From: Zardoz ® 04/11/2001 19:37:47 Subject: re: COSMOLOGY FAQ post id: 484073 Credits: NASA, Donald Walter (South Carolina State University), Paul Scowen and Brian Moore (Arizona State University) How does a star form? From: Greg L. ® A star is born by the collapse of a cloud of molecular hydrogen gas under gravity. Initially the cloud is very thin and dispersed throughout space, but under the influence of external perturbations and shock waves from violent nearby events such as supernova explosions, the cloud begins to collapse on itself. As it does so it begins to heat up and radiate away energy. Eventually the cloud becomes hot enough to glow, becoming a 'protostar.' Depending on the initial conditions present during the collapse, the cloud may further fragment into two or more stars, forming a open star cluster. Many of these stars will have disks of dust and gas around them, and these will go on to form planets. Eventually the cloud collapses to the point where the internal temperatures and pressures are high enough to initiate hydrogen burning at the core via a series of nuclear fusion reactions. Soon after this occurs the cloud stops collapsing and a new star is born. The lifetime of the newborn star will depend on its mass. Small stars will slowly burn up their stocks of hydrogen, whilst bigger stars will burn their stocks much quicker, and may not live longer than a few million years.