The Lives of Stars Chapter 12

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

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

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 stars about 150 around our Galaxy –very distant from Sun (>10,000 LY) –sizes 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

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

TheoreticalNGC 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 {

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 “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

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 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)