Evolutionary Stages of Stars

Variable Stars  

Stars that fluctuate in brightness
Types of variable stars  
Pulsating variables
Fluctuate regularly in brightness
Expand and contract in size
Eruptive variables
Explosive event
Sudden brightening
Called a nova

Stellar Evolution  

Stars exist because of gravity
Two opposing forces in a star are
Gravity – contracts
Thermal nuclear energy – expands

Stellar Evolution Stages

Birth
In dark, cool, interstellar clouds
Gravity contracts cloud and temperature rises
Radiates long-wavelength (red) light
Becomes a protostar

Protostar
Gravitational contraction of gaseous cloud continues
Core reaches 10 million K
Hydrogen nuclei fuse
Become helium nuclei
Process is called hydrogen burning  
Energy is released
Outward pressure increases
Outward pressure balanced by gravity pulling in
Star becomes a stable main-sequence star

Main-sequence stage
Stars age at different rates
Massive stars use fuel faster and exist for only a few million years
Small stars use fuel slowly and exist for perhaps hundreds of billions of years
90 percent of a star’s life is in the main sequence

Red giant stage
Hydrogen burning migrates outward
Star’s outer envelope expands
Surface cools
Surface becomes red
Core is collapsing as helium is converted to carbon
Eventually all nuclear fuel is used
Gravity squeezes the star

Burnout and death
Final stage depends on mass
Possibilities
Low-mass star
0.5 solar mass
Red giant collapses
Becomes a white dwarf

Burnout and death
Final stage depends on mass
Possibilities
Medium-mass star
Between 0.5 and 3 solar masses
Red giant collapses
Planetary nebula forms
Becomes a white dwarf
Final stage depends on mass
Possibilities
Massive star
Over three solar masses
Short life span
Terminates in a brilliant explosion called a supernova
Interior condenses
May produce a hot, dense object that is either a neutron star or a black hole