The Lives of Stars

The Lives of Stars

Chapter 12

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Life on Main-Sequence

• Zero-Age Main Sequence (ZAMS) – main sequence location where stars are born

• Bottom/left edge of main sequence• H fusion begins

• As star ages– Energy source is H fusion

• composition changes• H -> He

– Location in H-R diagram slowly changes• begins to move away (right/up) from ZAMS• broadens (smears out) main sequence

Stellar Lifetimes• 90% of star’s life spent in main sequence• Lifetime depends on mass

Main Sequence to Red Giant

• H in core used up– He “ash” in core– no more fuel for energy

• Gravity begins to win – core contracts, gets hotter– start H fusion in shell

surrounding core– outer layers expand

• Star becomes Red Giant


Further Evolution

• As core contracts– temperature increases– becomes hot enough– Begins to fuse

He into C• Energy production

– stops core collapse– star is stable again

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Beginning of the End

• When He exhausted, star out of fuel again– core collapse resumes– He shell burning begins

• outer layers expand

• Star becomes Red Supergiant– strong mass loss occurs via stellar wind

Stellar Evolution

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Evolution of Massive Stars• Up to C-O core, evolution same for all stars• From then on, different paths• Low-mass stars:

– no C burning– core energy generation complete– star dies

• High-mass stars:– C burning begins in core– Eventually fuse heavier

and heavier elements


Making Heavy Elements• High-mass stars fuse heavier elements in cores

C -> Ne O -> Si -> Fe– at each step, core collapses further

• This nucleosynthesis produces most elements up to iron (Fe)


Evolutionary Tracks

Star Clusters

• Star clusters – Stars born same location, same time– contains stars with different masses – permits study of stellar evolution

• Age of cluster – determined by which stars have departed main

sequence

Globular Clusters

• spherical “ball” of stars– concentrated toward center– 10,000 - 100,000 stars

• about 150 around our Galaxy– very distant from Sun

(>10,000 LY)– sizes 50-300 LY diameter


Open Clusters

• 100’s of stars (up to 1000)– smaller than Globular clusters – no central concentration

• Found within the Galaxy– 1000’s known– diameters < 30 LY



H-R Diagrams of Clusters

• Cluster ages are different– globular clusters oldest– open clusters relatively young

• H-R diagrams indicate age– interpret using stellar evolution

theory

Cluster Evolution

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Estimating Cluster Ages

• Make H-R diagram for cluster• Have all stars arrived at ZAMS?

– if not, cluster extremely young• Have some stars departed Main Sequence?

– cluster is older– main sequence turn-off point

• determines cluster age• the farther down the turn-off, the older the

cluster


Theoretical NGC 2264

Theoretical 47 Tuc

Ages of Clusters

• Globular clusters– only lowest part of main sequence is present– typical age: 15 billion yrs

• Open clusters– much younger than globulars– all ages: 1 million yrs up to a few billion yrs

Stellar Death

Death of Stars

• Two possibilities– Low mass stars < 5 Msun

• end is planetary nebula WHITE DWARF– High mass stars

• end is type II supernova either:NEUTRON

STAR or BLACK HOLE

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Fate of Low Mass Stars

• During end of red supergiant phase– large mass loss

• star loses entire envelope, revealing core– core becomes white dwarf

• white dwarf slowly cools• eventually becomes “black dwarf”

Planetary Nebulae

• During transition to white dwarf– outer layers expanding– exposes hot core; – shell material heated; begins to

glow• Result is a planetary nebula

– tens of thousands known in our Galaxy



Planetary Nebulae

Evolution to White Dwarf• Low mass stars

– cannot fuse carbon– lose energy source

• gravity wins• core contracts

• Core contraction– produces very high density– electron degeneracy pressure

• stops core collapse– remnant core becomes white dwarf

White Dwarf Stars

• Properties – diameter ~ same as Earth– very dense

(1 tsp = several tons!)– very hot on surface

• Chandrasekhar limit– Maximum mass = 1.4 Msun– larger stars collapse

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“Diamond stars”??

Fate of Massive Stars• High-mass stars fuse heavier elements in cores

C -> Ne O -> Si -> Fe– at each step, core shrinks further– fusion stops when iron (Fe) produced

• This nucleosynthesis produces most elements up to iron


Collapse and Explosion

• When core mass exceeds 1.4 Msun– collapse continues unabated– all protons converted into neutrons– collapse abruptly halted by neutron

degeneracy pressure – results in shock wave & explosion– Produces type II supernova

• Some material falls onto core– M < 2.5 Msun neutron star remains– M > 2.5 Msun black hole produced

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Supernovae

• Supernovae as bright as entire galaxy • Ejection velocities

– millions of miles/hr (~10,000 km/s)• Supernova explosion

– heavy elements (C, N, O, Fe) returned to interstellar medium for recycling

– also produces elements heavier than iron• elements such as gold, silver, uranium



Pulsars• Pulsating radio sources

– Periods .001-10 seconds– Very regular– also observed in optical (crab nebula)

• Pulsars = spinning neutron stars– fast period requires very small objects– neutron stars only possibility

• Radiation and particles beamed out from magnetic poles– spinning lighthouse effect results in

observed “pulses”



Novae

• Novae NOT same as Supernova– less energetic; not as bright

• Binary system with mass transfer onto WD– material accumulates on WD surface– eventually nuclear detonation occurs– result is a nova


White Dwarf Supernovae• As mass accumulates, WD exceeds Chandrasekhar limit

– rapid core collapse occurs– Resulting explosion = Type I supernova

• Properties somewhat different than Type II SN (caused by massive star explosions)


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The Lives of Stars