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Average Star Death

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Once a star has used the core's hydrogen and converted it to helium, it can no create energy that way, and must use other resources. This point is the trigger for the advanced evolution of the star, and over the next few billion years, it will evolve to a red giant star, and then decay into a planetary nebula.

Red Giant Ascent

After about ten billion years, a main sequence star has converted approximately 10% of its hydrogen to helium. Although this might seem as though it could still undergo hydrogen fusion for another 90 billion years, this is not the case. Remember that there are immense pressures at the core of stars, and it is only because of these pressures that the fusion can occur -- in a fixed volume, increased pressure leads to increased heat. Outside of the range of pressures there is still mostly hydrogen, but it cannot be used because the pressures are not high enough to initiate fusion.

The helium core is not hot nor dense enough to fuse to create energy, so the outward pressure is stopped, and gravity takes over again. Gravity will contract the star, and eventually a shell of hydrogen around the helium core will become hot enough to fuse H -> He. This shell will produce more energy than the previous hydrogen core phase did, so the luminosity will rise. Not all of the energy will escape, though, and it will go into expanding the star. This expansion will result in a surface temperature drop. The star will be in the subgiant star, and the cooler surface will have changed from yellow to orange-red. This cooling is due to the energy spreading over a larger surface area, so each unit of area receiving less energy.

The helium "ash" from the hydrogen fusion in the shell will effectively fall onto the core, which will result in the star continuing to contract to maintain pressure to hold up the star. Once the mass in the core is approximately 8% of the sun (the Schonberg-Chandrasekhar Limit), the density will be so great that the core will no longer act as a perfect gas, and it will become degenerate.

Now the core will be held up by the Pauli Exclusion Principle, AKA it will be supported by electron degeneracy. This phase will still have the hydrogen burning shell, but the star's outer layers will continue to expand, causing it to cool. This begins the star's Red Giant Ascent.

Red Giant Branch

In this phase, H- can form when neutral H takes on a free electron. Radiation is easily absorbed by H-, and so the outer layers will have a high opacity. This high opacity and high energy generation will lead to convection, where the whole outer envelope will become convective and the material from the core can rise to the surface in a process called "dredge-up".

As the outer layers continue to expand in this red giant phase, the ionization drops so there are fewer free electrons and fewer H- ions, leading to an opacity drop. The luminosity will still rise due to the core contracting. In this phase, the luminosity is approximately 100 times what it was during the main part of its life, the radius between 30-100 times, and the effective temperature will be approximately 60%. Examples of this are Arcturus and Aldebaran.

Degenerate PressureIn the core, the temperature continues to rise. When it approaches 100,000,000 K (180,000,000 °F), helium will begin to fuse into carbon in the triple alpha process. However, since the core is degenerate, when the temperature rises, the pressure does not, for degenerate pressure is only a function of the density (right). Therefore, the core cannot expand and cool, so the energy raises the temperature, which raises the energy which raises the temperature, etc. When this actually happens, 1011 times the luminosity of the star during its main sequence life will be released in a few seconds in what is known as the Helium Core Flash.

Horizontal Branch

None of the energy from the helium core flash will make it out of the star. It will act to revert the core back to an ideal gas state, and expand it. Thus, the star will have a helium burning core, a hydrogen burning shell which will provide most of the luminosity, and a large expanding envelope of outer atmosphere. The star will now become a Horizontal Branch Star, for as the core expands, it cools and the energy generation in the hydrogen burning shell will drop; so the luminosity decreases, the star will shrink, and the surface temperature rise.

During the course of the horizontal giant branch, a carbon and oxygen ash core will begin to build up. In a star such as the sun, carbon fusion cannot occur because the temperature and density are too low. Thus, it will contract and heat, heating the layers outside the core. This will cause the new helium shell to start to fuse, and the star will begin to expand again. This repeats the previous process where there is more energy, a higher opacity, convection and a dredge-up phase. The again-expanding photosphere and higher luminosity combine to move the sun into the Asymptotic Giant Branch (AGB), AKA a red supergiant (cross section to the right).

Asymptotic Giant Branch (AGB)

In the AGB phase, the star will undergo periodic instabilities. One cause of this are helium shell flashes. This comes from the inert helium shell continually having mass added to it from the hydrogen shell. Although it will be in a partially degenerate state, when the mass gets too high, it will "ignite" in a flash similar to the first one. This will cause it to drop in luminosity and contract in size, repeating on a timescale of approximately 100,000 years. Instabilities in the outer envelope can cause AGB stars to pulsate on periods of several hundred days. Mass is lost during this phase at a rate of approximately 0.01% the mass of the sun per year in a process that is not well understood.

Planetary Nebula - IC 418 AKA SpirographIn the AGB phase, the outer layers of the star will be greatly extended and will not be strongly bound. Mass loss, pulsations, and a low binding energy of the outer layers can cause them to be released from the star, turning this phase into the Planetary Nebula.

Planetary Nebula

As the outer layers expand, their density will drop, and would allow future civilizations to view the hot carbon/oxygen (C/O) core that will be left behind. The C/O core will initially be hot at 100,000 K (180,000 °F). However, it will be dead, with no nuclear reactions to power it. It will be a white dwarf. A current example of a white dwarf is the star Sirius B.

The outer layers that will form the planetary nebula will shine and become visible to outside observers, as is the nebula to the left of IC 418 (AKA The Spirograph Nebula), as taken by the Hubble Space Telescope. Typically, planetary nebulae are around 0.3 parsecs (1 light-year), expand at a rate of 10-30 km/s, and last only 10,000 years. For more information about this nebula, see the Hubble Heritage Archive - 2000.

When the star runs out of heat, it will be a huge, black, chunk of carbon and oxygen floating in space. It will be called a black dwarf - a dead star.

Dwarf Death

Red dwarfs are the only active (undergoing hydrogen fusion) type of dwarf (other types are brown, white, and black). Red dwarfs range between 1/3 and 1/12 the sun's mass, and shine only 1/100 to 1/1,000,000 as brightly. Proxima Centauri, Earth's closest extrasolar star, is a red dwarf 1/5 the size of the sun, and if it were to trade places with the sun, it would shine on Earth only 1/10 as much as the sun currently does on Pluto.

Red dwarfs, because of their small size, undergo fusion much less quickly than a solar mass star. Therefore, they use up their supply of hydrogen much less quickly than a main sequence star, and can live for more than 100 trillion years.

When they die, they simply wink out of existence, for they do not have enough pressure to fuse helium. Thus, they simply grow dimmer and cooler as they float through the void of space.

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