Monday, April 4, 2011

The Formation of a Star

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The formation of a star

Formation

The space between stars contains gas and dust at a very low density. This interstellar matter tends to gather into clouds. Sometimes the density becomes high enough so that gravity causes contraction, leading to the formation of a protostar. As a protostar slowly contracts, its pressure and temperature increase, the temperature rise being the result of the release of gravitational energy. Any hot object radiates energy, and the protostar eventually becomes hot enough to shine, although temperatures are not yet great enough to sustain nuclear reactions. The pressure builds up enough almost to balance gravity, but the radiation emitted drains energy and does not allow the internal pressure to complete the balance. Therefore the contraction (and heating) slowly continue.

The temperature at the center of the protostar finally becomes high enough to initiate nuclear reactions and the subsequent release of nuclear energy. This is what we call Hydrostatic Equilibrium. Hydrogen is the most abundant element, and hydrogen-burning reactions (thermonuclear fusion), in which hydrogen is converted to helium with a release of large amounts of energy (this is how the sun shines), are the first ones to become important. When the nuclear energy released exactly balances the pressure input by gravity, the protostar finally enters a state of balance, and contraction ceases. At this point the object becomes a true star.


Hydrogen-Burning and Helium-Burning Stages

A star that is in balance and burning hydrogen in its core is called a main sequence star. e. All stars begin their careers in the main-sequence phase. If a main-sequence star has a large mass, it will have a high surface temperature and will therefore be very luminous. If it has a small mass, it will be rather cool and faint. The Sun is a main-sequence star above average in mass, surface temperature, and radiant energy output.

When the hydrogen fuel in the core is used up, the star loses its main-sequence status. This can happen in less than a million years for the most luminous stars but takes many trillions of years for the faintest. The Sun has a main-sequence lifetime of about 10 billion years, of which half is over.

When the core hydrogen has all been converted to helium through nuclear reactions, the nuclear energy release stops. The star falls out of energy balance, and the central portions contract further under gravity and grows hotter. As referred by Dr. Adkins “ there’s no more fuel to burn” The end of nuclear reactions does not stop a star from radiating energy into space. Stars shine because they are hot, and a post-main-sequence star is still hot.

As the central parts of a star get hotter, nuclear reactions can be resumed. This may be in the form of hydrogen burning in the regions just outside the helium core, or else in the form of helium-burning reactions in the core itself. Any nucleus can undergo nuclear reactions if conditions are violent enough. Hydrogen burning takes place at temperatures of about 10 million K, but it requires about 100 million K to ignite helium. Hydrogen burning produces helium, while helium burning produces carbon, oxygen, and other rather heavy nuclei. The heavier the nucleus, the higher is the temperature required to bring it into nuclear reaction.

The star is no longer on the main sequence, is now what astronomers call a giant, or, if the luminosity is extremely great, a supergiant.


DEATH OF A STAR

The death of a star depends on its mass. Gravitational pressure will make the core temperature to increase at the point that thermonuclear reaction will ignite. This will burn hydrogen outside the core and it will convert helium into heavier elements.


MASS EIGHT TIMES GREATER THAN THE SUNS OR LESS

The core gains a high enough temperature to fuse elements up to oxygen, which contains eight protons. A white dwarf is created. A process called degenerate electron pressure does not allowed the white dwarf to collapse.


MASS GREATER THAN EIGHT SOLAR MASSES

In this situation the core can reach a hot enough temperature to fuse elements into iron, which has twenty-six protons. Eventually the pressure applied by gravity is so high that the counterbalancing degenerate electron pressure is not enough to avoid a collapse. In this situation, electrons are pushed into the proton, the core decreases into a much smaller ball and finally explodes becoming a Supernova. The results of this are Neutron Stars or Black Holes.


LESS THAN 25 SOLAR MASSES - NEUTRON STAR

GREATER THAN 25 SOLAR MASSES - BLACK HOLE



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