Have you ever wondered why the area at the back of your elbow is nicknamed the “funny bone”? For starters, it’s really not a bone. It’s the Ulnar nerve. and when you bump it, it’s not funny at all! In the same way, some of the most beautiful objects in space have been given a name that doesn’t fit very well.
A planetary nebula has nothing to do with planets, yet it’s stuck with the name. When astronomers first noticed them in their telescopes, they thought they resembled dim planets, so the nebulas were given a name that seemed to fit. Bigger and better telescopes showed they were expanding clouds of gas that had been ejected from unstable stars. The star at the center of the nebula is usually extremely hot and shines brightly with ultraviolet light. The UV light causes the gas around the star to glow.
Some of these stars rotate while ejecting gas, in a way that creates elaborate and colorful designs. Photography can reveal these designs in detail. Looking at them through a backyard telescope reveals some hints of the structure, but the colors are usually not noticeable. It’s helpful to use a light pollution filter to improve contrast. Planetary nebulas emit light in only one or two narrow slices of the spectrum. An OIII or UHC filter can allow that light to pass through while blocking all other unwanted light.
About 1,500 planetary nebulas have been found in the Milky Way galaxy. The closest ones make good targets for small telescopes. They often have a high surface brightness compared to other types of nebulas, although they usually appear small. Appearances can be deceptive though. The commonly observed ones like the Dumbbell Nebula, Helix Nebula, and Ring Nebula are all over one light-year across.
The Helix Nebula is big enough to fit over 4,000 of our solar systems across its almost 6 light-year diameter! Since its light is spread out over a large area, it’s hard to see with a telescope.
The Dumbbell Nebula was the first to be discovered. Charles Messier noticed it and added it to his list of things that looked like comets but weren’t. You can discover it for yourself, too, by pointing a telescope up into the Milky Way on a summer evening. It’s less than ten degrees to the east of the head of Cygnus the Swan.
On the opposite side of the head of Cygnus and less than ten degrees from it, you can find another planetary nebula. The star that blew off this shell of gas and dust did it evenly compared to others. Through a telescope it looks like a smoke ring. It was named the Ring Nebula, a name that actually does fit.