Average Star Life

Overview

A star's life is determined almost exclusively by its mass. Large stars (above about three solar masses) live much shorter and violent lives than most other, average-sized stars (such as our sun). Even the processes by which they create energy are different.

This page details what occurs during the life of the latter type of star.

Energy Production During Normal Life

Average-sized stars create energy by three main processes of hydrogen fusion. The basic idea is to combine light atoms into heavier ones, but the mass of the heavier ones is slightly less than the sum of the lighter ones. The extra mass is lost as energy and radiated into space; the energy is in Einstein's equation of .

The following process is known as the PPI chain and it occurs approximately 69% of the time in an average star (it is also depicted in the picture below it):

In the picture to the right, two protons join together to form a deuterium nucleus, which is also known as "heavy water." A positron and a neutrino are released as by-products. The deuterium nucleus is bombarded by another proton, creating a helium-3 nucleus. The by-product of this is a photon in the form of a gamma ray (a very high-energy form of light). Then, the helium-3 nucleus in bombarded by another helium-3 nucleus, creating a normal helium-4 nucleus. The by-product of this are two protons, which are free to start the whole process over again. The positron will be destroyed and form another gamma ray; the energy from this in the form of gamma rays is radiated out of sun's core.

A second hydrogen fusion process that occurs approximately 30.907% of the time in an average star is called the PPII chain. After line 2 of the PPI chain:

The third - and much more rare - form of fusion occurs only 0.093% of the time in an average star and is called the PPIII chain. After line 1 of the PPII chain:

Together, this looks like:

Compositional Changes

In a star such as the sun, this process lasts for time scales of about 10 billion years before the star begins to die. However, before that happens, all of the nuclear processes that have been occurring act to change the chemical composition of the star, which causes it to alter some properties.

As the hydrogen is turned to helium, the mean molecular weight of the core rises. Given the Ideal Gas Law (right), if the mean molecular weight rises, then the pressure drops, and if the pressure drops, then the core contracts, so the density rises. This must happen for the star to stay in hydrostatic equilibrium.

As the core contracts, the virial theorem says that half the gravitational energy released by contraction must go to heating the star. And, as the heat rises, energy production rises as . The end result of all this is for the star to become more luminous as it ages. For example, the sun is currently about 140% as luminous now as it was when it began fusion about 5 billion years ago.

All these changes build up, and eventually, the core runs out of hydrogen to fuse. When this happens, the star moves off the Main Sequence and begins its journey to death.