100%(1)1 out of 1 people found this document helpful
This preview shows page 1 - 2 out of 8 pages.
Life Cycles of StarsHow Stars are BornNo one has ever observed stars go through their life cycle, but astronomers can observe manystars at different stages in their life cycles. In addition, astronomers can calculate what wouldhappen to stars under various conditions, and attempt to match the predictions against actualstars. The overall outlines of stellar evolution are probably accurately known, but there are manyunanswered questions, some of them big ones, and very likely there are surprises waiting forastronomers as well.Stars are believed to form when clouds of interstellar gas and dust start to contract under theinfluence of gravity. In interstellar space, far from other stars, a cloud of gas can be very thin andstill be dense enough to begin contracting. Many astronomers believe violent explosions of olderstars create shock waves that help start the contraction process, at the same time enriching thegas in heavy chemical elements. In all likelihood, a cloud will condense into many stars and formastar cluster.Initially, a cloud destined to become a star will be roughly spherical. The cloud will almostcertainly have a slight rotation, simply because of random gas motions in the cloud as it startedcontracting. As the cloud contracts, its rotation will speed up, causing the cloud to become disk-shaped. The cloud is still far larger than our solar system, and its outer reaches very thin. Most ofthe mass of the cloud falls to the center, adding energy to the center and heating it up. The centerof the cloud becomes aprotostar, emitting mostly infrared radiation. Finally the temperaturesand pressures within the star reach the point where nuclear reactions begin, and the star "turnson". Matter in the surrounding disk may accrete to form planets, or, if the condensations aremassive enough, companion stars.Stars With CompanionsMultiple StarsMany, perhaps all stars, have companions. Many stars arebinary starsormultiple stars, withseveral stars orbiting around one another. Some multiple stars may form when a rapidly rotatingprotostar becomes unstable and splits; others may form as separate protostars. Our nearestneighbor, Alpha Centauri, is a triple star, with aprimarystar much like our sun, a smaller andcoolersecondarystar orbiting about as far away as Uranus orbits our sun, and a very faintcompanion many times farther away from the primary than pluto is from the sun. Binary stars areof great value to astronomers because the orbital periods of binary stars depend on the masses ofthe stars, and binary stars enable astronomers to determine the masses of starsDoes the sun have a companion star? There has been speculation from time to time that the sunmight have a companion, most recently the "nemesis" hypothesis. The "nemesis" hypothesisargued that an undiscovered companion of the sun periodically caused comets to sweep throughthe solar system, triggering among other things the extinction of the dinosaurs. If any