INTERSTELLAR MATTER

PRESENTED BY

MOHAN CHANDRA SHARMA

21UMPY10

DISTRIBUTION FEATURE OF INTERSTELLAR MATTER

• Clouds of matter of various dimension swim across the vast ocean of the more tenuous and almost uniform distribution of matter pervading the glactic plane.
• The uniformly distributed components mainly participate in the galactic rotation.
• Some clouds extend much higher above the galactic plane.
• The gas and dust have an identical distribution.
• The major part of the matter is concentrated along the spiral arm of galaxy.

INTERSETLLAR LINES

• The first observed interstellar lines were the h and k lines of ca ii in the spectrum of the spectroscopic binary delta Orionis.
• Higher resolving powers were achieved in subsequent years resulting in fainter lines of other atom,ions,molecules.
• These lines are absorbed from the ground state and so are sharp

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INTERSTELLAR RADIO LINES

• Radio astronomy has ensured a great advancement in the knowledge of physics and chemistry of interstellar space.
• 21 cm radio line of neutral hydrogen has played a important role in enhancing the understanding the structure of galaxies.
• The first interstellar molecular lines to be detected in the radio range were those of hydroxyl.
• The ground level of oh radicle gets separated in doublets and each component of this splits into two hyperfine levels leading to four transitions.

INTERSTELLAR CLOUD

• It is generally accumulation of gas plasma and dust in galaxies .
• It can be large upto 10^6 solar masses and are often said to be most massive entities in galaxy.
• they have invariably lower space velocities.
• In many of these associations of hot stars are embedded which create large h ii region around themselves which push the h I gasses with velocities of the order of 10km/sec.

VELOCITY DISTRIBUTION OF CLOUDS

• First of all radial velocity dispersion is determined from observations like adam observation of h and k lines of ca ii.
• The most natural suggestion was the gaussian distribution of radial velocities.
• The observed radial velocity of cloud deviate simply from gaussian distribution.
• Firstly the observed number of higher velocity cloud greatly exceeds that predicted by the distribution.
• Secondly among the high velocities cloud two third possess negative while one third possess positive velocities.

BLAAUW LAW

• To rectify the above shortcoming Blaauw deduced a distribution law for the radial velocities of the cloud which fitted best with observational results.
• The law is the

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STROMGRENS SPHERE

• It is a spherical volume of the h ii region where the rate of ionization balances the rate of recombination.
• Such regions are strong and characteristic emitters of radiation at radio wavelength and their dimension are well calibrated in terms of luminosity of central star.
• We may find the volume of stromgren sphere by equating the incident flux of ionizing photons from the star with the exiting flux of photons made by recombination of levels other than the ground state.

RADIUS OF STROMGREN SPHERE

INTERSTELLAR CLOUD COLLISION

• Interstellar h I clouds move at random in space with an average velocity of 8km/sec.
• There are 50,000 such clouds per cubic kpc so they are sure to suffer collision.
• Such collisions are highly compressible and generate shock waves which will heat the gas to few thousand degree kelvin ad compress it behind the shock.
• The shocked gas will eventually cool down in infrared radiation and thus the kinetic energy of the cloud motion is thus dissipated in the infrared lines.

THE INTERCLOUD MEDIUM

• We saw that clouds hardly occupy 10 percent of galactic disk.the remaining percent of space between the clouds has been called intercloud region.
• The very tenous matter pervading this region is called intercloud medium.
• Since the density of matter in intercloud space is very low and distance concerned are very great nothing practically can be observed of the intercloud medium.
• Its existence was proposed originally to explain between the optical observations idealized with standard cloud model and the 21 cm observation which suggest a smoother gas distribution.

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INTERSTELLAR GRAINS

• Tiny dust grains floating aimlessly around the stars.they are formed from materials ejected by stars.
• They absorb and scatter light thus shielding some regions from uv radiations .
• Grains in the cloud reflect light from the associated bright star.
• Polarisation measurement of light from the distant star led to the conclusion that the grains are elongated particles oriented in some fashion bybthe galactic magnetic field.
• The total mass of the dust component is only 1%of the gas and average size f particle is less then 1mu.

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