Class of Nebulosity: What are the nebula? The answer is very simple in first sight that it is glowing gas and dust of a dead stars, which is either re-excitation of gas by the radiation of near by star or reflection of electromagnetic wave by its dust. But really they are rebirth places of new stars and most fascinating state of matter like neutron stars and black holes. We will discuss those bodies in next article where we will talk about the high temperature state of matter with tremendous gravitational force acting on those systems. But here it is limited only up to class of nebulas. The Galactic Nebula divided into two classes Diffused Nebula (irregular)Planetary Nebula (Regular & Emission)The Diffused Nebula again divided into two classes Dark Nebula Luminous Nebula The Luminous Nebula again divided into two classes Reflection Nebula Emission Nebula The Emission Nebula again divided into two classes Super Nova Remnants Normal H II Regions So the question still remains what is behind the distribution of gas and dust in regular or irregular shape. The answer is nova out burst. As we know that the tremendous gravitational force keeps the nuclear chain reaction on in the stars. In the fusion, hydrogen nuclei get converted in helium and other isotopes. The production of these isotopes increases the internal pressure of star and when it reaches to the critical mass where gravitational force and this outward pressure balanced, and the out burst condition become ready and star burst. This is called super nova explosion. And exploded star becomes different class of nebulae as mentioned above. If the parent star is of O / B0 class (Sorry it will be covered in other section), then the radiation from the star is so energetic that it ionizes all portion of gas and dust. This gas further emit the spectral lines fully different from the parent star is known as “gas bounded” nebula. On the other hand if the radiation emits by the near by star is no so energetic, then the all radiation just reflect by the nebular gas and dust and this gas and dust portion is called “radiation dominant” nebula. In radiation dominant nebula the spectral lines are same as of the parent star. Now let us talk about the gas bounded nebula. If gas is in regular shape like oval or circle after burst of parent star and further gas is re excited by the central residual part of parent star (also known as white dwarf) is called planetary nebula. The famous examples are NGC 7393(Helix Nebula) and M57 (Ring Nebula).The nebula which are far far beyond the reach of near by star are called dark nebula. The great rift part of our Milky way galaxy is believed to be home of dark nebula. The Horse-head nebula, IC434 are best examples. On the other hand lagoon nebula and Great Orion nebula are best examples of the emission nebula. If there is a star of spectral class above B1, then there will be emission nebula associated with it. It will ionize all hydrogen beyond Lyman head. These region are called H II or H beta regions, whereas neutral hydrogen is being called as H I region or H alpha region. The H II region is surrounded by H I region. These nebula are called emission nebula. The star below B1 class are poor producers of energetic radiation and failed to activate the gas and dust and only reflection effect will come into picture and so nebula is called reflection nebula. The M45, Pleiades is best example of reflection nebula. All emission and reflection nebulae are radiation dominant. No gas dominant diffused nebula found yet.