Chapter 29.3

A. Star Size Mass – the mass of a star determines the size, temperature, and brightness of the star. - The greater the mass, the greater the gravity. - The greater the gravity the hotter and brighter the star burns. - Hotter and brighter stars burn up fuel at a faster rate.

B. Stellar Evolution 1. Beginning - Stars form from a large rotating cloud of gas and dust called a nebula. - As gravitational forces cause the cloud to contract, a center, called a protostar, begins to form. - When the core temperature of the protostar reaches a high enough temperature, fusion of hydrogen begins and a star is born. Public Domain –

2. Small Stars (Sun-Sized) - Sun-sized stars take about 10 billion years to expend their hydrogen fuel. - When the hydrogen fuel in the core is exhausted, the star swells to become a red giant. - The helium in the core fuses to form carbon until the helium is used up. - At this point the star shrinks to form a white dwarf. Wikipedia Image of Sirius A and Sirius B taken by the Hubble Space Telescope. Sirius B, which is a white dwarf, can be seen as a faint dot to the lower left of the much brighter Sirius A

3. Large Stars - These stars use up their fuel much quicker and therefore do not last as long as smaller stars. - When their fuel is consumed, the internal gravitational forces may cause electrons & protons in the core to fuse together to form neutrons, thus forming a neutron star. - Neutron stars may form pulsars, or rotating beacons of light. Public Domain –

- Others may begin to fuse heavier elements until an iron core develops and explodes in a supernova. - Very massive stars may be too large to form neutron stars and collapse in on themselves to from a black hole. Video – Fusion Produces Elements (3:42 min)