Stars

Stars are super hot gaseous body that emits radiation generated within itself by nuclear fusion. Mainly composed of hydrogen and helium, a star is a fine balancing act between the force produced within itself and the gravitational force. Many stars belong to binary or multiple systems.

A star begins its life when the temperature and pressure within a collapsing cloud of interstellar material cause nuclear reactions to start. Below about 0.08 solar mass, conditions do not reach the point where nuclear fusion can occur. Above about 150 solar masses, a star becomes unstable. The mass of a star has a great influence on its evolution. A star’s evolution is dictated throughout its life by the balancing act between gravitation and the energy produced by the nuclear reactions within the star. As a star evolves its chemical composition changes. Stars belonging to close binary systems often have their evolution altered by interactions with their companion.

The composition of stars is initially mainly hydrogen and helium, with a proportion of heavier elements, depending on the material from which it is forming. Stars produce energy within their core by nuclear fusion. The energy released has to force its way out through opaque gas, and at every point within the star the gravitational pull of the parts within exactly balances the outward thrust and the star is in hydrostatic equilibrium.

The internal structure of a star cannot be observed directly. The equations of stellar   structure, on which computer models of the conditions within stars depend, are based on known laws of physics. Physical data such as mass, luminosity, radius, chemical composition and effective temperature can be measured for some stars, and the observed stellar conditions are satisfactorily predicted by the equations of stellar structure.

Stars can be broadly classified by their mass and stage of evolution and also by their effective temperature. A star’s spectral type is an indication of its temperature, but it also gives information about its chemical composition. Stars are also classified by the percentage of heavy elements in their composition. The percentage of heavy elements affects a star’s evolution.

Many stars are observed to fluctuate in brightness. These fluctuations can be due to a star belonging to an eclipsing binary system or a close binary system, or they may be a result of instabilities caused by its evolutionary phase.

Questions to Ponder

• How can you calculate distances between stars?
• How do stars move in the Galaxy?
• Why don't astronomers use everyday units to measure distances (what is an AU or a pc)?
• Why do constellations look the same after several years even though all the stars are moving?