in optical thickness from the visual spectrum.<ref name="ODelletal2002"> {{cite journal | author=O'Dell, C. R. | author2=Balick, B. | author3=Hajian, A. R. | author4=Henney, W. J. | author5=Burkert, A. | date=2002 | title=Knots in Nearby Planetary Nebulae | journal=[[Astronomical Journal]] | volume=123 | issue=6 | pages=3329–3347 | bibcode=2002AJ....123.3329O | doi=10.1086/340726 | doi-access=free }}</ref><ref name="ODelletal2003"> {{cite journal | last1=O'Dell | first1=C. R. | last2=Balick | first2=B. | last3=Hajian | first3=A. R. | last4=Henney | first4=W. J. | last5=Burkert | first5=A. | date=2003 | title=Knots in Planetary Nebulae | journal=Revista Mexicana de Astronomía y Astrofísica, Serie de Conferencias | volume=15 | pages=29–33 | bibcode=2003RMxAC..15...29O }}</ref>
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