Catch a Star 2004

As a matter of fact the clouds can be divided into diffuse clouds, which are thin enough for us to observe stars behind them, and dark clouds (pic.1), which are so opaque that stars behind them cannot be seen. The intercloud region (between clouds) contains a high-temperature, low-density gas, much of it ionized hydrogen, and wispy-structured dust regions.

In photographs of the Milky Way, our view of the starry background is partly or wholly blocked by dark interstellar clouds, sometimes called dark nebulae. They contain denser concentrations of interstellar dust than occur generally in the Galactic plane. One such dark region is a long, chainlike complex composed of dozens of isolated and connected dark interstellar clouds that stretches about halfway around the Milky Way from the constellation Cygnus to Crux. This obscuring strip forms the Great Rift dividing the Milky Way into two branches. In many regions along its length this dark nebulosity separates into tangled lanes of absorbing material that partially cover bright, glowing, gaseous nebulae.

Diffuse nebulae radiate light and can be seen as large bright clouds without definite shape. They are divided into three types:

    1. Emission Nebulae - Emission nebulae are clouds of high temperature gas. The atoms in the cloud are energized by ultraviolet light from a nearby star and emit radiation as they fall back into lower energy states (in much the same way as a neon light). These nebulae are usually red because the predominant emission line of hydrogen happens to be red (other colors are produced by other atoms, but hydrogen is by far the most abundant). Emission nebulae are usually the sites of recent and ongoing star formation. (M 42 shown – pic.2)
   

2. Reflection Nebulae - Reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. Reflection nebulae are also usually sites of star formation. They are usually blue because the scattering is more efficient for blue light. Reflection nebulae and emission nebulae are often seen together and are sometimes both referred to as diffuse nebulae. (NGC 7023 shown – pic.3)