So let's begin with what these three different types of nebulae are. We can firstly divide nebulae into two types: nebulae which we see because they give out light and nebulae which we see because they are blocking light. We can then further subdivide the bright nebulae into nebulae that give out light for two different reasons.
|Pipe Nebula - Dark|
Dark nebulae are pretty easy to understand. There is something bright in the background and a dense are of gas/dust is blocking that light.
There are some purely dark nebulae. These nebulae are not associated with star birth or star death. The stuff they are made of has probably been hanging around the galaxy since its very beginning and has not done much of note except slowly clumping together slightly under what little gravity there may be there. Generally we will see these because they are blocking background stars.
Large star-forming nebulae will often have some dark components - areas of dust associated with the main nebula but not close enough to what is going on in there to glow. And so we see them because they block light from the glowing area of the nebula.
In our daily life we are used to things glowing because they are hot. In the dark depths of space, only stars have this honour. Nebulae are cold objects and glow for other reasons.
|Dumbell Nebula - Emission|
Most of the nebulae you have seen or heard of are emission nebulae. They are bright, common and easy to get good images of. They glow for the same reason we get the arora on Earth: gas atoms are excited by being bombarded with high energy particles given out by a nearby star, and as the atoms de-excite they emit their energy as light.
This light is of a very specific and predictable wavelength (colour) depending on what element of atom is involved. Hydrogen gives out red light. Oxygen gives out green. The narrowband filters (H-alpha and OIII) on galaxy cam select for just these wavelengths of light improving the contrast with the background light.
|M87 - Reflection (and dark)|
Nebulae not in a position to be excited by nearby stars, but still relatively close to nearby stars, might still have a chance to shine. They can do this by reflecting some of the stars light to us. These nebulae are very very hard to observe. The dust reflecting the light to us may be a light-year or more from the nearest star (that's 10,000 times the distance from the Sun to Pluto). Starlight is very weak at these distances and only some of that starlight is actually reflected to us. In most cases, any exposure long enough to see the nebulosity will result in CCD bleeding from the brightness of the source star itself. Many exposures must be taken and stacked to have any chance at all. And since reflection nebulae are reflecting star light, which is a mixture of all wavelengths of light, narrowband filters can't be used to give greater contrast as with emission nebulae.
The Triffid Nebula
The Triffid Nebula shows all three types of nebulosity. The main bright, red region is emission and shows up very clearly in a hydrogen alpha filter. Between us and the bright region are three distinct bands of dark nebula, obscuring that light. And off to one side a slightly bluish region can be seen which an area of reflection nebula.
Other features of note include the dense pillars of gas (such as the prominent feature in the lower right of the image above), which is known to be an area where new stars are being formed.
What filters you use for you image of the nebula will be determined by which region you are interested in. A colour images will give a good all-round impression of the nebula. Hydrogen-alpha gives stunning contract for the red regions. A blue filter will enhance the reflection areas.
- Try H-alpha, 120 000 ms
- Or Blue, 180 000 ms
- Or BVR, 120 000 ms