04-10-2012, 05:22 PM
Stellar evolution
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Giant Molecular Clouds – The Birth Place of a Star.
Stellar evolution occurs exclusively in Giant Molecular Clouds (GMC).It is vast assemblages of molecular gas (mainly H2), plasma and dust. Typical GMC’s are roughly 100 light- years (9.5×1014 km) and contain up to 6 million solar masses (1.2×1037 kg) and an average interior temperature of 10K.The clouds have an average density of 102-103 particles per cubic cm. So far the physical appearance of a GMC is concerned; they have substructures in a complex pattern of filaments, sheets, bubbles and irregular clumps. The densest parts of the clumps are called “molecular cores”, while the densest part of the molecular cores are called dense “molecular cores” having densities of 104-106 particles/cm3. Observations have revealed that the molecular cores are traced with carbon monoxide and the dense cores are traced with ammonia. The concentration of dust within molecular cores is sufficient to block light from background such that they appear as dark nebulae.
Within our own galaxy Milky Way molecular cloud accounts for less than 1% of the volume comprising the interstellar medium. The bulk of the molecular gas is present in the spiral arms of the galaxy. In the Milky Way there are estimated 6000 molecular clouds each with more than 1 million solar masses. The nearest nebula to the Sun where massive stars are being formed is the Orion Nebula 1300Ly away. Lower mass star formation is occurring 400-450 Ly distance in the ρ Ophiuchi cloud complex. A more compact site of star formation is the opaque clouds of dense gas and dust known as Bok Globules.These are formed in association with collapsing molecular clouds and sometimes independently.
Stellar Birth-Protostar Formation.
The protostellar phase is an early stage in the process of star formation. However not all molecular cloud leads to protostar formation. Only those cloud that is massive enough that the gas pressure is sufficient to support it, the cloud will undergo gravitational collapse. The mass above which a cloud will undergo gravitational collapse is called the Jeans Mass discovered by the scientist James Jeans.
A molecular cloud with mass above the Jeans Mass will undergo gravitational collapse. During this contraction the molecules collide with each other-the gas heats up and the energy is released in the form of radiation. However, the collapsing cloud will eventually become opaque to its own radiation. So, the dust within the cloud also gets heated up to temperatures of 60-100K and these particles radiate in the far infrared where the cloud is transparent. Thus the dust results in further collapse of the cloud. The density of the cloud increases at the centre and a core region, called the First Hydrostatic Core is formed. The gas falling towards the core creates shock waves which further heats the core.
When the core temperature reaches about 2000K, the thermal energy dissociates the H2 molecules. This is followed by the ionization of the hydrogen and helium atoms. After the density of in falling material has dropped below about 10−8 g cm−3, the material becomes sufficiently transparent to allow radiated energy to escape. The combination of convection within the protostar and radiation from the exterior allow the star to contract in radius. This continues until the gas is hot enough for the internal pressure to support the protostar against further gravitational collapse-a state called hydrostatic equilibrium. The resulting object is known as a protostar. For a one solar-mass star the protostellar phase lasts for about 100,000 years.
Maturity of stars.
After a protostar has formed, it creates energy at the hot, dense core region through the nuclear fusion of hydrogen atoms into helium. All stars at this stage are in hydrostatic equilibrium, where outward thermal pressure from the hot core is balanced by the inward gravitational pressure from the overlying layers. The strong dependence of the rate of energy generation in the core on the temperature and pressure helps to sustain this balance. Energy generated at the core makes its way to the surface and is radiated away at the photosphere. The energy is carried by either radiation or convection.
A star at this stage can be divided into two categories, based on the dominant process that a star uses to generate energy. Stars below about 1.5 times the mass of the Sun primarily fuse hydrogen atoms together in a series of stages to form helium, a sequence called the proton-proton chain. Above this mass the nuclear fusion process mainly uses atoms of carbon, nitrogen and oxygen as intermediaries in the CNO cycle that produces helium from hydrogen atoms
Eventually, the core exhausts its supply of hydrogen, and without the outward pressure generated by the fusion of hydrogen to counteract the force of gravity, it contracts until either electron degeneracy becomes sufficient to oppose gravity or the core becomes hot enough (around 100 megakelvins) for helium fusion to begin. Which of these happens first depends upon the star's mass.
The Fate of Stars.
When a star runs out of its hydrogen and other nuclear fuel, it starts to cool off and undergoes gravitational collapse as there is no outward force from inside to balance the inward gravitational tug. As the star becomes small the matter particles get very close to each other. However there is a limit to how close the matter particles can get. This limit is provided by a very well known principle of quantum mechanics called the Pauli Exclusion Principle. The principle states that two or more matter particles cannot have the same position in space. So the repulsion generated due to the exclusion principle makes the matter particles move away from each other and this makes the star expand. This repulsion balances the inward gravitational tug.
However an Indian scientist named Subrahmanyam Chandrasekhar found that a star with more than 1.5 times the mass of the sun will not be able to support itself against its own gravity and may explode in a supernova or somehow manage to reduce their mass below the limit. More massive stars collapse into a black hole. This limiting mass is known as the Chandrasekhar Limit. A star having mass below the Chandrasekhar limit will eventually stop contracting and become a white dwarf or a red dwarf in case of very low mass star.
Fate of Stars with very low mass.
If the mass of a main-sequence star is lower than approximately half a solar mass, it will never become hot enough to fuse helium at its core such a star will eventually burn all its hydrogen and end its evolution as a helium white dwarf composed chiefly of helium-4 nuclei.
Very low mass stars are also destined to become red dwarf. Red dwarfs are very low-mass stars with no more than 40% of the mass of the Sun. They have relatively low temperatures in their cores and energy is generated at a slow rate through nuclear fusion of hydrogen into helium by the proton-proton chain mechanism. Thus these stars emit little light, sometimes as little as 1/10,000th that of the Sun. The second nearest star to earth Proxima Centauri is a red dwarf star.