Life Cycle of a Star The changes that a star goes through is determined by how much mass the star has. Two Types of Life Cycles: Average Star- a star with relatively low mass Massive Star- a star with relatively high mass Life Cycle of Stars http://www.seasky.org/cosmic/sky7a01.html Stellar Nebula All stars begin in a cloud of gas and dust called a stellar NEBULA. Gravity will cause the nebula to contract. The nebula will break into smaller pieces. These pieces will eventually form stars. Protostars Protostar – after a few million years, the gas forms into a disk with a small dense core ~1500K The Life of an Average Star An Average Star (low mass star) is condensed in a nebula and begins a nuclear reaction that causes hydrogen to form helium, releasing energy in the form of heat and light. A low mass star will stay in this MAIN SEQUENCE phase for a long time, until it begins to use up all of it’s hydrogen. Red Giant Towards the end of it’s MAIN SEQUENCE phase, a star begins to burn all of its hydrogen. The outer layers will collapse, become heated by the core and expand out forming a red giant. Planetary Nebula The star begins to quickly blow off its layers forming a cloud around the star called a planetary nebula. The star in the center of the nebula is very hot but not very bright. White Dwarf When a star has burned all it’s fuel it will collapse under the pressure of gravity. The white dwarf that forms is very small and dense. Star with Similar Mass of the Sun Life of a Massive Star p. 388&404 Stellar Nebula All stars begin in a cloud of gas and dust called a stellar NEBULA. Gravity will cause the nebula to contract. The nebula will break into smaller pieces. These pieces will eventually form stars. Life of a Massive Star Stars with more mass than the sun (high mass stars) burn their hydrogen faster than low mass stars, so their MAIN SEQUENCE phase is much shorter. These stars burn hotter and brighter than low mass stars. Red Supergiant When the high mass star burns off it’s hydrogen its outer layers begin to expand rapidly. Temperatures at the core are much higher than a red giant. Nuclear fusion causes elements to combine into an iron core at amazing speeds. Supernova The iron core collapses on it’s self under the intense gravity at very high speeds. The energy released is called SUPERNOVA. Neutron Star or Black Hole After the incredible release of energy from the SUPERNOVA a dense core (1 trillion times denser than a white dwarf) is all that remains of the Massive Star. If the mass is too dense it will continue to collapse on itself forming a black hole. The gravitational pull of a black hole is so great, light can not escape.