Helix Nebula by Hubble Postcard

Helix Nebula by Hubble. The Helix Nebula is thought to be shaped like a prolate spheroid with strong density concentrations toward the filled disc along the equatorial plane, whose major axis is inclined about 21° to 37° from our vantage point. The size of the inner disc 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 disc. Spectroscopically, the outer ring's expansion rate is 40 km·s−1, and about 32 km·s−1 for the inner disc.