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13.15 Evolved Stars

Stars are born, evolve and die. The nuclear reactions converting mass to energy in stars have a beginning and an end. The Sun, for example, spent some 107 years in its formative stages; it will spend about 1010 years on the main sequence, roughly 109 years in the giant state, and a shorter time in later unstable states consuming heavier elements until energy generation stops. This evolutionary sequence might be likened to the periods of human life: 9 months in the womb, 70 years of normal life, 6 years of rapid aging, and perhaps a year of terminal illness. Stars do evolve to strange terminal states, involving incredible forms of dense matter and exotic physical processes.

 

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Composite image of the Crab pulsar in optical (red) and x-ray (blue). Click here for original source URL


A star's destiny is controlled by two competing forces. Gravity is the force trying to make the star collapse inward, and the energy-producing fusion reactions create pressure that provides an outward force. When the core of a main-sequence star runs out of hydrogen, the balance between these forces — hydrostatic equilibrium — is disrupted. But as hydrogen fusion diminishes, so does the pressure that supports the outer layers, and the core begins to collapse. The rapid production of energy in the collapsing core can cause an expansion of the outer layers so that the star temporarily balloons to a giant size. At first, the core may burn its way into layers just above it that still have hydrogen. Eventually, however, the hydrogen-burning reactions wind down. This means that there is less outward pressure generated by heat released in nuclear reactions. Thus the core must continue to contract due to gravitational forces pulling inward. Although some of the upper layers may be blown outward, most of the star's mass contracts to a small size.

What happens after the hydrogen is exhausted and the core starts to shrink? During the process of gravitational contraction, as a mass of gas contracts, it must grow hotter. Thus the atoms in the core move faster and collide even harder than before. They reach a condition where the helium atoms in the core collide hard enough to start fusion reactions. At this point the core has stopped consuming hydrogen and is now consuming helium. After the helium is used up, more contraction occurs and still other elements may become fuels. Eventually, there are no more fuels to react, and no new energy release to create a pressure that can oppose gravity. The star must therefore contract to a very dense state. There are four main stages of stellar old age: the giants, variables, explosions of several types, and terminal entombment in a state of very high density.

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The size of Betelgeuse compared to our solar system. Click here for original source URL.

Note that the most common stars — those about one solar mass or less — go through the giant phase and then contract rather smoothly to a small configuration of high density. But the rare, high mass stars above about 8 solar masses go through explosive instabilities. Eventually they reach even higher density stages than Sun-like stars because they have more mass and stronger gravity. They produce some of the most exotic objects yet discovered by astronomers, such as pulsars and black holes.
 

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M57, The Ring Nebula. A planetary nebula surrounding a white dwarf star, the end of the line for a low mass star like our sun. Click here for original source URL.

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Eta Carinae is about 100 times more massive than the Sun. Click here for original source URL.

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Three rings of glowing gas around supernova 1987a. Click here for original source URL.

All good things must pass. Every star in the night sky will one day exhaust its nuclear fuel and die. In the battle between gravity and the pressure caused by nuclear reactions, gravity is the final victor. As a result, every star dies as a compact object. Low-mass stars die a quiet death, fading away slowly as white dwarfs to become dark embers. High-mass stars die with a pyrotechnic flourish, lighting up the sky one final time as supernovae.

Stars are factories for creating heavy elements. Along the way, they also release a tiny fraction of the frozen mass-energy of matter in the form of radiation. This radiation travels through space never to return. In the case of our star the Sun, the radiation provides the heat and light that supports life on Earth. Our atoms were all once part of previous generations of stars, and the atoms that animate biology were created in those stars. The evolution of stars concerns us all.