STELLAR EVOLUTION
Stellar evolution is the process in which a star undergoes a sequence of radical changes during its lifetime. Depending on the stars’ mass, this lifetime ranges from only a few million years (for the most massive) to trillions of years (for the least massive). Stars evolve because they lose energy by radiating from their hot surfaces. As a result, the star tends to contract under gravity. This contraction is partly stopped by the release of energy from nuclear reactions. However, once the nuclear fuel is depleted, gravity gets the upper hand and the star ends its life as a compact object: a white dwarf, a neutron star or a black hole, the last representing the ultimate victory of gravity.
The energy radiated from the star must be supplied from changes in the stellar interior. A star essentially has three sources of energy: the release of gravitational energy through contraction which makes the star more tightly bound; cooling through emission of the internal thermal energy; the generation of energy through nuclear fusion whereby the lighter nuclei fuse into heavier, more tightly bound nuclei. During most of the life of a star, the nuclear processes dominate the energy production; this requires that the temperature is sufficiently high that the nuclei can overcome their mutual Coulomb repulsion. Fusion leads to the build-up of successively heavier elements in the core of the star. These changes in the composition of the deep interior cause dramatic changes in the star’s structure and surface properties such as luminosity and surface temperature, reflected in the observable characteristics of the star.
Stars form from condensations in interstellar clouds. During the initial contraction the energy output comes from the gravitational energy released as a result of the contraction. At the same time, this leads to heating of the stellar interior. When the temperature becomes sufficiently high, nuclear reactions set in. Generally speaking, the first reactions are those between nuclei of the smallest charges, and hence with the lowest Coulomb barrier. Thus the initial reaction is the fusion of hydrogen into helium. Once the nuclear energy generation balances the energy loss from the stellar surface, the contraction essentially stops. Evolution then takes place on a nuclear time scale, as a result of the gradual change in the structure of the star as hydrogen is converted into helium.
An important consequence of stellar evolution is the fusion of lighter elements into heavier, starting from the lightest, hydrogen. These processes take place in the deep interiors of the star; however, during late stages of evolution the products of the reactions may be spread through mixing by convective motions throughout the star, hence becoming visible in its observable surface composition. Also, mass loss from the star, either gradual or explosive, may distribute the elements generated in the stellar interior in the interstellar matter where it can be incorporated into newly formed stars and planets. Indeed, there is strong evidence that essentially all elements heavier than helium have been formed in this manner, from the mixture of hydrogen and helium resulting from the big bang. Thus to understand the origin of the chemical elements and, finally, our own origins, we need to understand the structure and evolution of stars.
Questions to Ponder
