Why would a planetary nebula look like a ring to every observer in the galaxy?

Answer 1

A simple way to think of it would be if you lay a doughnut flat on a table and measure the thickness from the outside edge to the hole. Let's say it's 2 inches thick. Then measure the distance across the entire doughnut on the right and left edges without touching the hole. The amount of dough there may be 5 inches thick when seen edge on. The same principle works for a shell of gas. The shell of gas in a planetary nebula reflects more light from the edges because you are seeing more gas. The center seems hollow because you are looking through less gas. No matter where you look from, when you look right into the center you see less reflected light than around the edges so it looks like a ring.

Answer 2

A planetary nebula appears ring-shaped to observers because of its structure, which is typically bipolar or even cylindrical in shape. The material expelled by the dying star forms a spherical shell that can sometimes appear as a ring when viewed edge-on. This structure is common for planetary nebulae due to the dynamics of the star's winds and the surrounding interstellar medium.