The
Ring Nebula (also catalogued as
Messier 57,
M57 and
NGC 6720) is a
planetary nebula in the mildly northern
constellation of
Lyra. Such a nebula is formed when a star, during the last stages of its
evolution before becoming a
white dwarf, expels a vast luminous envelope of ionized gas into the surrounding
interstellar space. M57 is of the class of such starburst nebulae known as
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 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 PNN; 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 planetary
IC 4406. Some of the knots do exhibit well-developed tails which are often detectable in optical thickness from the visual spectrum.
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