Stellar Evolution, The Lives of Stars

Where stars are born?
Astronomers believe that molecular clouds, dense clouds of gas lies in the spiral arms of galaxies are the birthplace of stars. Dense regions in the clouds collapse and form 'protostars, "said a star begins his life as a large mass of relatively cool gas. The contraction of the gas and the subsequent rise in temperature continues until the internal temperature of the star reached a value of approximately 1000000 ° C (about 1800000 ° F).
At this point a nuclear reaction takes place in which the nuclei of hydrogen atoms combine with heavy hydrogen deuterons (cores of so-called heavy hydrogen atoms) form the core of the inert gas helium. The latter reaction liberated large quantities of nuclear energy, and the further contraction of the star is stopped. Once the star has begun nuclear fusion, there will be a "main sequence" star.

Main sequence stars
Main sequence stars are stars like our sun, burning hydrogen into helium in their veins. For a certain chemical composition and age of stellar, stars' luminosity, the total energy radiated by the star per unit of time depends only on its mass. Stars, ten times more massive than the sun are more than a thousand times more luminous than the sun. But we should not be embarrassed by the sun low luminosity: It is ten times brighter than a star half its mass. The more massive a main sequence star, the brighter and blue it is.
For example, Sirius - the dog star is located on the lower left corner of the constellation Orion, is more massive than the sun and blue is palpable. On the other hand, Alpha Centauri, our nearest neighbour, is less massive than the sun, and is therefore redder and less light intensity.
Since stars have a limited supply of hydrogen in their cores, they have a limited life span as main sequence stars. Life is proportional to the FM / L, where f is the fraction of the total mass of the star, M, available for nuclear fusion in the core areas and L is the average brightness of the star during his lifetime main sequence. Due to the heavy dependence on the luminosity of mass stellar life depends sensitively on earth. Thus, it is fortunate that our sun is nothing more than massive, it is high since stars quickly exhaust their core competencies hydrogen supply.
Once a star exhausts its core hydrogen supply, the star is redder, more and more lights Strength: It is a red giant star. This relationship between mass and life allows astronomers to a lower limit for the age of the universe.

Death of a "normal" Star
After a low mass stars like the sun exhausted the supply of hydrogen in its core, there is no longer a heat source to support the core against the force of gravity. The core of the star collapse under the force of gravity drag until it reaches a high enough density to begin converting helium to carbon. Meanwhile, the stars' outer envelope and expands the star develops into a red giant. When the sun into a red giant, the atmosphere is the shape the earth and our planet will be consumed in a fiery death. The sun will ultimately lead to a red supergiant, as they exhausted the helium in its core. At this stage, there will be an extension outer envelope in the direction of Jupiter. During this short period of its existence, the last just a few tens of thousands of years, the sun is losing mass in a powerful winds.
Eventually, the sun loses all the mass in his bag and left behind a hot core of carbon embedded in a fog of gas expelled. Radiation from the hot core is ionise the fog creates a striking "planetary nebula", like the fog to the remains of other stars. The carbon-core will cool and finally to a white dwarf, the dense remnant dim once a bright star. The final fate of low-mass dwarfs is not known, except that they cease to radiate noticeable. Probably they will be incinerated ash, or black dwarfs.

The death of a massive star
Massive stars burn brighter and lost more than most dramatically. When a star ten times more massive then To exhaust the helium in the core of nuclear fusion cycle. The carbon-core treaties, and reached a high enough temperature to burn carbon to oxygen, neon, silicon, sulfur and finally to iron.
Iron is the most stable form of the nuclear issue and there is no energy to gain by participating in any heavier element. Without any heat source to compensate for gravity, the iron core collapses until it reaches the nuclear densities. This high density core resists further cause of the collapse in falling matter to "bounce" from the core.
This sudden bounce core (including the release of energetic neutrinos from the core) creates a supernova explosion. For a brilliant month, a single star burns brighter than an entire galaxy of a billion stars. Supernova explosions inject carbon, oxygen, silicon and other heavy elements up to iron in the interstellar space. They are also the place where most of the elements heavier than iron are produced.
Future generations of stars formed from this major element enriched gas is therefore start life with a richer supply of heavier elements than the previous generations of stars. Without supernova, the fiery death of massive stars, there would be no carbon, oxygen or other elements that make life possible.
The fate of the hot neutron star core depends on the mass of the progenitor star. If the progenitor mass is about ten times the mass of the sun, neutron star core will cool for a neutron star. Neutron stars are potentially detectable as "pulsars", powerful beacons of radio emission. There is a limit for the size of neutron stars, but beyond which such stars are gravitationally bound to keep contracting until it is a black hole from which light rays can not escape.
If the progenitor mass is greater, then the resulting core is so difficult that not even the nuclear forces can resist the pull of gravity and the core collapses to a black hole.