Evolution of stars

The actual process of star formation remains shrouded in mystery because stars form in dense, cold molecular clouds whose dust obscures newly formed stars from our view. For reasons which are not fully understood, but which may have to do with collisions of molecular clouds, or shockwaves passing through molecular clouds as the clouds pass through spiral structure in galaxies, or magnetic-gravitational instabilities (or, perhaps all of the above) the dense core of a molecular cloud begins to condense under its self-gravity, fragmenting into stellar mass clouds which continue to condense forming protostars. As the cloud condenses, gravitational potential energy is released - half of this released gravitational energy goes into heating the cloud, half is radiated away as thermal radiation. Because gravity is stronger near the center of the cloud (remember Fg ~ 1/distance2) the center condenses more quickly, more energy is released in the center of the cloud, and the center becomes hotter than the outer regions. As a means of tracking the stellar life-cycle we follow its path on the Hertzsprung-Russell Diagram. The initial collapse occurs quickly, over a period of a few years. As the star heats up, pressure builds up following the Perfect Gas Law: where, most importantly P=pressure and T=Temperature. The outward pressure nearly balances the inward gravitational pull, a condition called hydrostatic equilibrium. Energy Source: Gravity The star is cool, so its color is red, but it is very large so it has a high luminosity and appears at the upper right in the H-R Diagram. 2. Pre-Main Sequence Once near-equilibrium has been established, the contraction slows down, but the star continues to radiate energy (light) and thus must continue to contract to provide gravitational energy to supply the necessary luminosity. The star must continue to contract until the temperatures in the core reach high enough values that nuclear fusion reactions begin. Once nuclear reactions begin in the core, the star readjusts to account for this new energy source Gravity releases its potential energy throughout the star, but due to the very high temperature dependence of the nuclear fusion reactions, the proton-proton chain is highly centrally concentrated. During this phase the star lies above the main sequence; such pre-main sequence stars are observed as T-Tauri Stars, which are going through a phase of high activity. Material is still falling inward onto the star, but the star is also spewing material outward in strong winds or jets as shown in the HST Photo below. Age: 10 million yrs R ~ 1. 33 Rsun Tcore = 10, 000, 000K Tsurface = 4500K Energy Source: P-P Chain turns on. 3. Zero Age Main Sequence It takes another several million years for the star to settle down on the main sequence. The main sequence is not a line, but a band in the H-R Diagram. Stars start out at the lower boundary, called the Zero-Age Main Sequence referring to the ...