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Ring Nebula
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==Properties== M57 is {{Convert|0.787|kpc|ly|abbr=in|lk=on}} from [[Earth]].<ref name="simbad" /> It has a [[Apparent magnitude|visual magnitude]] of 8.8 and a dimmer [[photographic magnitude]], of 9.7. Photographs taken over a period of 50 years<ref name=karttunen/> show the rate of nebula expansion is roughly 1 [[Minute and second of arc|arcsecond]] per century, which corresponds to spectroscopic observations as 20–{{val|30|u=km/s}}. M57 is illuminated by a central [[white dwarf]] of 15.75v [[Apparent magnitude|visual magnitude]].<ref name=apj134/> All the interior parts of this nebula have a blue-green tinge that is caused by the doubly ionized [[oxygen]] [[emission line]]s at 495.7 and 500.7 [[nanometer|nm]]. These observed so-called "[[forbidden line]]s" occur only in conditions of very low density containing a few atoms per cubic centimeter. In the outer region of the ring, part of the reddish hue is caused by [[hydrogen]] emission at 656.3 nm, forming part of the [[Balmer series]] of lines. Forbidden lines of ionized [[nitrogen]] or [[N II]] contribute to the reddishness at 654.8 and 658.3 nm.<ref name=karttunen> {{cite book | first=Hannu | last=Karttunen | date=2003 | title=Fundamental Astronomy | url=https://archive.org/details/fundamentalastro00kart_973 | url-access=limited | pages=[https://archive.org/details/fundamentalastro00kart_973/page/n315 314] | publisher=[[Springer (publisher)|Springer]] | isbn=978-3-540-00179-9 }}</ref> ===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> ===Central star=== The central star was discovered by Hungarian astronomer Jenő Gothard on September 1, 1886, from images taken at his observatory in Herény, near [[Szombathely]]. Within the last two thousand years, the central star of the Ring Nebula has left the [[asymptotic giant branch]] after exhausting its supply of [[hydrogen]] fuel. Thus it no longer produces its energy through [[nuclear fusion]] and, in [[stellar evolution|evolutionary]] terms, it is now becoming a compact [[white dwarf]] star. The central star now consists primarily of [[carbon]] and [[oxygen]] with a thin outer envelope composed of lighter elements. Its mass is about {{Solar mass|0.61–0.62}}, with a surface temperature of {{val|fmt=commas|125,000|5,000|ul=K}}. Currently it is 200 times more luminous than the [[Sun]], but its [[apparent magnitude]] is only +15.75.<ref name=apj134> {{cite journal | last=O'Dell | first=C. R. |author2=Sabbadin, F. |author3=Henney, W. J. | date=2007 | title=The Three-Dimensional Ionization Structure and Evolution of NGC 6720, The Ring Nebula | journal=[[Astronomical Journal]] | volume=134 | issue=4 | pages=1679–1692 | bibcode=2007AJ....134.1679O | doi=10.1086/521823 | doi-access=free| url=https://www.openaccessrepository.it/record/99251/files/fulltext.pdf| archive-url=https://web.archive.org/web/20240220232803/https://www.openaccessrepository.it/record/99251/files/fulltext.pdf| url-status=dead| archive-date=February 20, 2024}}</ref> In 2025 [[James Webb Space Telescope|JWST]] observed a dust disk around the central star.<ref>{{cite arXiv |eprint=2504.01188 |last1=Sahai |first1=Raghvendra |author2=Griet Van de Steene |author3=Peter van Hoof |last4=Zijlstra |first4=Albert |last5=Volk |first5=Kevin |last6=Dinerstein |first6=Harriet L. |last7=Barlow |first7=Michael J. |last8=Peeters |first8=Els |last9=Manchado |first9=Arturo |last10=Matsuura |first10=Mikako |last11=Cami |first11=Jan |last12=Cox |first12=Nick L. J. |last13=Aleman |first13=Isabel |last14=Bernard-Salas |first14=Jeronimo |last15=Clark |first15=Nicholas |last16=Justtanont |first16=Kay |last17=Kaplan |first17=Kyle F. |last18=Kavanagh |first18=Patrick J. |last19=Wesson |first19=Roger |title=JWST observations of the Ring Nebula (NGC 6720): III. A dusty disk around its Central Star |date=2025 |class=astro-ph.SR }}</ref>
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