LIFECYCLES OF STARS

M.R. Burleigh 2601/Unit 5

DEPARTMENT OF PHYSICS AND ASTRONOMY

LIFECYCLES OF STARSLIFECYCLES OF STARS

Option 2601Option 2601


Stellar PhysicsStellar Physics

Unit 1 - Observational properties of Unit 1 - Observational properties of starsstars

Unit 2 - Stellar SpectraUnit 2 - Stellar Spectra Unit 3 - The SunUnit 3 - The Sun Unit 4 - Stellar StructureUnit 4 - Stellar Structure Unit 5 - Stellar EvolutionUnit 5 - Stellar Evolution Unit 6 - Stars of particular interestUnit 6 - Stars of particular interest



Unit 5Unit 5

Stellar EvolutionStellar Evolution



Star formationStar formation Main sequenceMain sequence Stellar clusters (open, globular)Stellar clusters (open, globular) Population I & II starsPopulation I & II stars Red GiantsRed Giants Planetary NebulaePlanetary Nebulae White DwarfsWhite Dwarfs SupernovaeSupernovae Neutron StarsNeutron Stars


SequenceSequence

ProtostarProtostar Pre-main Sequence (PMS)Pre-main Sequence (PMS) Main SequenceMain Sequence Post-main SequencePost-main Sequence


ProtostarsProtostars

Stars born by gravitational contraction Stars born by gravitational contraction of interstellar clouds of gas and dustof interstellar clouds of gas and dust

Gravitation energy Gravitation energy 50% thermal & 50% thermal & 50% radiative50% radiative

Cloud is a Cloud is a ProtostarProtostar before hydrostatic before hydrostatic equilibrium is establishedequilibrium is established



Collapse starts in “free fall”Collapse starts in “free fall”– Particles do Particles do notnot collide during collapse collide during collapse– i.e. P=0, gravity is only force involvedi.e. P=0, gravity is only force involved

Collapse is unevenCollapse is uneven– Core collapses more rapidly forming a Core collapses more rapidly forming a

small central condensationsmall central condensation– Core then Core then accretes accretes materialmaterial



Low mass objects accrete all (most) of Low mass objects accrete all (most) of materialmaterial

High mass objects behave similarly, High mass objects behave similarly, butbut– Fusion begins before end of accretionFusion begins before end of accretion– Some material then blown away by Some material then blown away by

radiation pressureradiation pressure


Effect of RotationEffect of Rotation

If angular momentum > 0If angular momentum > 0– Cloud flattens into a diskCloud flattens into a disk

In some cases several central blobs In some cases several central blobs form, which can coalesce into fewer…form, which can coalesce into fewer…

Multiple star systemsMultiple star systems


Cloud CollapseCloud Collapse


Star FormationStar Formation


Pre-main sequence for a solar mass star


Evolution of a high mass star


Star Formation Star Formation


Star FormationStar Formation


Stellar LifecycleStellar Lifecycle


The Main SequenceThe Main Sequence

Start of nuclear burning Start of nuclear burning zero-age zero-age main sequence main sequence

As H As H He composition ( He composition () changes, ) changes, structure changesstructure changes

Rates of evolution depend on two Rates of evolution depend on two thingsthings

1.1. Initial massInitial mass2.2. CompositionComposition


The Main SequenceThe Main Sequence

High mass stars are hotter & more High mass stars are hotter & more luminousluminous

Use their energy faster, i.e. evolve Use their energy faster, i.e. evolve fasterfaster

Spend less time on the main sequenceSpend less time on the main sequence O & B stars evolve faster than M starsO & B stars evolve faster than M stars


Mass-luminosity relation:

3.3

**

SunSun MM

LL

3.2

***

*

SunLLMM

Sun MM

tt

Sun

Sun

Giving star lifetime:

QuantitativelyQuantitatively


Eagle NebulaEagle Nebula


Rosette NebulaRosette Nebula


TT


The PleiadesThe Pleiades


Population I StarsPopulation I Stars

Accreting from the ISM now! (i.e. recent past)Accreting from the ISM now! (i.e. recent past) Typical stars are young, in galactic spiral Typical stars are young, in galactic spiral

arms where gas and dust foundarms where gas and dust found Typically reside in open star clustersTypically reside in open star clusters ~2% of mass elements heavier than H or He ~2% of mass elements heavier than H or He

(ISM enriched by supernovae)(ISM enriched by supernovae) If MIf M* * a little > Ma little > M energy generation is by CNO energy generation is by CNO

cyclecycle Sun is population ISun is population I


Post main-sequence for a solar mass star


Evolutionary phases of a solar mass star, post main-sequence

H-R positionH-R position StageStage Physical processesPhysical processes

33 ZAMSZAMS Core hydrogen burning beginsCore hydrogen burning begins

44 Evolution on main-Evolution on main-sequencesequence

Core hydrogen burning ceases; shell Core hydrogen burning ceases; shell hydrogen burning beginshydrogen burning begins

55 Evolution off main-Evolution off main-sequencesequence

Shell hydrogen burning continues; Shell hydrogen burning continues; convection dominates energy transportconvection dominates energy transport

66 Red giantRed giant Helium flash occurs; core helium burning Helium flash occurs; core helium burning beginsbegins

77 SubgiantSubgiant Core helium burning continues along with Core helium burning continues along with shell hydrogen burningshell hydrogen burning

Red giant againRed giant again Thermonuclear reactions then end; shell Thermonuclear reactions then end; shell helium and hydrogen burning continues helium and hydrogen burning continues

88 Planetary nebulaPlanetary nebula Star enters the planetary nebula stageStar enters the planetary nebula stage

99 White dwarfWhite dwarf All thermonuclear reactions stop; slow All thermonuclear reactions stop; slow coolingcooling


End of Main SequenceEnd of Main Sequence


Post main-sequence for a solar mass star


Population II StarsPopulation II Stars

First stars to be formed in UniverseFirst stars to be formed in Universe Have only 0.01% heavy elementsHave only 0.01% heavy elements Typically found in galactic bulge and globular Typically found in galactic bulge and globular

clustersclusters Similar sequence of evolution but occupy Similar sequence of evolution but occupy

different region of H-R diagram during core different region of H-R diagram during core He burningHe burning

Significant temperature changes, heating and Significant temperature changes, heating and then coolingthen cooling


Late in the life of a solar mass star


Red Giant > PN Red Giant > PN


Evolutionary phases of a solar mass star, post main-sequence

H-R positionH-R position StageStage Physical processesPhysical processes

33 ZAMSZAMS Core hydrogen burning beginsCore hydrogen burning begins

44 Evolution on main-Evolution on main-sequencesequence

Core hydrogen burning ceases; shell Core hydrogen burning ceases; shell hydrogen burning beginshydrogen burning begins

55 Evolution off main-Evolution off main-sequencesequence

Shell hydrogen burning continues; Shell hydrogen burning continues; convection dominates energy transportconvection dominates energy transport

66 Red giantRed giant Helium flash occurs; core helium burning Helium flash occurs; core helium burning beginsbegins

77 SubgiantSubgiant Core helium burning continues along with Core helium burning continues along with shell hydrogen burningshell hydrogen burning

Red giant againRed giant again Thermonuclear reactions then end; shell Thermonuclear reactions then end; shell helium and hydrogen burning continues helium and hydrogen burning continues

88 Planetary nebulaPlanetary nebula Star enters the planetary nebula stageStar enters the planetary nebula stage

99 White dwarfWhite dwarf All thermonuclear reactions stop; slow All thermonuclear reactions stop; slow coolingcooling


Late in the life of a solar mass star


PN > White DwarfPN > White Dwarf


White DwarfsWhite Dwarfs


For a perfect gas: nkTP TP From hydrostatic equilibrium:

31

1

MR Greater mass, smaller radius

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): 35

KP Constant

Chandrasekhar LimitChandrasekhar Limit


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

From :

R 7 10-3RSun

= 3 109kg m-3

424 TRL Sun 3 10-3LSun


Massive StarsMassive Stars

Stars with masses > 7 MStars with masses > 7 M

Masses greater than ~ 50 MMasses greater than ~ 50 M

– Affected by mass loss (i.e. winds)Affected by mass loss (i.e. winds)– As mass of star changes so does the As mass of star changes so does the

structure and luminositystructure and luminosity


StageStage Physical processesPhysical processes

ProtostarProtostar Dust and gas cloud collapses rapidly, Dust and gas cloud collapses rapidly, accompanied by heating of the interior and accompanied by heating of the interior and ionisation of atomsionisation of atoms

PMSPMS Semihydrostatic equilibrium; contraction and Semihydrostatic equilibrium; contraction and heating continueheating continue

ZAMSZAMS Hydrogen burning commencesHydrogen burning commences

Initial evolution on the main Initial evolution on the main sequence sequence

Hydrogen consumed in the core; some Hydrogen consumed in the core; some contraction occurscontraction occurs

Evolution off the main sequenceEvolution off the main sequence Hydrogen depleted in the core, isothermal helium Hydrogen depleted in the core, isothermal helium core and hydrogen-burning establishedcore and hydrogen-burning established

Evolution to the right in the H-R Evolution to the right in the H-R diagramdiagram

Core rapidly contracts, envelope expands, Core rapidly contracts, envelope expands, hydrogen-burning shell narrowshydrogen-burning shell narrows

Red giantRed giant Energy output increases, convective envelope Energy output increases, convective envelope forms, helium burning beginsforms, helium burning begins

CepheidCepheid Convective shell contracts, core helium burning Convective shell contracts, core helium burning becomes the major energy sourcebecomes the major energy source

SupergiantSupergiant Helium-burning shell formsHelium-burning shell forms

Evolutionary phases of a massive star


SupernovaeSupernovae

Absolute magnitudes from –16 to –20 Absolute magnitudes from –16 to –20 (energy ~10(energy ~104444J)J)– e.g. China, SN of 1054 reached me.g. China, SN of 1054 reached mVV=-6 =-6

(remnant is Crab Nebula)(remnant is Crab Nebula) Two types… Type I & Type IITwo types… Type I & Type II Both types eject a large fraction of Both types eject a large fraction of

original mass with v~5000-10000 km soriginal mass with v~5000-10000 km s-1-1

Explosion of stellar Explosion of stellar interiorinterior


Type II SupernovaeType II Supernovae

Seen in spiral galaxies only, especially in Seen in spiral galaxies only, especially in spiral arms… Population I starsspiral arms… Population I stars

Explosions in cores of Blue/Red Supergiants Explosions in cores of Blue/Red Supergiants (10-100M(10-100M))

Implosion of stellar core to form neutron starImplosion of stellar core to form neutron star– Core reaches density > electron pressureCore reaches density > electron pressure

Violent rebound > explosion > ejects outer Violent rebound > explosion > ejects outer layerslayers


Type II SupernovaeType II Supernovae


SN 1987ASN 1987A


Type I SupernovaeType I Supernovae

Seen in both elliptical and spiral Seen in both elliptical and spiral galaxies… Population II starsgalaxies… Population II stars

Progenitors are H-deficient, highly Progenitors are H-deficient, highly evolved starsevolved stars

Mechanism not well understoodMechanism not well understood– Accretion onto a WD increasing MAccretion onto a WD increasing MWDWD > >

Chandrasekhar limitChandrasekhar limit– Merger of two WDs to give M > 1.4MMerger of two WDs to give M > 1.4M


Type Ia SupernovaeType Ia Supernovae


Supernovae: Key PointsSupernovae: Key Points

SN responsible for nucleosynthesis of SN responsible for nucleosynthesis of element above element above 5656FeFe

Remnant neutron stars… sometimes Remnant neutron stars… sometimes revealed as pulsarsrevealed as pulsars

Shockwave heating of interstellar Shockwave heating of interstellar medium… medium… Supernova RemnantsSupernova Remnants


Supernova RemnantsSupernova RemnantsVelaVela

Crab NebulaCrab Nebula


Supernova RemnantsSupernova Remnants

Cassiopiea ACassiopiea A


Supernova expansion


Schematic H-R diagram showing the spectral classification of stars


H-R diagram for stars near the Sun


H-R diagram from Hipparcos data


Cluster H-R DiagramsCluster H-R Diagrams

In a cluster, compared to evolutionary In a cluster, compared to evolutionary timescales, the stars are all (roughly) timescales, the stars are all (roughly) the same agethe same age

H-R Diagram can reveal the age of the H-R Diagram can reveal the age of the clustercluster

Need to identify the “turn-off”, mass Need to identify the “turn-off”, mass above which all stars have evolved above which all stars have evolved away from the main sequenceaway from the main sequence


H-R diagram showing open cluster (pop I) ages


Globular ClustersGlobular Clusters


Globular Cluster AgesGlobular Cluster Ages

Population II stars, few heavy elementsPopulation II stars, few heavy elements Older than open clustersOlder than open clusters Also have different tracks due to Also have different tracks due to

composition differencescomposition differences


H-R diagram for a globular cluster



Star formationStar formation Main sequenceMain sequence Stellar clusters (open, globular)Stellar clusters (open, globular) Population I & II starsPopulation I & II stars Red GiantsRed Giants Planetary NebulaePlanetary Nebulae White DwarfsWhite Dwarfs SupernovaeSupernovae Neutron StarsNeutron Stars



Unit 5Unit 5



Stellar PhysicsStellar Physics

Unit 1 - Observational properties of Unit 1 - Observational properties of starsstars

Unit 2 - Stellar SpectraUnit 2 - Stellar Spectra Unit 3 - The SunUnit 3 - The Sun Unit 4 - Stellar StructureUnit 4 - Stellar Structure Unit 5 - Stellar EvolutionUnit 5 - Stellar Evolution Unit 6 - Stars of particular interestUnit 6 - Stars of particular interest



STELLAR PHYSICSSTELLAR PHYSICS

Option 2607Option 2607


Mass-radius relationship for white dwarfs

Marked is the best fitting mass and radius for V471 Tau, with 1 and 2 sigma uncertainty contours


DA Hydrogen dominated Non-DA Helium dominated

PROGENITORS – SdB, SdOB, SdO, H-rich

PNN?

Hottest DO stars

DO pulsationsDO cooling

sequenceCoolest DO

stars

No known

PROGENITORS – He-rich SdO

and PNNLate helium thermal pulse

Settling of He and CNO

Dredge up of helium

Hottest DA stars

DA cooling sequence

DA pulsations

DO or DBs

DB pulsations

DB cooling sequence

70,000K

13,000K

10,000K

30,000K

45,000K

150,000K


White Dwarf CoolingWhite Dwarf Cooling

LIFECYCLES OF STARS

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