stellar evolution

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The changes that occur to a star during its lifetime, from birth to final extinction. A star is believed to form from a condensation of interstellar matter, which collects either by chance or for unexplained reasons, and grows by attracting other matter towards itself as a result of its gravitational field. This initial cloud of cold contracting matter, called a protostar, builds up an internal pressure as a result of its gravitational contraction. The pressure raises the temperature until it reaches 5−10 × 106 K, at which temperature the thermonuclear conversion of hydrogen to helium begins. In our sun, a typical star, hydrogen is converted at a rate of some 1011 kg s−1 with the evolution of some 6 × 1025 J s−1 of energy. It is estimated that the sun contains sufficient hydrogen to burn at this rate for 1010 years and that it still has half its life to live as a main-sequence star (see Hertzsprung–Russell diagram). Eventually, however, this period of stability comes to an end, because the thermonuclear energy generated in the interior is no longer sufficient to counterbalance the gravitational contraction. The core, which is now mostly helium, collapses until a sufficiently high temperature is reached in a shell of unburnt hydrogen round the core to start a new phase of thermonuclear reaction. This burning of the shell causes the star's outer envelope to expand and cool, the temperature drop changes the colour from white to red and the star becomes a red giant or a supergiant if the original star was very large. The core now contracts, reaching a temperature of 108 K, and the helium in the core acts as the thermonuclear energy source. This reaction produces carbon, but a star of low mass relatively soon runs out of helium and the core collapses into a white dwarf, while the outer regions drift away into space, possibly forming a planetary nebula. Larger stars (several times larger than the sun) have sufficient helium for the process to continue so that heavier elements, up to iron, are formed. But iron is the heaviest element that can be formed with the production of energy and when the helium has all been consumed there is a catastrophic collapse of the core, resulting in a supernova explosion, blowing the outer layers away. The current theory suggests that thereafter the collapsed core becomes a neutron star or a black hole, depending on its mass.

Subjects: Astronomy and Astrophysics — Physics.

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