What is Nebula? Formation and classification of Nebulae.

If we talk about mysterious objects of outer sky, most glittering celestial one is Nebula, or Nebulae(more than one nebula).

A Nebula, named for the Greek word for 'Cloud' is in fact an interstellar cloud of dust, hydrogen, helium and other ionized gases. Some Nebulae come from the gas and dust thrown out by the explosion of a dying star, such as a Supernova. Other nebulae are regions where new stars are beginning to form. For this reason, some nebulae are called 'Star Nurseries'.

Most Nebulae are of vast size, some are hundreds of light years in diameter. A nebulae that is visible to naked eye from Earth would appear larger, but no brighter, from close by.The 'Orion Nebula' the brightest nebula in the sky and occupying an area twice the diameter of the full Moon, can be viewed with the naked eye but was missed by early astronomers. Although most Nebulae are completely invisible to the naked eye, modern telescopes are able to give us close up view of some of the most beautiful objects in the galaxy. Nebulae are often star-forming regions, such as in the "Pillers of Creation" in the 'Eagle Nebula'. In these regions, the formations of gas, dust, and other materials clump together to form denser regions, which attract further matter, and eventually will become dense enough to form stars. The remaining material is then believed to form planets and other planetary system objects.

FORMATION:

                        There are a variety of formation mechanisms for the different types of nebulae. Some nebulae form from giant molecular clouds, the coldest, densest phase of interstellar gas that is formed by the cooling and condensation of more diffuse gas. And others are produced from material shed by a star in late stages of its stellar evolution.
Star forming regions are a class of emission nebula associated with giant molecular clouds. These form as a molecular cloud collapses under its own weight, producing stars. Massive stars may form in the center, and their ultraviolet radiation ionizes the surrounding gas, making it visible at optical wavelengths.

CLASSIFICATIONS:

                                  There are five types of cloudy or nebulous objects in the sky such as,
      1. Planetary Nebulae.
      2. Emission Nebulae.
      3. Reflection Nebulae.
      4. Dark Nebulae.
      5. Supernova remnants.

1. Planetary Nebulae: 

A planetary nebula is an astronomical object consisting of a glowing shell of gas and plasma formed by certain types of stars at the end of their lives. They are unrelated to the planets, the name originates from a supposed similarity in appearance to giant planets. They are short lived phenomenon, lasting few tens of thousands of years. In other galaxies, planetary nebulae may be the only objects observable enough to yield useful information about chemical abundances.

X-ray/optical composite image of the

                            Cat's eye Nebula

             

2. Emission Nebulae: 

Emission Nebulae are clouds of ionised gas that, as the name suggests, emit their own light at optical wavelengths. A common example of emission Nebulae is planetary Nebulae.

  Ring Nebula is a example of Emission Nebulae.

      

3. Reflection Nebulae: 

A reflection Nebulae is created when light from a star is scattered or reflected off a neighbouring dust cloud. The scattered light is slightly polarised and has a blurred spectrum similar to that of the illuminating star. This shift in colour arises because the typical size of dust grains in the cloud are comparable to the wavelength of blue light. The result is that blue light is scattered more efficiently than longer, red wavelengths giving the characteristic blue colour for these nebulae

  The witch head reflection Nebula.

     

4. Dark Nebulae: 

Dark Nebulae are interstellar clouds that contain a very high concentration of dust grains, which allows them to scatter and absorb all incident optical light, making them completely opaque at visible wavelengths.

A visual image of Barnard 68, Dark Nebulae.

5. Supernova Remnants:

 A Supernova remnant (SNR) is the remains of a Supernova explosion. SNRs are extremely important for understanding our galaxy. They heat up the interstellar medium, distribute heavy elements throughout the galaxy, and accelerate cosmic rays.

Crab Nebula.