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.