Emission Nebula -- Part 3 to Part 6

It is a nebula that shines and emits its own light as a result of the  union of electrons in protons to form hydrogen atoms. This happens when  the electron approaches the proton and generates energy that appears in  the form of a red light, and since this process occurs for the majority  of atoms inside the nebula, it appears red.
This  nebula is created by the emission of ultraviolet radiation from a very  hot star on a cloud of hydrogen gas. As a result, the process of  ionizing the atoms (extracting the electrons from the atoms) occurs.  Free electrons can then begin in the process of union and fusion, It is the remnant of the birth of massive stars

It  is also called the hydrogen plasma regions (HII), and the relativity is  that it is mostly composed of hydrogen ionized plasma and the electrons  are free. Hydrogen atoms in the interstellar medium are ionized by the  ultraviolet radiation from one or more stars located near the nebula. Only  the very hot stars of the O, B or A star rating, usually the young  stars, are the ones that cause it. They have the ability to produce  enough ultraviolet radiation to ionize the hydrogen, and the excess  energy after hydrogen ionization turns into kinetic energy of the  electrons. Eventually, during collisions, energy becomes common with other gas molecules. There is a balance in the nebula when the kinetic energy temperature is between 7,000 kelvin and 20,000 kelvin.
Their  mass typically ranges from 100 to 10,000 solar masses, and can spread  in less than one light year to several hundred light years. That  is why their densities vary widely, ranging from millions of atoms per  cubic centimeter to only a few atoms per cubic centimeter depending on  the pressure of the nebula. The density is usually about 1,000 atoms per cubic centimeter, with an average temperature of about 10,000 Kelvin.
The  color of the nebula depends on its chemical composition and its degree  of ionization. The most common is red. This is due to the spread of  hydrogen between the stars. Its energy is relatively low, but if more  energy and other elements are available, the nebula may appear in other  colors such as green and blue. Most of this type is composed of 90% hydrogen, the rest of helium, oxygen, nitrogen and other elements.
The  most common nebula is where a cloud of interstellar gas is found,  dominated by neutral hydrogen atoms, and these atoms come from one or  more stars close to O or B. These very warm and bright stars emit large  amounts of energy-rich UV rays, Photons by separating neutral hydrogen atoms into the hydrogen nucleus from their electrons and electrons become free. The neutral hydrogen atom returns to its lowest state of energy,  emitting photons at optical wavelengths corresponding to the red  wavelength of the visible spectrum, which gives the nebula a distinctive  red color
The other common type of radiation nebula is the planetary nebula. As  they include a white dwarf in the center surrounded by clouds of gas as  a result of the development of the original star to the stage of the  white dwarf. In  this case, the evaporated gas is not necessarily neutral hydrogen, but  it can contain large amounts of helium (HeII) and thus the nebula  appears in blue, or oxygenated (OIII) and then the nebula is green. Since  the energy needed to ionize helium far more than hydrogen, the blue  regions of planetary nebula are hot zones and point to higher excitation  gas areas 

  image source   1 

Nebula is the birthplace of stars  -- Part 1   

Planetary Nebula   -- Part 2  

Emission Nebula   -- Part 3   The Post has been added   

Reflection Nebula   -- Part 4  

Dark Nebula   -- Part 5 

Supernova Remnants   -- Part 6