Helix Nebula facts for kids
The Helix Nebula is a planetary nebula in the constellation of Aquarius. It is about 700 light-years away from Earth.
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). We see the outer-most ring as flattened on one side due to its colliding with the ambient interstellar medium.
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. Spectroscopically, the outer ring's expansion rate is 40 km·s−1, and about 32 km·s−1 for the inner disk.
The Helix Nebula was the first planetary nebula discovered to contain cometary knots. Its main ring contains knots of nebulosity, which have now been detected in many nearby planetaries. These knots are highly radially symmetric (from the PNN) and are described as "cometary", each centered on a core of neutral molecular gas and containing bright cusps (local photoionization fronts) towards the central star and tails away from it. All tails extend away from the PNN in a radial direction. Excluding the tails, they are (very approximately) the size of the Solar system, while each of the cusp knots are optically thick due to Lyc photons from the PNN. There are more than 20,000 cometary knots estimated to be in the Helix Nebula.
The excitation temperature varies across the Helix nebula. 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 central PNN, calculated at about 900 K in the outer region at the distance of 5.6′.