Most nebulae is widespread and that means that they are very large and do not have well-defined boundaries. In visible light, these nebulae can be divided in emission nebulae and reflection nebulae based on how you create the light we see. The emission nebulae contain ionized gas (mostly ionized hydrogen) that produces spectral lines of emission. They are often called H II regions derived from the language of the professional astronomers referring ionized hydrogen.
Friends of NASA - Google+ | Moon-Venus-Jupiter Conjunction | "Celestial Junction: When the sun sets tonight, go outside and face west. There, splayed across the sunset sky, is a beautiful lesson in geometry. Venus, Jupiter, and the crescent Moon have converged to form a scalene triangle--a three-sided polygon with sides of unequal length. In the Adirondack mountains of New York, artist Sally J. Smith created a special sculpture to frame the event."
Cometary Globule CG4: Wide-field view | This wide-field image shows a rich region of the sky in the constellation of Puppis (The Poop). At the center lies the strange cometary globule CG4. Other interesting objects are also seen, including several much more distant spiral galaxies.