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Our Our Place Place in in the the Cosmos Cosmos

Lecture 12 Stellar Evolution

Stellar Evolution

• We saw in the last lecture that all main

sequence stars generate their energy by fusing hydrogen into helium in their cores

• The Sun, a typical main sequence star, fuses
• ver 4 billion kg of hydrogen to helium each

second

• Eventually (in about 5 billion years time) that

hydrogen will be exhausted, and the Sun will evolve off the main sequence

• The same will happen to all main sequence

stars

Stellar Evolution

• Just as mass determines the other properties
• f a main sequence star (luminosity, size,

temperature), the eventual fate of a star is also pre-ordained by its mass, which is locked in place when the star forms

• Chemical composition also plays a secondary

role in a star’s properties and fate

• Each star is thus unique - minor differences in

mass and composition can result in significant differences in fate

• Low and high mass stars evolve differently

Main Sequence Lifetime

• If one were to add mass to the Sun, the

extra weight of material pushing down would compress its core, driving up temperature and density

• Nuclear reaction rate would increase as nuclei

collide more frequently and with higher energy

• A modest increase in pressure can lead to a

dramatic increase in energy production and hence luminosity

Main Sequence Lifetime

• This is why main sequence is primarily a

sequence of masses

• More mass stronger gravity higher

temperature and pressure faster nuclear reaction rates higher luminosity

• Hence more massive stars lie further up

and to the left on the main sequence

Main Sequence Lifetime

• The length of time a star can continue fusing

hydrogen to helium, its main sequence lifetime, depends on

• The amount of hydrogen available
• The rate at which hydrogen is fused into helium
• More massive stars contain more fuel, but the

rate at which the fuel is used up, as measured by the luminosity, is also higher

Main Sequence Lifetime

• The main sequence lifetime is equal to the

amount of fuel available (proportional to mass) divided by rate at which fuel is used (proportional to luminosity)

• The Sun has an estimated MS lifetime of ten

billion years

• For other MS stars, lifetime is given by

Main Sequence Lifetime

• Luminosity increases very

rapidly with mass

• The most massive stars (about 60 times more

massive than the Sun) have luminosities 794,000 times greater and lifetimes below 1 million years

• In general, more massive stars are shorter-

lived than less massive stars

Main Sequence Evolution

• As the hydrogen within a main sequence star is fused

into helium, its composition gradually changes and it evolves slowly up the main sequence, gaining in luminosity

• The Sun’s luminosity will roughly double between first

joining and leaving the main sequence

• Helium cannot be fused into heavier nuclei in a main

sequence star due to four times stronger electrostatic repulsion between helium nuclei compared with hydrogen nuclei

• Helium simply accumulates within the core of a star,

like the ash in a fireplace Helium “ash” accumulates most rapidly in the centre of a star where reaction rates are highest Sun was initially 30% helium throughout Today, the Sun’s centre consists of 65% helium, 35% hydrogen In about 5 billion years time no hydrogen will remain at the centre

Farewell Main Sequence

• Once all hydrogen is exhausted in the core of

a star, no more energy is generated and the star is no longer in equilibrium and is no longer on the main sequence

• How a star subsequently evolves depends on

its mass

• The rest of this lecture will discuss the

evolution of low mass stars, those less massive than about 3 M

• The evolution of massive stars will be

discussed in the next lecture

Degenerate Helium Core

• No hydrogen burning lower pressure

gravity wins

• Core becomes extremely dense until it

becomes electron degenerate (around 1000 kg per cubic centimetre)

• Although core is “dead”, hydrogen can still

burn in a shell around the core - shell burning

• As shell deposits more helium “ash” on the

core the core shrinks due to extra weight of material

Post-Main Sequence Evolution

• Just outside helium core the gravitational

acceleration is given by gcore = G Mcore/r2core

• As more helium “ash” is deposited onto core it

both increases in mass Mcore and decreases in radius rcore

• Both cause gravity at core’s surface to

increase

• Stronger gravity increases pressure within

hydrogen-burning shell thus increasing shell burning rate and hence luminosity of star

• As a star runs out of nuclear fuel it becomes

more luminous!

Subgiant Stage

• As the degenerate core grows and the shell-

burning rate increases, the surrounding layers

• f the star are heated and expand
• As the star’s surface expands it becomes

more luminous but also cooler

• The star becomes a subgiant - luminous but

red in colour

• The star moves above and to the right of the

main sequence on the H-R diagram until its surface temperature has dropped by about 1000K

Red Giant

• At this point the formation of H- ions

(hydrogen atoms with 2 electrons) regulates how much radiation can escape - just as greenhouse gases in the Earth’s atmosphere trap in heat

• The star is now a red giant - it can no

longer cool and moves almost vertically up the red giant branch on the H-R diagram as it grows larger and more luminous but remains at the same surface temperature

Giant Evolution

• It takes around 200 million years for a star

like the Sun to evolve from the main sequence to the top of the red giant branch, starting slowly, then evolving at a rapidly increasing rate

• In the first 100 million years the star’s

luminosity increases to about 10 L to become a subgiant

• In the remaining 100 million years the

luminosity skyrockets to almost 1000 L

• As mass accumulates on core hydrogen shell

burning rate increases (positive feedback)

As core gains mass and shrinks, shell burning increases and the

• uter parts of the star swell

up

Helium Burning

• As core shrinks it becomes hotter
• Eventually the helium nuclei are moving

energetically enough to combine together to form carbon

Helium Flash

• The degenerate core behaves more like a solid than a

gas so heat from helium burning rapidly spreads through core with no accompanying expansion

• Rapidly rising temperature within core leads to a

chain reaction known as the helium flash

• Within just a few seconds the core explodes
• The energy released lifts the outer layers and core is

no longer degenerate

• The expanded core now has a much weaker surface

gravity lower pressure slower nuclear reactions lower luminosity

Horizontal Branch Star

• The star is now stably burning helium to

carbon in a non-degenerate core and burning hydrogen to helium in a surrounding shell

• Stars in this phase have a narrow range of

luminosities, about one hundredth of their luminosity at the time of the helium flash, but still much more luminous than their main sequence stage

• They are know as horizontal branch stars

from their locations on the H-R diagram, where they remain for about 50 million years

Asymptotic Giant Branch Star

• As helium is depleted in the core, gravity

again wins over pressure and degenerate carbon core is surrounded by helium and hydrogen burning shells

• As before, star grows larger and redder,

following a path close to the red giant branch known as the asymptotic giant branch (AGB)

• Core temperatures are too low for carbon

fusion and AGB star starts to lose outer envelope

• This is heated by hot degenerate core and

glows as a spectacular planetary nebula

Planetary nebulae are so-called because they look like planets when seen through small telescopes HST reveals their beautiful structure