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H-R Diagram
(Hertzsprung-Russell)
 Shows the relationship between the
absolute magnitude and temperature of
stars
 So what?
 It shows stars of different ages and in
different stages, all at the same time. It is a
great tool to check your understanding of
the star life cycle.
 Hey, let’s look at the life cycle of a
star
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Star Life Cycle
 1. Beginning: Nebula
 1. Gravity pulls gas and dust inward toward the core.
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(accretion) this is a protostar
2. Inside the core, temperature increases as gas
atom collisions increase.
3. Density of the core increases as more atoms try to
share the same space.
4. Gas pressure increases as atomic collisions and
density (atoms/space) increase.
5. The protostar’s gas pressure RESISTS the
collapse of the nebula.
6. When gas pressure = gravity, the protostar has
reached equilibrium and accretion stops
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Protostar: two options
 if critical temp. is not reached: ends up as
a brown dwarf
 if critical temp is reached: nuclear fusion
begins and we have a star
 Hydrogen in the core is being fused into
helium
 H-R Diagram: main sequence star
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2. Main sequence stars
 90% of life cycle as a main sequence
 fusing Hydrogen into Helium
 **always present is the “equilibrium”
battle between gravity and gas
pressure
 how long a star lives depends on its
initial mass
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3. Crisis (after H used up)
 fuel (hydrogen) runs out/fusion stops so
gravity begins to win
 gravity compresses core repeating initial
process of increase temperature, gas
pressure, and density
 2nd critical temperature: He to C
 Higher core temperature causes outer
layers begin to expand, cool off and turn
reddish in color : become Red Giants
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4. Death: two branches
 a.)
lower mass stars
 period of instability
 outer layers lifting off (planetary nebula)
 collapse under own weight creating a white
dwarf
 *this is what will happen to our sun
 slowly fades away since no new energy
produced until black as space (black
dwarfs)
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4. Death: two branches
 b) massive stars
 Repeated process of gravity vs. gas pressure w/new
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critical temperatures
He → C → Ne → O → Si → Fe
This continues until Fe (iron created)
Fusion stops (no more energy created)
core collapses creating a supernova
because of tremendous pressure, electrons join
protons to become neutrons
creates a neutron star
no space between atoms; extremely dense
 *Super Massive stars can become black holes
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