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Stellar Death

How stars form, and how they live depends largely on their mass. It should be no surprise that how they die also depends on their mass.
Why do stars die? Because temperatures in their cores cannot be reached to fuse the next element. The star then slowly fades away, or dies in a spectacular supernova explosion which can be seen from other galaxies!
Stars in close binary systems may die differently due to the partner having a substantial impact on its' counterpart's fate.
Dwarf stars will eventually fade away, but stars the size of the sun, and slightly larger will eventually evolve into red giants, and then shed off their outer layers as a planetary nebula, and leave a hot white dwarf behind, which is an earthsized object consisting of helium, carbon or oxygen, in the state of plasma. In time, this white dwarf will also fade away, becoming a black dwarf. However, this black dwarf is only a theoretical object: The Universe isn't old enough to allow the existence of black dwarfs!
If the white dwarf is located in a binary system, the star may not fade away as quickly as it would have if it would have been single..

Supernovae Type Ia, Ib and Ic

The difference between a type II, and a type I supernova is that if a supernova's spectrum contains a line of hydrogen (aka the "Balmer series" in the visual portion of the spectrum) it is classified Type II else, it is Type I. Further, a supernova of type Ia is often the result of a white dwarf exploding in a binary star system, in which it is annihilated. During this explosion, the total energy output is consistently between 1 - 2 x 1044 Joules. During the explosion, a shockwave propagates through space, sometimes at a speed of as much as 30 000 km/s! This explosion may be enough to trigger star formation in nearby nebulae. The white dwarf in this supernova will consist mainly of helium, but also of carbon and oxygen. Supernovae type Ia can be thought of as a giant hydrogen bomb going off in space, with the bomb being the size of the earth, and the mass being at most close to 1.44 solar masses.

The spectra of type Ib and Ic lacks lines of silicon. Also, this kind of explosion is a result of the collapse of the core of a massive star. This star, known as the "Progenitor" is at the end of its' life like an onion in its' interior, consisting of layers; hydrogen at the surface, then helium, carbon, neon, oxygen, silicon and finally, iron. The stars that result in these supernovae are believed to have been stripped of their outer layers of hydrogen and helium. Type Ib has lost it's hydrogen layer, while type Ic has also lost its' helium layer. This has occurred when the star has been in a normal, evolutionary phase (in this phase the star falls under the category of Wolf-Rayet stars, if the star has atleast 20 solar masses) of huge mass loss, where the mass has been lost to space through very strong stellar wind, or ripped by a companion of it the star is in a multiple system.

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Supernovae Type II

This kind of explosion also have progenitors with atleast 8-9 solar masses. In other words, massive stars. At the end of their lives these stars have an onion-like interior, with the heaviest element at the very heart. As explained here other fusion reactions a massive star will eventually build up an iron core (through the decay of nickel-56). Iron cannot fuse into heavier elements unless energy is transported to the reaction. The iron core is supported only by the electron degeneracy pressure. When the mass of the core becomes larger than 1.44 solar masses, the star collapses. The outer layers may fall into the core at speeds as high as 70 000 km/s! The core starts shrinking very fast as it heats up. This leads to the creation of neutrons (by merging protons with electrons, resulting in massive neutrino and gamma ray output) which help stopping the implosion. As it stops the matter bounces off the core and all the layers except for the core are shed into space in a violent explosion.
The core, which is left behind has an initial temperature of 100 000 000 000 degrees Kelvin (100 billion K), and a radius of tens of kilometres only. This means the object has a density in the order of 1017Kg/m3! When the star explodes it will shine stronger than the light of the entire galaxy combined! The total energy output will be in the order of 1046Joules! In this process heavier elements that iron will be created and spread out into space. Depending on the mass of the progenitor, the remnant of the explosion will either be a neutron star, weighing atleast 1.44 solar masses, or a black hole, from which no light can escape. Thanks to explosions like this, life has been possible on earth, as we know it.

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Next: Stellar Remnants.


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Supernovae Type Ia, Ib and Ic - Illustration (P)
Supernovae Type II - Illustration (P)

Space Art

Above: A massive star exploding in an event which will outshine the rest of the entire galaxy, combined.

This illustration is available upon request, as a print (5000x3000 pixels, 300 dpi).

Space Art

Above: The process of a supernova type II explosion, at the very end of the star's life, and at the very beginning of its' death.

This illustration is available upon request, as a print (5000x3500 pixels, 300 dpi), and as a .PSD file, customizable.


All content Copyright , 2005- by Fahad Sulehria, unless stated otherwise.
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