§ Nebula Latin: "mist"; pl. nebulae
1. From the Online Etymology Dictionary: astronomical meaning "cloud-like patch in the night sky" first recorded c.1730. As early as Herschel (1802) astronomers realized that some nebulae were star clusters, but distinction of gas clouds from distant galaxies was not made until c.1930.
- You Should Know Nebula Terminology
In This Section You Should Know Definitions from the Astronomical Science of Nebulae
- The Cradle of Star Formation
Emission Nebula The Birthplace of Stars
- The Nebulae
Reflections on a Nebula From the Pleiades to IC 2118
- Planetary Nebula
From a Universe of Beauty Stars that Shape Forms
- Remnants of a Massive Explosion
Supernova Remnants Titanic Forces Shape a Nebula
- Dark or Absorption Nebulae
Seeing In the Dark Backdrops of Illumination
- Imaging the Nebula Under Edit
The Protoplanetary Nebulae Amazing Images of Nebulae
- Subsequent chapters under construction
» Nebula - an interstellar cloud of dust, gas and plasma. Originally, nebula was a general name for any extended astronomical object, including galaxies beyond the Milky Way. Some examples of the older usage still survive, for example, the Andromeda Galaxy is sometimes referred to as the Andromeda Nebula.
» Classification - unlike the morphological classification scheme of galaxies, nebulae are classified by how they are or how they are not illuminated. The source of it's illumination, it's chemical makeup and the source's interaction with the gaseous cloud combine to define the type of nebula it is. How this system works is explained in the following examples:
- Diffused Nebulae, like the Omega Nebula left, are illuminated nebulae that are extended, containing no well-defined boundaries. These nebulae are usually classified into Emission and Reflection types. Emission Nebulae, like NGC 2174 at right, are internally illuminated clouds of ionized gas. Two of the most common types of emission nebula are H II Regions and Planetary Nebulae.
- Reflection Nebulae are illuminated as a result of light reflection from nearby stars. An example is the nebulosity surrounding the Pleiades star cluster, seen at left and known as NGC 1435. Planetary Nebulae, like PN Mz3 at right, are compact shells of gas around a dead star or an intermittantly active star. They form as a result of a star's outer layers being expelled via pulsations or strong stellar winds.
- Supernova Remnants like Cas A at left result from the massive and cataclysmic detonation of a star. The resulting debris from the blast, the star's remnants, move away at high speed and are heated by colliding with (relatively) slow moving dust and gas. Dark or Absorption Nebulae, like LDN 1622 at right, are unilluminated. They can be detected only when they obscure stars or other nebulae. Famous examples include the Horsehead nebula in Orion, and the Coalsack Nebula in the Southern Cross.
- Protoplanetary Nebulae like Gomez's Hamburger at left, form during a star's rapid stellar evolution between the late asymptotic giant branch (LAGB) phase and the subsequent planetary nebula (PN) phase. However, the name "protoplanetary" may be a slight misnomer. Another name suggested for this type of nebula, "Preplanetary Nebula" has been suggested by Sahai, Sánchez Contreras & Morris 2005, fn.1 as it does not overlap with any other disciplines of astronomy. Image left of IRAS 19024+0044 from Sahai et al. The Astrophysical Journal, 620:948960, 2005 February 20
» H II Region - a region of interstellar dust and gas with a high content of hydrogen gas. The notation H II refers to the fact that the hydrogen atoms (H) are ionized (H I is neutral, un-ionized hydrogen). Within the interstellar cloud are young, hot, blue stars which are emitting large amounts of ultraviolet light ( photons ), which strike the gas and plasma within the nebula and in turn, cause the electrons in the hydrogen nuclei to shift their orbits. When the electrons return to their normal orbit they emit photons at the optical wavelength of of 656.3 nanometers which is that part of the spectrum that is responsible for the RED color of these nebula. 
» Proplyds or Protoplanetary Discs - a rotating disk of dust and gas that surrounds the core of a developing solar system. It may eventually develop into orbiting celestial bodies such as planets and asteroids or young newly formed stars like a T Tauri or Herbig star. The protoplanetary disk may be considered an accretion disk because gaseous material may be falling from the inner edge of the disk onto the surface of the star, but this process should not be confused with the accretion process thought to build up the planets themselves. 
» Gravitational Accretion - a dense core of gas and dust which becomes more and more dense by drawing material into the core. This is called accretion. This process forms accretion disks of which many have been identified with the Hubble Telescope. The core heats up (to about 1500 K) because of the compression, caused by gravity, and begins to shine in the infrared and at radio wavelengths. After about 100,000 years of accretion the protostar has as much mass as the Sun. Eventually, the wind of radiation and particles flowing from the proto-star prevents further material from accreting and it thereafter becomes a pre-main sequence star. 
» White Dwarf Star - As a star like our sun is running out of fuel in its core it begins to bloat into a red giant. This will happen to our sun in 5 Billion years. Then after a few million years the outer layers of the red giant will begin to puff off and form a planetary nebula. Leaving behind only the dead core of the star made of mostly carbon and oxygen. This is the white dwarf. 
» Astronomers, Astrophysicists & Researchers - below are just a few of the individuals historically or currently associated with nebulae. They include professional or amateur astronomers, scientists, astrophysicists and researchers the world over:
Johannes Hevelius John Herschel William Herschel E. E. Barnard
Richard A. Shaw Lubos Kohoutek William Huggins Charles Messier
Williamina P. Fleming
2. The Cradle of Star Formation
3. The Nebulae
- Definition of H II Region from Encyclopedia.com
- Protoplanetary disk at Wikipedia
- Formation of Stars from Harrison B. Prosper at the Florida State University
- White Dwarfs 2006 by Travis Metcalfe. From the website Windows to the Universe, © The Regents of the University of Michigan.
- Takao Nakagawa - Dramatic Birth and Death of Stars from JAXA Japan Aerospace Exploration Agency, Copyright 2007
- Vulpecula Article from Wikipedia, the free encyclopedia. June 27, 2009.
- ESA - Space Science - AKARI delivers its first images 22 May 2006, European Space Agency, Copyright 2000 - 2009 © European Space Agency. All rights reserved.
- Census Finds Unknown Young Stars of Orion By SPACE.com Staff, 20 April, 2009. © 2007 Imaginova Corp. All rights reserved
- Pleiades Mythology by Steven J. Gibson, astronomer at Arecibo Observatory in Puerto Rico.
- The 10 Brightest Stars - 7. Rigel By Pedro Braganca. From SPACE.com -- The 10 Brightest Stars, 15 July 2003
- Spitzer Space Telescope Research Program for Teachers and Students from Young Stars in IC 2118 Round 2 - Proposal News, from Coolcosmos.com
- Barnard's Merope Nebula IC 349 in M45 from the SEDS web site by Hartmut Frommert & Christine Kronberg, 2001.
Additional Resources of Interest
- Astronomical Nebula & Nebulae Simplified, from Doug Snyder Planetary Nebulae Observer's Home Page
- Nebula Encyclopedia Online
- Wikipedia Nebula the free encyclopedia
- AKARI: current status and recent science highlights
- The bright stars and associated nebulosities in the Pleiades star cluster M45
- Revised New General Catalogue and Index Catalogue Wolfgang Steinicke Version: March 26, 2009; catalogue lists 14002 entries.