TheHelix Nebula (also known asNGC 7293 orCaldwell 63) is aplanetary nebula (PN) located in theconstellationAquarius. Discovered byKarl Ludwig Harding, most likely before 1824, this object is one of the closest of all the bright planetary nebulae to Earth.[3] The distance, measured by theGaia mission, is 655±13 light-years.[4] It is similar in appearance to theCat's Eye Nebula and theRing Nebula, whose size, age, and physical characteristics are in turn similar to theDumbbell Nebula, differing only in their relative proximity and the appearance from the equatorial viewing angle.[5] The Helix Nebula has sometimes been referred to as the "Eye of God" inpop culture,[6] as well as the "Eye of Sauron".[7][8]
The Helix Nebula is an example of a planetary nebula, formed by an intermediate to low-mass star, which sheds its outer layers near the end of its evolution. Gases from the star in the surrounding space appear, from Earth's perspective, ahelix structure. The remnant central stellar core, known as the central star (CS) of the planetary nebula, is destined to become awhite dwarf star. The observed glow of the central star is so energetic that it causes the previously expelled gases to brightlyfluoresce.
The nebula is in the constellation ofAquarius, and lies about 650light-years away, spanning about 0.8parsecs (2.5 light-years). Its age is estimated to be10600+2300 −1200 years, based on the ratio of its size to its measured expansion rate of 31 km·s−1.[5]
A 3 dimensional map of carbon monoxide in NGC 7293[9]Structure and cometary knots are prominent in this Infrared false-color image taken by theSpitzer Space Telescope[10]The location of NGC 7293 (labelled in red)
The Helix Nebula is thought to be shaped like aprolate spheroid with strong density concentrations toward the filled disk along theequatorial plane, whosemajor axis is inclined about 21° to 37° from our vantage point. The size of the inner disk is 8×19 arcmin in diameter (0.52 pc); the outer torus is 12×22 arcmin in diameter (0.77 pc); and the outer-most ring is about 25 arcmin in diameter (1.76 pc). The outer-most ring appears flattened on one side due to it colliding with the ambientinterstellar medium.[11]
Expansion of the whole planetary nebula structure is estimated to have occurred in the last 12,100 years, and 6,560 years for the inner disk.[2] Spectroscopically, the inner disk's expansion rate is 40 km/s, and about 32 km/s for the outer ring.
The Helix Nebula was the first planetary nebula discovered to containcometary knots.[12] Its main ring contains knots of nebulosity, which have now been detected in several nearby planetary nebulae, especially those with a molecular envelope like theRing nebula and theDumbbell Nebula.[13]
These knots are radially symmetrical (from the CS) and are described as "cometary", each centered on a core of neutral molecular gas and containing bright localphotoionization fronts or cusps towards the central star and tails away from it.[14] All tails extend away from the Planetary Nebula Nucleus (PNN) in a radial direction. Excluding the tails, each knot is approximately the size of theSolar System, while each of the cusp knots are optically thick due toLyc photons from the CS.[2][5][15] There are about 40,000 cometary knots in the Helix Nebula.[16]
The knots are probably the result ofRayleigh-Taylor instability. The low density, high expansion velocity ionized inner nebula is accelerating the denser, slowly expanding, largely neutral material which had been shed earlier when the star was on theAsymptotic Giant Branch.[17]
Theexcitation temperature varies across the Helix nebula.[18] The rotational-vibrational temperature ranges from 1800K in a cometary knot located in the inner region of the nebula are about 2.5'(arcmin) from the CS, and is calculated at about 900 K in the outer region at the distance of 5.6'.[18]
A light curve for the Helix Nebula central star, adapted from Iskandarli et al. (2024)[19]
The central star of the Helix Nebula is awhite dwarf of spectral type DAO.[19] It has the designations WD 2226-210, PHL 287, and GJ 9785.[1] The star has a radius of 0.025 solar radii (17,000 km), a mass of 0.678 M☉, a temperature of 120,000Kelvin and has an apparent magnitude of 13.5.[19]
A mid-infrared excess suggest a disk with a size of 35 to 150 AU, formed fromKuiper-belt like objects.[20] The size was later revised to be a ring between 30 and 100 AU. The non-detection at longer wavelengths allowed a research team to reject a series of scenarios. The researchers think the mid-IR excess comes from a replenishment of dust particles from thousands ofexocomets at high eccentricities, with an origin from anOort cloud-like structure.[21]
A 2024 study hypothesized that the central star might be orbited by a planet based on periodic variations in itslight curve, but it cannot be ruled out that these variations are due to intrinsic stellar variability. Assuming an inclination of 25° (aligned with the nebula itself), this hypothetical planet is estimated to have a radius of 0.021 solar radii (15,000 km), or about 2.3 times theradius of Earth.[19]
Another study from 2025 found from X-ray observation that the central star may be accreting the remains of a Jupiter-like planet. This would be closer than the planet found via optical variability.[22]
This zoom sequence starts with a wide-field view of the rather empty region of sky around the constellation of Aquarius.
This video compares a new view of the Helix Nebula acquired with the VISTA telescope in infrared light with the more familiar view in visible light from the MPG/ESO 2.2-metre telescope at ESO's La Silla Observatory.
A 3D model of the Helix Nebula from the Galaxy Map app (iOS/Android)
^Huggins, Patrick; Bachiller, Rafael; Cox, Pierre; Forveille, Thierry (1992). "CO in the globules of the Helix nebula".The Astrophysical Journal Letters.401:L43 –L46.Bibcode:1992ApJ...401L..43H.doi:10.1086/186666.
^O'Dell, C. R.; Balick, B.; Hajian, A. R.; Henney, W. J.; et al. (2003). "Knots in Planetary Nebulae". In Arthur, Jane; Henney, William (eds.).Winds, Bubbles, and Explosions: A Conference to Honor John Dyson, Pátzcuaro, Michoacán, México, September 9–13, 2002. Revista Mexicana de Astronomía y Astrofísica, Serie de Conferencias. Vol. 15. Instituto de Astronomía, Universidad Nacional Autónoma de México. pp. 29–33.Bibcode:2003RMxAC..15...29O.
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