Our Place Our Place in in the the Cosmos Cosmos The Sun, a - PDF document

Stellar Evolution • We saw in the last lecture that all main sequence stars generate their energy by fusing hydrogen into helium in their cores Our Place Our Place in in the the Cosmos Cosmos • The Sun, a typical main sequence star, fuses over 4 billion kg of hydrogen to helium each second Lecture 12 • Eventually (in about 5 billion years time) that Stellar Evolution hydrogen will be exhausted, and the Sun will evolve off the main sequence • The same will happen to all main sequence stars Stellar Evolution Main Sequence Lifetime • Just as mass determines the other properties • If one were to add mass to the Sun, the of a main sequence star (luminosity, size, extra weight of material pushing down would temperature), the eventual fate of a star is compress its core, driving up temperature and also pre-ordained by its mass, which is locked density in place when the star forms • Nuclear reaction rate would increase as nuclei • Chemical composition also plays a secondary collide more frequently and with higher role in a star’s properties and fate energy • Each star is thus unique - minor differences in • A modest increase in pressure can lead to a mass and composition can result in significant dramatic increase in energy production and differences in fate hence luminosity • Low and high mass stars evolve differently Main Sequence Lifetime Main Sequence Lifetime • This is why main sequence is primarily a • The length of time a star can continue fusing hydrogen to helium, its main sequence lifetime, sequence of masses depends on • More mass � stronger gravity � higher • The amount of hydrogen available temperature and pressure � faster • The rate at which hydrogen is fused into helium nuclear reaction rates � higher • More massive stars contain more fuel, but the luminosity rate at which the fuel is used up, as measured by the luminosity, is also higher • Hence more massive stars lie further up and to the left on the main sequence

Main Sequence Lifetime Main Sequence Lifetime • The main sequence lifetime is equal to the • Luminosity increases very amount of fuel available (proportional to mass) rapidly with mass divided by rate at which fuel is used (proportional to luminosity) • The Sun has an estimated MS lifetime of ten billion years • The most massive stars (about 60 times more massive than the Sun) have luminosities • For other MS stars, lifetime is given by 794,000 times greater and lifetimes below 1 million years • In general, more massive stars are shorter- lived than less massive stars Helium “ash” Main Sequence Evolution accumulates most rapidly in the centre of a star where reaction • As the hydrogen within a main sequence star is fused rates are highest into helium, its composition gradually changes and it evolves slowly up the main sequence, gaining in Sun was initially 30% luminosity helium throughout • The Sun’s luminosity will roughly double between first joining and leaving the main sequence Today, the Sun’s centre • Helium cannot be fused into heavier nuclei in a main consists of 65% helium, sequence star due to four times stronger 35% hydrogen electrostatic repulsion between helium nuclei compared with hydrogen nuclei In about 5 billion years • Helium simply accumulates within the core of a star, like the ash in a fireplace time no hydrogen will remain at the centre Farewell Main Sequence Degenerate Helium Core • Once all hydrogen is exhausted in the core of • No hydrogen burning � lower pressure � a star, no more energy is generated and the gravity wins star is no longer in equilibrium and is no • Core becomes extremely dense until it longer on the main sequence becomes electron degenerate (around 1000 kg • How a star subsequently evolves depends on per cubic centimetre) its mass • Although core is “dead”, hydrogen can still • The rest of this lecture will discuss the burn in a shell around the core - shell burning evolution of low mass stars, those less massive • As shell deposits more helium “ash” on the than about 3 M � core the core shrinks due to extra weight of • The evolution of massive stars will be material discussed in the next lecture

Post-Main Sequence Subgiant Stage Evolution • Just outside helium core the gravitational • As the degenerate core grows and the shell- acceleration is given by g core = G M core / r 2core burning rate increases, the surrounding layers • As more helium “ash” is deposited onto core it of the star are heated and expand both increases in mass M core and decreases in • As the star’s surface expands it becomes radius r core more luminous but also cooler • Both cause gravity at core’s surface to • The star becomes a subgiant - luminous but increase red in colour • Stronger gravity increases pressure within hydrogen-burning shell thus increasing shell • The star moves above and to the right of the burning rate and hence luminosity of star main sequence on the H-R diagram until its • As a star runs out of nuclear fuel it becomes surface temperature has dropped by about more luminous! 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, Helium Burning shell burning increases and the outer parts of the star swell up • 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 Asymptotic Giant Branch Star • As helium is depleted in the core, gravity • The star is now stably burning helium to again wins over pressure and degenerate carbon in a non-degenerate core and burning carbon core is surrounded by helium and hydrogen to helium in a surrounding shell hydrogen burning shells • Stars in this phase have a narrow range of • As before, star grows larger and redder, luminosities, about one hundredth of their following a path close to the red giant branch luminosity at the time of the helium flash, known as the asymptotic giant branch (AGB) but still much more luminous than their main • Core temperatures are too low for carbon sequence stage fusion and AGB star starts to lose outer envelope • They are know as horizontal branch stars from their locations on the H-R diagram, • This is heated by hot degenerate core and glows as a spectacular planetary nebula where they remain for about 50 million years

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