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29.3 Stellar Evolution
Basic Structure of Stars
The mass of a star governs its temperature, luminosity, and diameter.
Mass effects
 The more massive a star is, the greater the gravity pressing inward and the hotter and more dense
it has to be in order to balance its own gravity.
Fusion
 The density and temp in a star increase toward the center, where energy is generated by nuclear
fusion.
Stellar Evolution
 As long as a star is changing hydrogen into helium, nuclear fusion, it will be a main sequence star.
Star formation
 The formation of a star begins with a cloud of dust and gas called a nebula.
 The nebula will then collapse on itself because of its own gravity.
 As the cloud contracts, its rotation forces it into a disk shape with a hot condensed object at the
center called a protostar.
 Friction from gravity will continue to increase the temp until it reaches the temp for nuclear
reactions to occur and then it becomes a new star.
Fusion begins
 When the temp inside a protostar becomes hot enough, nuclear fusion begins.
 The first reaction is the conversion of hydrogen to helium
 Once this happens, the star becomes stable because it has enough internal heat to produce the
pressure to balance gravity.
 The object is now a true star and takes its place on the main sequence according to its mass.
Life Cycle of Stars Like the Sun (low mass)
 What happens next depends on the stars mass.
 It takes about 10 billion years for a star with a mass of the Sun to convert all of the hydrogen to
helium, therefore the life cycle is about 10 billion years.
 The next step is to become a red giant.
Red Giant
 When the hydrogen in the stars core is gone, the star will now have a helium center with outer
layers of hydrogen.
 The energy in a thin layer at the outer edge of the helium core will force the outer layers of the star
to expand and cool.
 The star then becomes a red giant because its luminosity increases while its surface temp
decreases due to the expansion
White Dwarf
 The helium in the core of a red giant will become really hot and react to form carbon.
 Eventually the helium is all used up leaving a core of carbon.
 Energy production ends because it is not hot enough.
 The star is now a white dwarf.
Black Dwarf
 Eventually the white dwarf will lose its luminosity and becomes an undetectable black dwarf.
Life Cycle of Massive Star
 Massive stars convert hydrogen to helium the same way as less massive stars do.
 But because they are much higher on the main sequence, their lifetime is shorter because the star
is very luminous and uses up its fuel quickly.
Supergiant
 The core heats up to much higher temps.
 Star will expand to a supergiant.
 Iron forms in the core and becomes too massive to be supported.
 The core collapses in on itself in a violent explosion.
 This is called a supernova.
 A supernova can go in two directions.
1. Neutron star
 When the collapsed core of a supernova is 1.4-3 times as massive as the sun, it will shrink to about
20km(12mi) in diameter.
 Only neutrons can exist here, thus the name neutron star.
 Neutrons are very dense. 1 teaspoon can weigh 600 million metric tons
2. Black hole
 If the remaining core of a supernova is more than 3 times as massive as the Sun, the core will
collapse.
 It is now called a black hole.
 Gravity is very strong now and nothing can escape its gravity.
 In order for something to get sucked into the black hole, it has to cross its event horizon.