The Cat’s Eye Nebula and the eleven rings nobody expected

The Cat's Eye was one of the first planetary nebulae ever identified, and it
remains one of the strangest. Hubble's detailed images show the bright inner
structure that gives it its name — and, around that, **eleven or more
concentric rings**, nested one inside another like the layers of an onion.

What a planetary nebula actually is

The name is a historical mistake worth clearing up. Planetary nebulae have
nothing to do with planets. Eighteenth-century observers saw small round
greenish discs through their telescopes, thought they looked like Uranus, and
the name stuck.

What they are is the end of a star like the Sun. Having exhausted its fuel, the
star sheds its outer layers over thousands of years, exposing the hot core
beneath. That core lights the expelled gas from inside, and the glowing shell is
what we see. The Sun will do this in about five billion years.

The rings are not rings

Each apparent ring is really a spherical shell of gas surrounding the star.
We see them as rings for the same reason a soap bubble looks like a bright
outline: at the edge of a sphere, your line of sight passes through far more
material than at its centre, so the edge appears brighter.

So the Cat's Eye is not eleven flat hoops. It is eleven nested bubbles, seen
from outside.

The timing is the puzzle

Measuring the spacing gives the interval between ejections: roughly
1,500 years, repeated at least eleven times, with striking regularity.

Something about the star was pulsing on that cycle, and there is no agreed
explanation. The candidates:

• Thermal pulses in the star's shell burning — these happen, but the
  expected interval is usually much longer.
• Magnetic cycles, an analogue of the Sun's eleven-year sunspot cycle
  operating on a far longer timescale.
• A companion star, whose orbit modulates the mass loss.

The companion idea has gained ground, partly because the inner region of the
Cat's Eye is markedly not spherical — it has jets and a complex axial
structure of the kind a binary system tends to produce. A single isolated star
losing mass should produce something round.

Why this object matters

Planetary nebulae are brief. This phase lasts perhaps ten thousand years
against a stellar lifetime of billions, so catching one is catching a moment.
The Cat's Eye is close enough and detailed enough to serve as the test case:
any model of how stars like the Sun end has to explain those rings, and none
yet does so completely.