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:

• Bright or emission nebulae - They are mostly made of Hydrogen, which is ionized by the ultraviolet light of stars in the area. Ionized hydrogen emits light in the H-alpha band, giving a typical red color. Many emission nebulae have star forming regions. Typical emission nebula: M16
  
• Reflection nebulae - They are essentially made of dust, which is illuminated by foreground stars, giving a typical blue color. Typical reflection nebulae: NGC 7023, M78
  
• Dark nebulae - They are also made of dust, but with no star illuminating them. Therefore they merely block the light coming from background stars. Typical dark nebulae: Horse Head south part, Cave eastern part, IC 1396 dark portion.

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.

Nebulae can also be seen in some galaxies, with of course lack of detail due to the huge distance. Some examples are the extra galactic emission nebulae in M33 and NGC 6822

Planetary Nebula

Another type of Nebula is known as Planetary Nebula, because sometimes it looks like a planet.

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 years.

My equipment, with 1.4 arc sec / pixel sampling is not adapted for Planetary Nebulae imaging. However, using Virtual Pixel, .7 arc sec / pixel sampling can be emulated when sky conditions are very good and stable.