12 April 2005 AST 2010: Chapter 21 1

Stars:Stars: From Adolescence From Adolescence

to Old Ageto Old Age

12 April 2005 AST 2010: Chapter 21 3

Mass Determines Life Stages• Mass determines stages stars go through and

how long they last in each stage – with just little bit of dependence on composition

• Massive stars evolve faster than small stars– Relationship between the luminosity and mass

determined by how compressed gases behave– Small increase in mass produces a large increase

in the luminosity of a star

12 April 2005 AST 2010: Chapter 21 4

Main Sequence: Lifetime vs. Mass

12 April 2005 AST 2010: Chapter 21 5

Old Age: Main Sequence to Red Giant

• Stage 5: Red Giant – collapse: fusion stops when the hydrogen in the core runs

out– shell burning: hydrogen shell surrounding the core ignites– star expands and becomes a subgiant, then a red giant

• Stage 6: Helium Fusion – helium fusion begins in the core– star passes through a yellow giant phase– equilibrates as a red giant or supergiant

• Stage 7: Stellar Nucleosynthesis – fusion of heavier elements (up to iron)– core fuel in stage 6 runs out and collapse resumes – fusion of heavier elements may ignite if star is sufficiently

massive

Stage 5, Part 1: Collapse• main sequence: inward gravity balanced

by the outward pressure – pressure due to fusion in core

• hydrogen in the core eventually converted to helium

nuclear reactions stop! • gravity takes over and the core shrinks• outside layers also collapse• layers closer to the center collapse

faster than those near the surface.• As the layers collapses, the gas

compresses and heats up

Stage 5, Part 2: Shell Burning • shell layer outside the core becomes

hot and dense enough for fusion to start• fusion in the layer just outside the core

is called shell burning• shell fusion is very rapid because the

shell layer is still compressing and increasing in temperature

• luminosity of the star increases from its main sequence value

• Gas surrounding the core puffs outward under the action of the extra outward pressure

• The star expands and becomes a subgiant and then a red giant– surface has a red color because star

is puffed out and cooler – red giant is very luminous because of

its huge surface area

time to reach main red giant stage

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

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

Stage 5: Shell Burning Red Giant

End of Life on Earth …

• When the Sun becomes a red giant, it will swallow Mercury,Venus and perhaps the Earth too.– Or conditions on Earth’s surface will become impossible for

life to exist. – Water oceans and atmosphere will evaporate away.

12 April 2005 AST 2010: Chapter 21 13

Star Clusters• We saw that stars tend to form in clusters

– The stars in the cluster have different masses but about the same age

– The different stars in a cluster provide a test for theories of stellar evolution

• Three types of clusters:– Globular clusters -- only contain very old stars– Open clusters -- contain relatively young stars– Stellar associations -- small groups of young stars

12 April 2005 AST 2010: Chapter 21 14

Testing the Theory: Relatively Young Stars

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

12 April 2005 AST 2010: Chapter 21 15

Testing the Theory: An Older Cluster

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

12 April 2005 AST 2010: Chapter 21 16

Stage 6: Helium Fusion

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

• reaction rate settles down • Fusion in the core releases more energy/second than

core fusion in main sequence – star is smaller and hotter, but stable! – hydrostatic equilibrium holds until the core fuel runs out

red giant: dead helium core plus hydrogen burning shellgravity plus inward pressure from burning shell heats corehelium fusion starts at 100 million K

triple alpha process: three 4He 12C

stage 6: helium flash yellow giant

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

12 April 2005 AST 2010: Chapter 21 19

Stage 6: Helium Fusion • hydrostatic equilibrium holds until the core fuel runs

out• star is a yellow/orange giant• dead carbon core shrinks under its weight• gravity pressure and heat • heats helium shell surrounding core• fusion of hydrogen surrounding helium shell• star again puffs out to red giant• Sun-like or smaller stars: terminal stage • heavier stars:

– helium shell flashes– pulsation (as in Cephied variable stars)– heavier elements fuse

stage 6: yellow giant red giant or supergiant

12 April 2005 AST 2010: Chapter 21 21

Pulsating Stars• In ordinary stars hydrostatic equilibrium works to

dampen (diminish) the pulsations• But stars entering and leaving stage 6 can briefly (in

terms of star lifetimes!) create conditions where the pressure and gravity are out of sync and the pulsations continue for a time

• Larger, more luminous stars will pulsate with longer periods than the smaller, fainter stars – because gravity takes longer to pull the more extended

outer layers of the larger stars back

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

Upper Main-Sequence Stars

12 April 2005 AST 2010: Chapter 21 25

Stage 7: Red Giant or Supergiant • When core fuel runs out

again, the core resumes its collapse

• If the star is massive enough, it will repeat stage 5

• The number of times a star can cycle through stages 5 to 7 depends on the mass of the star

• Each time through the cycle, the star creates new heavier elements from the ash of fusion reactions in the previous cycle

Red Supergiant• core radius earth-sized• heavy element fusion in shells• envelope 5 AU

Betelgeuse

12 April 2005 AST 2010: Chapter 21 27

12 April 2005 AST 2010: Chapter 21 28

Planetary Nebula• Planetary nebula got their name

because some looked like round, green planets in early telescopes

• Now known to be formed when old, low-mass stars are unable to fuse heavier elements, and their cores collapse– The outer layer of the star is ejected by

wind• About one or more light years across

– much larger than our solar system!

12 April 2005 AST 2010: Chapter 21 31

12 April 2005 AST 2010: Chapter 21 33

Stellar Nucleosynthesis• Fusion creates heavier elements from lighter

elements • Very massive stars produce elements up to iron in

the core– nuclear fusion releases energy for elements lighter than iron – past iron, fusion absorbs energy

• Stars like our Sun produce elements up to carbon and oxygen

• Heavier elements are produced in supernova explosions of very massive stars– density gets so great that protons and electrons are

combined to form neutrons (+ neutrinos)– outer layers are ejected in a huge supernova explosion– elements heavier than iron are formed and ejected