Helix Nebula

Template:Short description Template:For Template:Redirect Template:Planetary nebula The Helix Nebula (also known as NGC 7293 or Caldwell 63) is a planetary nebula (PN) located in the constellation Aquarius. Discovered by Karl Ludwig Harding, most likely before 1824, this object is one of the closest of all the bright planetary nebulae to Earth.<ref name="Horaetal2006">Template:Cite journal</ref> The distance, measured by the Gaia mission, is 655±13 light-years.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> It is similar in appearance to the Cat's Eye Nebula and the Ring Nebula, whose size, age, and physical characteristics are in turn similar to the Dumbbell Nebula, differing only in their relative proximity and the appearance from the equatorial viewing angle.<ref name="ODelletal2002">Template:Cite journal</ref> The Helix Nebula has sometimes been referred to as the "Eye of God" in pop culture,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> as well as the "Eye of Sauron".<ref>Template:Cite journal</ref><ref>Template:Cite news</ref>

General informationEdit

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, a helix structure. The remnant central stellar core, known as the central star (CS) of the planetary nebula, is destined to become a white dwarf star. The observed glow of the central star is so energetic that it causes the previously expelled gases to brightly fluoresce.

The nebula is in the constellation of Aquarius, and lies about 650 light-years away, spanning about 0.8 parsecs (2.5 light-years). Its age is estimated to be Template:Val years, based on the ratio of its size to its measured expansion rate of 31 km·s⁻¹.<ref name="ODelletal2002" />

StructureEdit

File:Helix3D.png

A 3 dimensional map of carbon monoxide in NGC 7293<ref>Template:Cite journal</ref>

File:Comets Kick up Dust in Helix Nebula (PIA09178).jpg

Structure and cometary knots are prominent in this Infrared false-color image taken by the Spitzer Space Telescope<ref name="Suetal2007">Template:Cite journal</ref>

File:NGC7293Location.png

The location of NGC 7293 (labelled in red)

The Helix Nebula is thought to be shaped like a prolate spheroid with strong density concentrations toward the filled disk along the equatorial plane, whose major 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 ambient interstellar medium.<ref>Template:Cite journal</ref>

Expansion of the whole planetary nebula structure is estimated to have occurred in the last 6,560 years, and 12,100 years for the inner disk.<ref name="ODelletal2004" /> Spectroscopically, the outer ring's expansion rate is 40 km/s, and about 32 km/s for the inner disk.

KnotsEdit

File:Close-Up of the Helix Nebula.jpg

A closer view of knots in the nebula

The Helix Nebula was the first planetary nebula discovered to contain cometary knots.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Its main ring contains knots of nebulosity, which have now been detected in several nearby planetary nebulae, especially those with a molecular envelope like the Ring nebula and the Dumbbell Nebula.<ref>Template:Cite journal</ref>

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 local photoionization fronts or cusps towards the central star and tails away from it.<ref name="Huggins1992">Template:Cite journal</ref> All tails extend away from the Planetary Nebula Nucleus (PNN) in a radial direction. Excluding the tails, each knot is approximately the size of the Solar System, while each of the cusp knots are optically thick due to Lyc photons from the CS.<ref name="ODelletal2004">Template:Cite journal</ref><ref name="ODelletal2002" /><ref name="ODelletal2003">Template:Cite book</ref> There are about 40,000 cometary knots in the Helix Nebula.<ref>Template:Cite journal</ref>

The knots are probably the result of Rayleigh-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 the Asymptotic Giant Branch.<ref>Template:Cite journal</ref>

The excitation temperature varies across the Helix nebula.<ref name="Matsuuraetal2007">Template:Cite journal</ref> The rotational-vibrational temperature ranges from 1800 K 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'.<ref name="Matsuuraetal2007" />

Central starEdit

File:HelixCentralStarLightCurve.png

A light curve for the Helix Nebula central star, adapted from Iskandarli et al. (2024)<ref name="Iskandarli2024"/>

The central star of the Helix Nebula is a white dwarf of spectral type DAO.<ref name="Iskandarli2024"/> It has the designations WD 2226-210, PHL 287, and GJ 9785.<ref name="simbad"/> The star has a radius of Template:Convert, a mass of Template:Solar mass, a temperature of 120,000 Kelvin and has an apparent magnitude of 13.5.<ref name="Iskandarli2024"/>

A mid-infrared excess suggest a disk with a size of 35 to 150 AU, formed from Kuiper-belt like objects.<ref name="Su2007"/> 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 of exocomets at high eccentricities, with an origin from an Oort cloud-like structure.<ref name="Marshall2023"/>

A 2024 study hypothesized that the central star might be orbited by a planet based on periodic variations in its light 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 Template:Convert, or about 2.3 times the radius of Earth.<ref name="Iskandarli2024"/>

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.<ref name="Estrada-Dorado2025"/>

Template:OrbitboxPlanet begin Template:OrbitboxPlanet hypothetical Template:OrbitboxPlanet hypothetical Template:OrbitboxPlanet disk Template:Orbitbox end