Emission nebulae are clouds of high temperature gas, named as such because they emit their own light rather than reflect it. The atoms in the cloud are energized by the 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 also called stellar nurseries.
Conversely, reflection nebulae are clouds of dust which are simply reflecting the light of nearby stars. 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. There are four other main categories of nebulae, Reflective, Planetary, Dark, and Supernova Remnant.
Stellar Nurseries
Protoplanetary disks in the heart of the Orion Nebula.
At the heart of an Emission Nebulae can be found the universes ‘cosmic incubators’. It is here where new stars and planets are formed. Emission Nebulae are created when the molecular hydrogen that is sparsely dispersed throughout interstellar space falls together due to gravitational forces. When the clouds become massive enough they begin to collapse, forming a central dense region. Warmer than the surrounding gases, this dense region further condenses into many smaller clumps. As each one of these clumps continues to collapse due to gravity, they heat up and become what are known as protostars. The internal pressure of the protostars will continue to mount and eventually nuclear reactions will ignite in their core. Stars are born. The remaining gas and dust forms a protoplanetary disk. It is out of this material that planets will eventually form.
The Orion Nebula – M42
Perhaps the finest example of an Emission Nebulae, and one of the few naked eye nebulas, M42 has intrigued mankind since ancient times. At a distance of 1500 light years, M42 is over 25 ly in width and contains several hundred new and still forming stars including the Trapezium, a cluster of four stars at the center of the nebula. Other examples of Emission Nebulae are the Lagoon Nebula (M8) and the Trifid Nebula (M20) both located in Sagittarius.
Did you know?
The Orion Nebula, as seen in the sky, is actually larger than the full moon.
It is a common misconception that astronomical objects are very tiny and telescopes are needed to make them bigger. In fact, with the exception of the planets, many of the objects that we commonly view are quite large.
While true, they are very far away, their huge size more then makes up for their distance. The problem lies in that they are very dim and our eyes are just too small to resolve them. So in fact we use telescopes not necessarily to make objects bigger, but we use their large aperture to make them brighter so they can become visible to our feeble eyes.