Stellar Evolution Essay

2389 words - 10 pages

Stellar Evolution

A star begins as nothing more than a very light distribution of interstellar gases and dust particles over a distance of a few dozen lightyears. Although there is extremely low pressure existing between stars, this distribution of gas exists instead of a true vacuum. If the density of gas becomes larger than .1 particles per cubic centimeter, the interstellar gas grows unstable. Any small deviation in density, and because it is impossible to have a perfectly even distribution in these clouds this is something that will naturally occur, and the area begins to contract. This happens because between about .1 and 1 particles per cubic centimeter, pressure gains an inverse relationship with density. This causes internal pressure to decrease with increasing density, which because of the higher external pressure, causes the density to continue to increase. This causes the gas in the interstellar medium to spontaneously collect into denser clouds. The denser clouds will contain molecular hydrogen (H2) and interstellar dust particles including carbon compounds, silicates, and small impure ice crystals. Also, within these clouds, there are 2 types of zones. There are H I zones, which contain neutral hydrogen and often have a temperature around 100 Kelvin (K), and there are H II zones, which contain ionized hydrogen and have a temperature around 10,000 K. The ionized hydrogen absorbs ultraviolet light from it’s environment and retransmits it as visible and infrared light. These clouds, visible to the human eye, have been named nebulae. The density in these nebulae is usually about 10 atoms per cubic centimeter. In brighter nebulae, there exists densities of up to several thousand atoms per cubic centimeter. This is extremely large in comparison to densities outside the nebulae, which is slightly less than .1 atoms per cubic centimeter.

Groups of stars begin to form from these larger, denser clouds of gas and dust. Again, any deviation that occurs in these clouds will break the entire cloud into a number of condensed groups. If a grouping of gas has over a certain mass, gravity, instead of pressure variances, will then be strong enough to condense it. First, a high mass cloud will contract, then begin breaking apart and each individual mass will break apart and contract at different rates. The rate at which it contracts is dependent on the mass of each condensation, it is basically the acceleration due to the gravity of each condensing cloud. There is evidence of this in the grouping of stars into clusters and that all the stars are the same age in these clusters, as well as the large number of binary star systems that exist. The cloud can condense at an unimpeded free-fall rate until its own infrared radiation can’t escape. When this happens, the cloud dims and greatly slows in its condensation, as the infrared radiation provides and outward force and starts heating the new protostar. The protostar finally...

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