February 21, 2006 Astronomy 2010 1 Chapter 21: Stars: From Adolescence to Old Age.

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Chapter 21: Stars: From Chapter 21: Stars: From Adolescence to Old AgeAdolescence to Old Age

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Recall: the H-R DiagramRecall: the H-R Diagram

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Mass Determines Life StagesMass Determines Life StagesMass determines stages stars go through and how Mass determines stages stars go through and how long they last in each stage long they last in each stage

with just little bit of dependence on compositionwith just little bit of dependence on composition

Massive stars evolve faster than small starsMassive stars evolve faster than small stars

Relationship between the luminosity and mass Relationship between the luminosity and mass determined by how compressed gases behave.determined by how compressed gases behave.

Small increase in mass produces a large increase Small increase in mass produces a large increase in the luminosity of a star.in the luminosity of a star.

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Lifetime vs. MassLifetime vs. Mass

star mass (solar masses)

time (years) Spectral type

60 3 million O3

30 11 million O7

10 32 million B4

3 370 million A5

1.5 3 billion F5

1 10 billion G2 (Sun)

0.1 1000's billions M7

main sequence: more mass main sequence: more mass hotter and shorter life hotter and shorter life

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Old Age:Old Age: Main Sequence to Main Sequence to Red GiantRed GiantStage 5Stage 5: Red Giant : Red Giant collapse:collapse: fusion stops when the hydrogen in the core runs out fusion stops when the hydrogen in the core runs outshell burningshell burning: hydrogen shell surrounding the core ignites: hydrogen shell surrounding the core ignitesstar expands and becomes a subgiant, then a star expands and becomes a subgiant, then a red giantred giant

Stage 6Stage 6: Helium Fusion : Helium Fusion helium fusion begins in the corehelium fusion begins in the corestar passes through a star passes through a yellow giantyellow giant phase phaseequilibrates as a equilibrates as a red giantred giant or or supergiantsupergiant

Stage 7Stage 7: Stellar Nucleosynthesis – : Stellar Nucleosynthesis – fusion of heavier elements fusion of heavier elements (up to iron)(up to iron)

core fuel in stage 6 runs out and collapse resumes core fuel in stage 6 runs out and collapse resumes fusion of heavier elements may ignite if star is sufficiently fusion of heavier elements may ignite if star is sufficiently massivemassive

Stage 5, part 1: Stage 5, part 1: CollapseCollapse

main sequence: inward gravity balanced by the main sequence: inward gravity balanced by the outward pressure outward pressure

pressure due to fusion in corepressure due to fusion in core

hydrogen in the core eventually converted to hydrogen in the core eventually converted to heliumhelium

nuclear reactions stop! nuclear reactions stop! gravity takes over and the core shrinksgravity takes over and the core shrinksoutside layers also collapseoutside layers also collapselayers closer to the center collapse faster than layers closer to the center collapse faster than those near the surface. those near the surface. As the layers collapses, the gas compresses and As the layers collapses, the gas compresses and heats upheats up

Stage 5, part 2:Stage 5, part 2: Shell BurningShell Burning shell layershell layer outside the core becomes hot and outside the core becomes hot and dense enough for fusion to startdense enough for fusion to startfusion in the layer just outside the core is fusion in the layer just outside the core is called called shell burningshell burningshell fusion is very rapid because the shell shell fusion is very rapid because the shell layer is still compressing and increasing in layer is still compressing and increasing in temperaturetemperatureluminosity of the star increases from its main luminosity of the star increases from its main sequence valuesequence valueGas surrounding the core puffs outward under Gas surrounding the core puffs outward under the action of the extra outward pressurethe action of the extra outward pressureThe star expands and becomes a The star expands and becomes a subgiantsubgiant and then a and then a red giantred giant..

surface has a red color because star is surface has a red color because star is puffed out and cooler puffed out and cooler red giant is very luminous because of its red giant is very luminous because of its hugehuge surface area surface area

time to reach red giant stage

short for big stars•as low as 10 million (107) years

long for little stars•up to 10 billion (1010) years for low mass

stage 5: shell burning stage 5: shell burning red giant red giant

End of Life on Earth …End of Life on Earth …

When the Sun becomes a red giant, it will swallow When the Sun becomes a red giant, it will swallow Mercury,Venus and perhaps the Earth too.Mercury,Venus and perhaps the Earth too.

Or conditions on Earth’s surface will become impossible for life to exist. Or conditions on Earth’s surface will become impossible for life to exist.

Water oceans and atmosphere will evaporate away. Water oceans and atmosphere will evaporate away.

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21.2 Star Clusters21.2 Star Clusters

We saw that stars tend to form in clustersWe saw that stars tend to form in clustersThe stars in the cluster have different masses but The stars in the cluster have different masses but about the same age.about the same age.

The evolution of the stars in a cluster should be The evolution of the stars in a cluster should be consistent with the theory.consistent with the theory.

Three types of clusters:Three types of clusters:Globular clusters -- only contain very old starsGlobular clusters -- only contain very old stars

Open clusters -- contain relatively young starsOpen clusters -- contain relatively young stars

Stellar associations -- small groups of young starsStellar associations -- small groups of young stars

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21.3 Checking Out the Theory21.3 Checking Out the Theory

Comparison of the prediction for the stars of a 3 Comparison of the prediction for the stars of a 3 million year old cluster (left) with measurements of the million year old cluster (left) with measurements of the stars in cluster NGC 2264 (right). stars in cluster NGC 2264 (right).

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An Older ClusterAn Older Cluster

Comparison of the model for a 4.24 billion year old Comparison of the model for a 4.24 billion year old cluster (left) with measurements of stars in 47 cluster (left) with measurements of stars in 47 Tucanae (right). Note the different scales.Tucanae (right). Note the different scales.

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Stage 6: Helium Fusion Stage 6: Helium Fusion

helium flashhelium flash: onset of helium fusion produces a burst of : onset of helium fusion produces a burst of energy energy

reaction rate settles down reaction rate settles down

Fusion in the core releases more energy/second than core Fusion in the core releases more energy/second than core fusion in main sequence fusion in main sequence

star is smaller and hotter, but stable! star is smaller and hotter, but stable!

hydrostatic equilibrium holds until the core fuel runs out. hydrostatic equilibrium holds until the core fuel runs out.

red giant: dead helium core plus red giant: dead helium core plus hydrogen burning shellhydrogen burning shell

gravity plus inward pressure from gravity plus inward pressure from burning shell heats coreburning shell heats core

helium fusion starts at 100 million K helium fusion starts at 100 million K triple alpha process: three triple alpha process: three 44He He 1212CC

stage 6: helium flash stage 6: helium flash yellow giant yellow giant

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Stage 6: Helium Fusion Stage 6: Helium Fusion hydrostatic equilibrium holds until the core fuel runs hydrostatic equilibrium holds until the core fuel runs outoutstar is a yellow/orange giantstar is a yellow/orange giantdead carbon core shrinks under its weightdead carbon core shrinks under its weightgravity gravity pressure and heat pressure and heat heats helium shell surrounding coreheats helium shell surrounding corefusion of hydrogen surrounding helium shellfusion of hydrogen surrounding helium shellstar again puffs out to red giantstar again puffs out to red giantSun-like or smaller stars: terminal stage Sun-like or smaller stars: terminal stage heavier stars: heavier stars:

helium shell flasheshelium shell flashespulsation (as in Cephied variable stars)pulsation (as in Cephied variable stars)heavier elements fuse heavier elements fuse

stage 6: yellow giant stage 6: yellow giant red giant or supergiant red giant or supergiant

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Pulsating StarsPulsating StarsIn ordinary stars hydrostatic equilibrium works to In ordinary stars hydrostatic equilibrium works to dampen (diminish) the pulsations. dampen (diminish) the pulsations. But stars entering and leaving stage 6 can briefly (in But stars entering and leaving stage 6 can briefly (in terms of star lifetimes!) create conditions where the terms of star lifetimes!) create conditions where the pressure and gravity are out of sync and the pressure and gravity are out of sync and the pulsations continue for a time. pulsations continue for a time. Larger, more luminous stars will pulsate with longer Larger, more luminous stars will pulsate with longer periods than the smaller, fainter stars periods than the smaller, fainter stars

because gravity takes longer to pull the more extended outer because gravity takes longer to pull the more extended outer layers of the larger stars back. layers of the larger stars back.

The The period-luminosity relationperiod-luminosity relation can be used to can be used to determine the distances of these luminous stars from determine the distances of these luminous stars from the inverse square law of light brightness. the inverse square law of light brightness.

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Upper main-Upper main-sequence starsequence star

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Stage 7: Red Giant or Supergiant Stage 7: Red Giant or Supergiant

When the core fuel runs out When the core fuel runs out again, the core resumes its again, the core resumes its collapse. collapse. If the star is massive enough, it If the star is massive enough, it will repeat stage 5. will repeat stage 5. The number of times a star can The number of times a star can cycle through stages 5 to 7 cycle through stages 5 to 7 depends on the mass of the depends on the mass of the star. star. Each time through the cycle, Each time through the cycle, the star creates new heavier the star creates new heavier elements from the ash of fusion elements from the ash of fusion reactions in the previous cycle. reactions in the previous cycle.

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Stellar NucleosynthesisStellar NucleosynthesisFusion creates heavier elements from lighter elements Fusion creates heavier elements from lighter elements

very massive stars produce elements up to iron in the very massive stars produce elements up to iron in the corecore

nuclear fusion releases energy for elements lighter than iron nuclear fusion releases energy for elements lighter than iron

past iron, fusion past iron, fusion absorbsabsorbs energy energy stars like our Sun produce elements up to carbon and stars like our Sun produce elements up to carbon and oxygenoxygenheavier elements produced in supernova explosions of heavier elements produced in supernova explosions of very massive starsvery massive stars

density gets so great that protons and electrons are density gets so great that protons and electrons are combined to form neutrons (+ neutrinos)combined to form neutrons (+ neutrinos)outer layers are ejected in a huge supernova explosionouter layers are ejected in a huge supernova explosionelements heavier than iron are formed and ejectedelements heavier than iron are formed and ejected

red supergiantcore radius earth-sizedcore radius earth-sized

heavy element fusion in shellsheavy element fusion in shells

envelope 5 AUenvelope 5 AU

Betelgeuse

Stage 5 begins: collapse of the star generates heat

hydrogen fuel in core runs out

post-main post-main sequence evolution sequence evolution of a 5 solar mass of a 5 solar mass starstar

Stage 5 continues: star collapses further as shell burning begins

Star adjusts as collapse continues•further collapse of core•exterior of star cools

Stage 5-6: Young Red Giant forms, Shell Burning starts

Shell burning starts•outer layers of star expand•core continues to contract

Stage 6: Core Burning drives further expansion

•strong new heat source from helium fusion•hotter more yellow

Stage 6-7: Red Giant matures

mature red giant region (AGB)

young red giant region (RGB)

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Planetary NebulaPlanetary Nebula

Planetary nebula got their name because some Planetary nebula got their name because some looked like round, green planets in early looked like round, green planets in early telescopes.telescopes.

Now known to be formed when old, low-mass Now known to be formed when old, low-mass stars are unable to fuse heavier elements, and stars are unable to fuse heavier elements, and their cores collapse.their cores collapse.

The outer layer of the star is ejected by wind.The outer layer of the star is ejected by wind.

About one or more light years across About one or more light years across much larger than our solar system!much larger than our solar system!

The Helix NebulaThe Helix Nebula

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