Basic Astronomy

Inter Stellar Medium

Page Editor: Deb Sankar Bhattacharya


The most of the part of a galaxy is empty. In a galaxy, stars take only 1% of the entire space and rest of 99% is filled with very rarefied dust and gas particles. This greater part of a galaxy is called Inter Stellar Medium (ISM).

Cats eye nebula
Cats eye nebula (Picture credit: NASA)

ISM has the characteristics of selective scattering and absorption. Beam of light coming from a star when pass through the dust cloud get scattered partly. According to Rayleighs law of scattering, light of lower wave length is scattered more than that of higher wave length (as scattering is inversely proportional to the fourth power of wavelength). Evidently blue light scattered within the cloud where as red light is allowed to pass and reach to the observer. So instead of visible range, lights belonging to mid-infrared and far-infrared are more likely to come from the galactic plane. This sometime is called interstellar reddening effect.

ISM is one of the main reasons which produce phenomena like Extinction, Polarization etc. Extinction is an incident which obscures stars and others radiation sources. Due to extinction a star may appear fainter and hence may produce a strong ambiguity in the measurement how distant a star is. Its not that the density of gases in the interstellar medium is constant; it may change with position as well as with time for a particular position. They follow a complex dynamics. So extinction differs according to position and direction in space.

When a dust cloud resides by the side of a star and not between the star and the observer, it scatters light as mentioned earlier but now the observer sees the scattered light instead of filtered one; means the cloud now appears bluer. This phenomenon may give rise what is called a Reflection Nebula. All these effects the Interstellar Reddening, Reflection, Extinction and Polarization mainly caused due to the interstellar grains or dusts which is expected to contain mostly by Elongated dry-ice grains (responsible for Polarization), Grains of graphite (responsible for extinction in UV and emission in IR), Particles with small cores and large icy mantles, Silicate particles (both causes extinction) and complex, large molecules called Polycyclic Aromatic Hydrocarbons (PAHs). They are even smaller than particles in Tobacco smoke. Up to now all we talked is about dust or interstellar grains but the massive part of ISM is consisted with gas; the total mass of the Gas in our Galaxy is 100 times more than that of Dust, not only that number density of Gas is greater by 1012 times.

In a binary system, the two stars revolve round their common centre of mass. Due to Doppler Effect the spectral lines from the stars are seen to oscillate between blue and red (also known as Blue Shift and Red Shift). But Astronomers found some of them are static. This was an indication of the existence of a static medium now known as ISM.

The main gas contain of ISM is the lightest gas, Hydrogen. Very Close to a star, the Hydrogen Gas is ionized due to the radiation emitted from the star. It is known as H-II region. This region ends sharply after some distance and H-I starts. H-I is neutral Hydrogen. Recombination process also goes on in those layers, which then behave like a strong source of Radio emission. Besides Hydrogen some simple molecules like CO, SO, NO, OH and organic molecules like formaldehyde, methanol etc are also found. This all together creates the illusions and mysteries of Heaven.






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