The Ring Nebula is not a ring

The Ring Nebula is in every introductory astronomy book, and it is the reason a
great many people picture a planetary nebula as a smoke ring: a bright ellipse
with a darker middle and a faint star at the centre.

Hubble observations established that the ring is an artefact of our viewing
angle. The object is not a ring. It is closer to a barrel, and we are looking
straight down the barrel.

A doughnut with jam in it

The astronomer who led the work, C. Robert O'Dell, put it memorably: the nebula
is not a bagel but a jelly doughnut — it is filled in the middle.

The darker centre is not empty space. It is material moving towards us and away
from us along our line of sight, which is faint because there is comparatively
little of it and because it is not being seen edge-on. The bright ellipse is
the denser equatorial band, where our sight line passes through far more gas.

Work out the three-dimensional structure and you get a roughly barrel-shaped
shell with a thick ring around its waist, oriented so that one end of the
barrel points almost exactly at Earth.

How you measure a shape you cannot see from the side

You cannot fly around it. The structure was reconstructed from three things:

• Velocity measurements. Gas moving towards us is blueshifted, gas moving
  away is redshifted. Mapping velocity across the face gives you depth.
• Emission from different elements. Different ions glow at different
  distances from the central star, so each traces a different layer.
• Comparison with nebulae we see from other angles. Objects of the same
  class seen edge-on show the barrel directly, confirming the inference.

Why the shape is not round

A single isolated star losing mass evenly should produce a sphere. Most
planetary nebulae are not spherical, and that is one of the standing problems
in the field.

The favoured explanations involve something breaking the symmetry: a companion
star, a strong magnetic field, or rapid rotation — each of which would make it
easier for gas to escape along the poles than at the equator, producing exactly
this barrel-with-a-waist geometry.

Why it matters beyond the picture

The Ring Nebula is what the Sun will become in about five billion years: the
outer layers drifting away, the exposed core cooling for billions of years as a
white dwarf.

Correcting the shape is not pedantry. Getting the geometry right changes the
estimate of how much mass the star shed, and that number feeds into how much
material stars like the Sun return to the galaxy — which is where the next
generation of stars, and planets, comes from.