BASIC IDEA
Stars spend their lifetimes trying to stay
in hydrostatic equilibrium and thermal
equilibrium. When stars don't generate
enough energy, they fall out of thermal
and hydrostatic equilibrium and they
evolve. The story of stellar evolution is
simply the story of a star in its eternal
struggle with gravity.
• Now consider a
“slab” of solar
material. Aka a
layer of the Sun.
• The Suns interior is
in hydrostatic
equilibrium.
• In general there is a
balance between
• Radiation pressure
and
• Gravitational
pressure
• If a star is in hydrostatic and
thermal equilibrium, and it derives
all of its energy from nuclear
reactions, then its structure is
completely and uniquely determined
by its total mass and by the
distribution of the various chemical
elements throughout its interior.
• In other words, the mass and
composition, the properties a star is
born with are just the properties
which determine its structure.
Figure from Foundations of Astronomy by M. Seeds
CLICK HERE
CNO CYCLE.mpg
• Stars that have
not quite
balanced out yet
are not on the
Main Sequence
• They are still very
energetic and
spew matter into
space in an effort
to stabilize (next
slide)
• May take as many
as 10 Million
years to stabilize
H–R Diagram
Supergiants
Luminosity (Lsun)
106
104
102
Giants
1
10-2
10-4
40,000
White Dwarfs
20,000
10,000
5,000
Temperature (K)
2,500
Relative Star Sizes
L  M4
Main Sequence
High
Mass
Luminosity (Lsun)
106
104
102
1
10-2
Low
Mass
10-4
40,000
20,000
10,000
5,000
Temperature (K)
2,500
• During the main sequence phase there is a
"feedback" process that regulates the energy
production in the core and maintains the star's
stability. The basic physical principles are:
• The thermal radiation law, L ~ T4, determines the energy
output, which fixes requirement for nuclear energy
production.
• The nuclear reaction rates are very strong functions of the
central temperature; Reaction Rate ~ T4 for the P-P Chain.
• The inward pull of gravity is balanced by the gas pressure
which is determined by the Ideal Gas Law: PV=NRT
Main Sequence Star Structure
• At a certain point a star has
burned enough hydrogen that
the chemical structure changes.
• The evolutionary path then
depends on MASS
Chandrasekhar,
Subrahmanyan (1910-1995)
An Indian-born American
astrophysicist renowned
for his theoretical work on
compact celestial objects,
notably white dwarfs and
neutron stars.
Evolution of Low Mass
Stars
(below the Chandrasekhar Limit)
Climbing the Red Giant Branch
Luminosity (Lsun)
106
104
Red Giant
Branch
102
H-core
exhaustion
1
10 -2
10 -4
40,000
20,000
10,000
5,000
Temperature (K)
2,500
Red Giant Star Structure
Red Giant Star
Inert
He
Core
H Burning
Shell
Cool, Extended
Envelope
Horizontal Branch
Helium
Flash
Luminosity (Lsun)
106
104
Horizontal Branch
102
Red Giant
Branch
H-core
exhaustion
1
10 -2
10 -4
40,000
20,000
10,000
5,000
Temperature (K)
2,500
Horizontal Branch Star
He
Burning
Core
H Burning
Shell
Envelope
The Asymptotic Giant Branch
Asymptotic
Giant Branch
Luminosity (Lsun)
106
104
Horizontal Branch
102
Red Giant
Branch
H-core
exhaustion
1
10 -2
10 -4
40,000
20,000
10,000
5,000
Temperature (K)
2,500
Asymptotic Giant Branch Star
H Burning
Shell
He Burning
Shell
Inert
C-O
Core
Cool, Extended
Envelope
Planetary Nebula Phase
Bare Core
Envelope Ejection
Luminosity (Lsun)
106
104
102
1
10 -2
White
Dwarf
10 -4
40,000
20,000
10,000
5,000
Temperature (K)
2,500
Evolution of High Mass
Stars
(above the Chandrasekhar Limit)
Red Supergiant Branch
Luminosity (Lsun)
106
Red
Supergiant
104
102
1
10 -2
10 -4
40,000
20,000
10,000
5,000
Temperature (K)
2,500
Red Supergiant Star
Inert
He
Core
H Burning
Shell
Cool, Extended
Envelope
Not to Scale
Blue Supergiant
Blue Supergiant
Helium
Flash
Luminosity (Lsun)
106
104
102
1
10 -2
10 -4
40,000
20,000
10,000
5,000
Temperature (K)
2,500
End of Helium Burning
Luminosity (Lsun)
106
104
102
1
10 -2
10 -4
40,000
20,000
10,000
5,000
Temperature (K)
2,500
End of Carbon Burning
Phase
H Burning
Shell
He Burning
Shell
Inert
O-Ne-Mg
Core
C Burning
Shell
Red Supergiant
Envelope
End of the Silicon
Burning Phase
H Burning
Shell
He Burning
Shell
Core Radius: ~1 Rearth
C Burning
Shell
Ne Burning
Shell
Inert
Fe-Ni
Core
O Burning
Shell
Si Burning
Shell
Envelope Radius: ~ 5 AU
Supernova Remnant and Neutron Star
Close up Image of Jet Near A Black Hole
A Black Hole?
Black Hole
• A pulsar is a neutron star which emits beams of
radiation that sweep through the earth's line of sight.
Like a black hole, it is an endpoint to stellar evolution.
• The "pulses" of high-energy radiation we see from a
pulsar are due to a misalignment of the neutron star's
rotation axis and its magnetic axis.
• Pulsars pulse because the rotation of the neutron star
causes the radiation generated within the magnetic
field to sweep in and out of our line of sight with a
regular period.
http://science.nasa.gov/NEWHOME/help/tutorials/pulsar.htm
Black hole pics to use
K. Schwarzschild
(1873-1916)
• Develop the
mathematics for
determining the
Schwarzschild radius
of Black Holes
• S’s already know about
escape velocity from
planets. Relate this to
black holes for light to
determine masses of
Black Holes