A planetarynebula is formed when a star, during the last stages of itsevolution before becoming awhite dwarf, expels a vast luminous envelope of ionized gas into the surroundinginterstellar space. The progenitor star for the ring nebula is now a carbon-oxygenwhite dwarf with anapparent visual magnitude of +15.75. Based onparallax measurements, this star is located at a distance of approximately 2,570light-years (790 pc) from the Sun. After expanding for 1,610 years, the nebula currently has a diameter of4.6 ly.
This nebula was discovered by the French astronomerCharles Messier while searching forcomets in late January 1779. Messier's report of his independent discovery ofComet Bode reached fellow French astronomerAntoine Darquier de Pellepoix two weeks later, who then independently rediscovered the nebula while following the comet. Darquier later reported that it was "...as large as Jupiter and resembles a planet which is fading" (which may have contributed to the use of the persistent "planetary nebula" terminology).[7] It would be entered intoMessier's catalogue as the 57th object. Messier and German-born astronomerWilliam Herschel speculated that the nebula was formed by multiple faint stars that were unresolvable with his telescope.[8][9]
In 1800, German CountFriedrich von Hahn announced that he had discovered the faint central star at the heart of the nebula a few years earlier. He also noted that the interior of the ring had undergone changes, and said he could no longer find the central star.[10] In 1864, Englishamateur astronomerWilliam Huggins examined the spectra of multiple nebulae, discovering that some of these objects, including M57, displayed the spectra of brightemission lines characteristic of fluorescing glowing gases. Huggins concluded that most planetary nebulae were not composed of unresolved stars, as had been previously suspected, but were nebulosities.[11][12] The nebula was first photographed by the Hungarian astronomerEugene von Gothard in 1886.[10]
Location of the Ring Nebula in the constellation Lyra
M57 is found south of the bright starVega, which forms the northwestern vertex of theSummer Triangle asterism. The nebula lies about 40% of the distance fromBeta (β) toGamma (γ) Lyrae, making it an easy target foramateur astronomers to find.[13]
The nebula disk has an angular size of1.5 × 1arcminutes, making it too small to be resolved with 10×50binoculars.[13] It is best observed using atelescope with anaperture of at least 20 cm (8 in), but even a 7.5 cm (3 in) telescope will reveal its elliptical ring shape.[14] Using a UHC or OIII filter greatly enhances visual observation, particularly inlight polluted areas. The interior hole can be resolved by a 10 cm (4 in) instrument at a magnification of 100×.[13] Larger instruments will show a few darker zones on the eastern and western edges of the ring and some faint nebulosity inside the disk.[15] The centralstar, at magnitude 14.8, is difficult to spot.[14]
M57 is 0.787 kpc (2,570light-years) fromEarth.[4] It has avisual magnitude of 8.8. Photographs taken over a period of 50 years show the rate of nebula expansion is roughly 1 arcsecond per century.[16] Spectroscopic observations show that the expansion velocity along the line of sight is 20–30 km/s. M57 is illuminated by a centralwhite dwarf with anapparent magnitude of 15.75.[17]
The interior parts of this nebula have a blue-green tinge that is caused by the doubly ionizedoxygenemission lines at 495.7 and 500.7 nm. These emission lines are so-called "forbidden lines" which occur only in regions of very low density containing no more than a few thousand atoms per cubic centimeter. In the outer region of the ring, part of the reddish hue is caused byhydrogen emission at 656.3 nm, forming part of theBalmer series of lines. Forbidden lines of ionizednitrogen orN II contribute to the reddishness at 654.8 and 658.3 nm.[16]
M57 is thought to be aprolate spheroid with strong concentrations of material along itsequator. From Earth, the symmetrical axis is viewed at about 30°. Overall, the observed nebulosity has been estimated to be expanding for approximately 1,610 ± 240 years.[citation needed]
The central star was discovered by Hungarian astronomer Jenő Gothard on September 1, 1886, from images taken at his observatory in Herény, nearSzombathely. Within the last two thousand years, the central star of the Ring Nebula has left theasymptotic giant branch. It no longer produces its energy throughnuclear fusion and, inevolutionary terms, it is now becoming a compactwhite dwarf star.
The central star now consists primarily ofcarbon andoxygen with a thin outer envelope composed of lighter elements. Its mass is about 0.61–0.62 M☉, with a surface temperature of125,000±5,000 K. Currently it is about 300 times more luminous than theSun,[18] but itsapparent magnitude is only +15.75.[17]
In 2025JWST observed a dust disk around the central star.[18]
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