Download Unit 5

DEPARTMENT OF PHYSICS AND ASTRONOMY
LIFECYCLES OF STARS
Option 2601
M.R. Burleigh 2601/Unit 5
Stellar Physics
 Unit 1 - Observational properties of
stars
 Unit 2 - Stellar Spectra
 Unit 3 - The Sun
 Unit 4 - Stellar Structure
 Unit 5 - Stellar Evolution
 Unit 6 - Stars of particular interest
M.R. Burleigh 2601/Unit 5
DEPARTMENT OF PHYSICS AND ASTRONOMY
Unit 5
Stellar Evolution
M.R. Burleigh 2601/Unit 5
Stellar Evolution
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Star formation
Main sequence
Stellar clusters (open, globular)
Population I & II stars
Red Giants
Planetary Nebulae
White Dwarfs
Supernovae
Neutron Stars
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Sequence
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Protostar
Pre-main Sequence (PMS)
Main Sequence
Post-main Sequence
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Protostars
 Stars born by gravitational contraction
of interstellar clouds of gas and dust
 Gravitation energy  50% thermal &
50% radiative
 Cloud is a Protostar before hydrostatic
equilibrium is established
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Protostars

Collapse starts in “free fall”
– Particles do not collide during collapse
– i.e. P=0, gravity is only force involved

Collapse is uneven
– Core collapses more rapidly forming a
small central condensation
– Core then accretes material
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Protostars
 Low mass objects accrete all (most) of
material
 High mass objects behave similarly, but
– Fusion begins before end of accretion
– Some material then blown away by
radiation pressure
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Effect of Rotation
 If angular momentum > 0
– Cloud flattens into a disk
 In some cases several central blobs
form, which can coalesce into fewer…
 Multiple star systems
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Cloud Collapse
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Star Formation
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Star Formation
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Star Formation
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Pre-main
sequence for a
solar mass star
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Evolution of a high mass star
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Star Formation
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Star Formation
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Stellar Lifecycle
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The Main Sequence
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Start of nuclear burning  zero-age
main sequence
As H  He composition () changes,
structure changes
Rates of evolution depend on two
things
1. Initial mass
2. Composition
M.R. Burleigh 2601/Unit 5
The Main Sequence
 High mass stars are hotter & more
luminous
 Use their energy faster, i.e. evolve
faster
 Spend less time on the main sequence
 O & B stars evolve faster than M stars
M.R. Burleigh 2601/Unit 5
Quantitatively
Mass-luminosity relation:
L*  M * 

 
LSun  M Sun 
Giving star lifetime:
t*
t Sun
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3.3
 MM *   M  2.3
Sun 
*


 L  
 *   M Sun 
 LSun 
Eagle Nebula
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Eagle Nebula
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Rosette Nebula
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T
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The Pleiades
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Population I Stars
 Accreting from the ISM now! (i.e. recent past)
 Typical stars are young, in galactic spiral
arms where gas and dust found
 Typically reside in open star clusters
 ~2% of mass elements heavier than H or He
(ISM enriched by supernovae)
 If M* a little > M energy generation is by
CNO cycle
 Sun is population I
M.R. Burleigh 2601/Unit 5
Post mainsequence
for a solar
mass star
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Evolutionary phases of a solar
mass star, post main-sequence
H-R position
Stage
Physical processes
3
ZAMS
Core hydrogen burning begins
4
Evolution on mainsequence
Core hydrogen burning ceases; shell
hydrogen burning begins
5
Evolution off mainsequence
Shell hydrogen burning continues;
convection dominates energy transport
6
Red giant
Helium flash occurs; core helium burning
begins
7
Subgiant
Core helium burning continues along with
shell hydrogen burning
Red giant again
Thermonuclear reactions then end; shell
helium and hydrogen burning continues
8
Planetary nebula
Star enters the planetary nebula stage
9
White dwarf
All thermonuclear reactions stop; slow
cooling
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End of Main Sequence
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Post mainsequence
for a solar
mass star
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Population II Stars
 First stars to be formed in Universe
 Have only 0.01% heavy elements
 Typically found in galactic bulge and globular
clusters
 Similar sequence of evolution but occupy
different region of H-R diagram during core
He burning
 Significant temperature changes, heating and
then cooling
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Late in the life of
a solar mass star
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Red Giant > PN
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Evolutionary phases of a solar
mass star, post main-sequence
H-R position
Stage
Physical processes
3
ZAMS
Core hydrogen burning begins
4
Evolution on mainsequence
Core hydrogen burning ceases; shell
hydrogen burning begins
5
Evolution off mainsequence
Shell hydrogen burning continues;
convection dominates energy transport
6
Red giant
Helium flash occurs; core helium burning
begins
7
Subgiant
Core helium burning continues along with
shell hydrogen burning
Red giant again
Thermonuclear reactions then end; shell
helium and hydrogen burning continues
8
Planetary nebula
Star enters the planetary nebula stage
9
White dwarf
All thermonuclear reactions stop; slow
cooling
M.R. Burleigh 2601/Unit 5
Late in the life of
a solar mass star
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PN > White Dwarf
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White Dwarfs
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Chandrasekhar Limit
White dwarfs form from stars with M  8MSun
Degenerate gas pressure prevents further gravitational
contraction
Chrandrasekhar limit: degeneracy pressure can only support
M  1.4MSun. Above this limit a neutron star is formed
For a degenerate gas (non-relativistic):
For a perfect gas:
P  nkT P  T 
From hydrostatic equilibrium:
 Greater mass, smaller radius
M.R. Burleigh 2601/Unit 5
P  K
R
1
M
1
3
5
3
Constant
White dwarf companions
e.g.
Sirius – companion Sirius B (Alvan Clark, 1862)
Procyon – Procyon B (1882)
In binaries we can measure the companion’s mass from
Kepler’s laws
MSirius B = 1.0MSun
TSirius A = 10,000K
;
MV = -1.5
TSirius B = 25,000K
;
MV = 8
2
From L  4RSun
T 4 :
R  7  10-3RSun
  = 3  109kg m-3
M.R. Burleigh 2601/Unit 5
3  10-3LSun
Massive Stars
 Stars with masses > 7 M
 Masses greater than ~ 50 M
– Affected by mass loss (i.e. winds)
– As mass of star changes so does the
structure and luminosity
M.R. Burleigh 2601/Unit 5
Evolution of a high mass star
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Evolutionary phases of a massive star
Stage
Physical processes
Protostar
Dust and gas cloud collapses rapidly,
accompanied by heating of the interior and
ionisation of atoms
PMS
Semihydrostatic equilibrium; contraction and
heating continue
ZAMS
Hydrogen burning commences
Initial evolution on the main
sequence
Hydrogen consumed in the core; some
contraction occurs
Evolution off the main sequence
Hydrogen depleted in the core, isothermal helium
core and hydrogen-burning established
Evolution to the right in the H-R
diagram
Core rapidly contracts, envelope expands,
hydrogen-burning shell narrows
Red giant
Energy output increases, convective envelope
forms, helium burning begins
Cepheid
Convective shell contracts, core helium burning
becomes the major energy source
Supergiant
Helium-burning shell forms
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Evolution of a high mass star
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Supernovae
 Absolute magnitudes from –16 to –20
(energy ~1044J)
– e.g. China, SN of 1054 reached mV=-6
(remnant is Crab Nebula)
 Two types… Type I & Type II
 Both types eject a large fraction of
original mass with v~5000-10000 km s-1
 Explosion of stellar interior
M.R. Burleigh 2601/Unit 5
Type II Supernovae
 Seen in spiral galaxies only, especially in
spiral arms… Population I stars
 Explosions in cores of Blue/Red Supergiants
(10-100M)
 Implosion of stellar core to form neutron star
– Core reaches density > electron pressure
 Violent rebound > explosion > ejects outer
layers
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Type II Supernovae
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SN 1987A
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Type I Supernovae
 Seen in both elliptical and spiral
galaxies… Population II stars
 Progenitors are H-deficient, highly
evolved stars
 Mechanism not well understood
– Accretion onto a WD increasing MWD >
Chandrasekhar limit
– Merger of two WDs to give M > 1.4M
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Type Ia Supernovae
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Supernovae: Key Points
 SN responsible for nucleosynthesis of
element above 56Fe
 Remnant neutron stars… sometimes
revealed as pulsars
 Shockwave heating of interstellar
medium… Supernova Remnants
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Vela
Supernova Remnants
Crab Nebula
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Supernova Remnants
Cassiopiea A
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Supernova
expansion
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Schematic H-R diagram showing the
spectral classification of stars
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H-R diagram
for stars near
the Sun
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H-R diagram
from
Hipparcos
data
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Cluster H-R Diagrams
 In a cluster, compared to evolutionary
timescales, the stars are all (roughly)
the same age
 H-R Diagram can reveal the age of the
cluster
 Need to identify the “turn-off”, mass
above which all stars have evolved
away from the main sequence
M.R. Burleigh 2601/Unit 5
H-R diagram showing open cluster
(pop I) ages
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Globular Clusters
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Globular Cluster Ages
 Population II stars, few heavy elements
 Older than open clusters
 Also have different tracks due to
composition differences
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H-R diagram for
a globular cluster
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Stellar Evolution









Star formation
Main sequence
Stellar clusters (open, globular)
Population I & II stars
Red Giants
Planetary Nebulae
White Dwarfs
Supernovae
Neutron Stars
M.R. Burleigh 2601/Unit 5
DEPARTMENT OF PHYSICS AND ASTRONOMY
Unit 5
Stellar Evolution
M.R. Burleigh 2601/Unit 5
Stellar Physics
 Unit 1 - Observational properties of
stars
 Unit 2 - Stellar Spectra
 Unit 3 - The Sun
 Unit 4 - Stellar Structure
 Unit 5 - Stellar Evolution
 Unit 6 - Stars of particular interest
M.R. Burleigh 2601/Unit 5
DEPARTMENT OF PHYSICS AND ASTRONOMY
STELLAR PHYSICS
Option 2607
M.R. Burleigh 2601/Unit 5
Mass-radius relationship for white dwarfs
Marked is the best fitting mass and radius for V471 Tau, with
1 and 2 sigma uncertainty contours
M.R. Burleigh 2601/Unit 5
DA Hydrogen dominated
Non-DA Helium
dominated
PROGENITORS
– SdB, SdOB,
SdO, H-rich
PNN?
PROGENITOR
S – He-rich SdO
and PNN
Late helium
thermal pulse
Hottest DO
stars
Settling of He
and CNO
 70,000K
Hottest DA
stars
DO pulsations
DO cooling
sequence
Coolest DO
stars
DA cooling
sequence
No known
Dredge up
of helium
DA
pulsations
10,000K
M.R. Burleigh 2601/Unit 5
45,000
K
DO or DBs
DB pulsations
13,000K

150,000K
DB cooling
sequence
30,000
K
White Dwarf Cooling
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M.R. Burleigh 2601/Unit 5