The Life of a Star!!

Stage 1 - Nebula  In a nebula, dust, gas, and other materials come together through gravitational attraction, and eventually form a star

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Stage 2 - Main Sequence Star Stars are in their main sequence phase for the majority of their life; at this stage, the star is undergoing Hydrogen fusion. That means that the star is hot enough for Hydrogen atoms to come close together, and thus for chemical reactions to occur; these stars stay together as the outward energy of nuclear fusion is balanced by the inward gravitational pull.

Image Credit: NASA/SDO

The Sun

Stage 3 - Red Giant/Red Super Giant Stars which are below the Chandrasekhar limit (that is, their remnant mass is less than 1.4 times that of our sun) turn into Red Giants, while stars which are above that limit turn into red supergiants (which are essentially the same, but more massive). 95% percent of stars will become red-giants, while only 5% will become red super-giants. Red giants from when the main sequence star is no longer able to carry out Hydrogen fusion; After the star uses up all Hydrogen in Hydrogen fusion, there is no force countering the gravitational pull of the planet. As a result, the star collapses, increasing the temperature in the core to a point where helium fusion can begin. Since as a result of helium fusion there is more energy in the star than before, the star expands in size, becoming a red giant. However, throughout the expansion the mass of the star remains constant, and thus the density decreases. Red Giants have relatively short lives, generally living no more than a hundred years.

Image credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA) !

V838 Monocerotis

Stage 4 - Planetary Nebula Low mass stars turn into planetary nebulae after becoming red giants; as a result of fusion, the star becomes extremely unstable and begins to pulsate; the outer material is ejected by stellar winds, and the Nebula expands. Planetary nebulae are gaseous, expanding shells of plasma which expand at a rate of 20-50 km per second, and are typically 1 light year across. Planetary nebulae have a relatively short life, living only tens of thousands of years.

Image Credit: C. R. O'Dell, (Vanderbilt) et al. ESA, NASA !!!!!

NGC 7293 Helix Nebula

Stage 5 - White Dwarf/Black Dwarf (below the Chandrasekhar limit) After undergoing multiple stages of fusion, the star will reach carbon, which it cannot fuse; as a result, the outer layer of the star will fade away with time, and the star will be left as a white dwarf. White dwarves are extremely dense, and, since fusion is no longer occurring, they are constantly decreasing in temperature. White dwarves are eventually destined keep losing temperature until they become black dwarves, that is, until they can’t emit a significant amount of heat; however, since the time it would take for a star to cool down into a black dwarf is longer than the life of the universe, there are currently none known to be in existence.

Sirius - B

Stage 4 - Supernova (above the Chandrasekhar limit)Stars above the Chandrasekhar limit will continue undergoing fusion until they reach their iron core; due to the immense amounts of gravity acting on the iron core, the core will contract. This contraction results in neutronization, where protons fuse with high energy electrons to form neutrons. The core eventually stiffens, and sends shock waves to material falling into the core, leading to the outer material being driven outwards, and a supernova resulting.  

http://en.wikipedia.org/wiki/Supernova

SN1994D

(brightness compared to a galaxy)

Stage 5 - Neutron Star (below Oppenheim-Volker limit)If a star is below the Oppenheim Volker Limit, that is, if it has a remnant mass greater than approximately 3 solar masses, but it is still above the Chandrasekhar limit, it will turn into a neutron star. At the end of neutronization, contraction stops, and the star turns into a stable neutron star. Neutron stars emit a wide variation of spectrum wavebands, and vary greatly from star to star. Neutron stars are extremely dense, in fact, 14 ounces of a neutron star is equal to the mass of the moon. Scientists are not sure of what consists at the core, but some theorize that it consists of a pool of free quarks, while others suggest that it is composed of particles like pions. Neutron Stars have intense magnetic fields which create x-rays and gamma rays. These stars often rotate at very high speeds, some as fast as 46,000 times per minute, and have jets of particles shooting out of their magnetic poles at all times.

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Stage 5 - Black Hole (above Oppenheim-Volker limit)If the star is massive enough, nothing is able to counter the stars gravity, and the star contracts to infinite density, thus turning into a black hole; since the escape velocity of a black hole is greater than the speed of light, nothing, not even light escapes. Black holes grow in size by absorbing surrounding stars, dust, and smaller black holes. Black holes are also known to emit a small amount of thermal radiation; since mass can be converted to energy, the blackhole may theoretically shrink over time, but since black holes absorb more energy from background radiation, no shrinking occurs.