Nebula

Nebula comes from the Latin word which means 'cloud' or we can say a 'fog'. The nebula is an interstellar cloud which is made up of dust, and a mixture of hydrogen and helium and other gasses which are ionized. Originally the term was used to describe an object which was diffused astronomically. These objects were galaxies which were beyond the Milky Way.     

The term which we have talked about was formerly applied to any object outside the solar system that had a diffused appearance which was rather a point like image as in the case of a star. 

This is the definition which was adopted at a time when very distant objects could not be resolved into great detail which unfortunately includes two classes which are of the unrelated objects. We can say that the extragalactic nebulae now known as the  galaxies are enormous collections of stars and gas. Along with that the galactic nebulae which are composed of the interstellar medium - the gas between the stars with its accompanying small solid particles within a single galaxy. Today we can say that the term nebula generally refers exclusively to the interstellar medium.

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Composition of Nebula

Nebulae are made of dust and gases which are mostly helium and hydrogen. The gases and the dust in a nebula are a result of gravity that can slowly begin to pull together clumps of dust and gas. As these lumps get bigger and bigger in size their gravity also gets stronger and stronger in nature.

Eventually, the clump of dust and gas gets so big that it collapses from its own gravity. The collapse generally causes the material at the centre of the cloud to heat up-and this hot core is the beginning of a star.

The star nebulae exists in the space between the other stars, also known as interstellar space. The closest space which is known as a nebula to Earth is the Helix Nebula. It is the remnant of a star dying that too possibly one like the Sun. It is said to be approximately 700 light-years away from Earth. That means that even if we could travel at the speed of light then also it would still take us 700 years to get there.

The astronomers who use very powerful telescopes are able to take pictures of faraway nebulae. Telescopes such as NASA's Spitzer Space Telescope and the Hubble Space Telescope have captured many images of nebulae which are faraway.

Nebula and Galaxy

In a spiral galaxy, the interstellar medium that generally makes up 3 to 5 percent of the galaxy’s mass but within an arm which is a spiral it increases mass fraction which is to about 20 percent.  We can say that about 1 percent of the mass of the interstellar medium is in the form of “dust” which is a small solid particle that is efficient in absorbing and scattering radiation. Much of the rest of the mass which is within a galaxy is concentrated in visible stars but there is also some form of dark matter that accounts for a substantial fraction of the mass in the outer regions.

Most conspicuous property of interstellar gas is its clumpy distribution on all scale sizes which is observed from the size of the entire Milky Way Galaxy that is about 1020 metres, or hundreds of thousands of light-years. This is equal to the distance from Earth to the Sun that is about 1011 metres, or a few light-minutes. The scale of which of the large variations are seen by direct observation. The small-scale variations are observed by fluctuations in the intensity of the radio waves. We can say that it is similar to the “twinkling” of starlight caused by unsteadiness in the atmosphere of the earth. Various regions exhibit an enormous range of densities and temperature as well.

Within the spiral galaxy’s arms which is about half the mass of the interstellar medium is concentrated in molecular clouds in which hydrogen occurs in molecular form H2. Here, hydrogen and temperatures are as low as 10 kelvins. These clouds are said to be optically inconspicuous and are detected principally by their carbon monoxide emissions in the millimetre wavelength range. Their densities in the studied regions by CO emissions are typically 1,000 H2 that is the molecules per cubic cm. 

Dark Nebula

All the stars of nebulae observed in the Milky Way Galaxy are forms of interstellar matter that is namely the gas which is between the stars that are almost always accompanied by solid grain cosmic dust. Their appearance generally differs widely depending not only on the temperature and density of the material but also on how the material is spatially situated with respect to the observer. Their composition of chemicals however is said to be fairly uniform. It is said to correspond to the composition of the universe in general. Approximately 90 percent of constituent atoms is hydrogen and nearly all the rest is helium. Oxygen and carbon, along with neon, and nitrogen, and the other elements that are together make up about two atoms that is per thousand.