furthering to what has already been explained (which is the main reason in anycase)
When imaging in Narrow band your capturing a very very small area of light and hence it separates it from the other colours and gives you a very distinctive line. Here is the difference between OSC broad band and OSC narrow band of the same image with the same equipment/time/location
ha
http://i700.photobucket.com/albums/w...a-high-res.jpg
broad band
http://i700.photobucket.com/albums/w...M-16-final.png
blended
http://i700.photobucket.com/albums/w...6-HaRGBadj.jpg
Like i said the details in the full colour (broad band) image are lacking but in the blended and cropped version they are very prominent
Do also remember this is a OSC DSLR all images are 10 min ISO 800 average of about 16 images with flats and darks subtracted.
hope that gives you a clearer idea of how it all goes