Nebula, plural Nebulae
Nebulae are made of diffuse matter comprising dust and gas. They are located inside the disk of our galaxy, the Milky Way. Therefore, we can only see nebulae which are at a relatively short distance from the sun (between 400 and 10,000 light years), the others being masked by the galaxy dust. There are 3 main types of nebulae:
Some nebulae combine several types (dark + emission, dark + reflection, emission + reflection, dark + emission + reflection).
Another less common type of nebula is known as "variable" nebula. Here gas and dust are illuminated by a variable star. These are the only deep sky objects changing shape from year to year. The only example here is the Gyulbudaghian's nebula. Objects of this type are also listed as Cohen objects.
With an H-alpha filter, which is a narrow band filter (4.5 nm) centered on
the Hydrogen Alpha emitting band (656.3 nm) it is possible to correctly image
emission nebulae with very bad sky conditions like full moon or light pollution.
Only the emitting portion of the nebula passes through the filter, surrounding
stars appear dimmed.
Planetary nebulae consist of an extremely hot star surrounded by a tenuous shell of ionized gas. Most of them have a very small apparent diameter of 1 arc min or less, which makes them very difficult to image with some details. There are many exceptions though, like M27 the Dumbbell planetary nebula (6' diameter) and NGC 7293 the Helix planetary nebula (15' diameter).
Visible Planetary Nebulae are located
inside our galaxy, the Milky Way, at distances between 400 and 50,000 light