Stellar Evolution Life After the Main Sequence

1. Stellar EvolutionLife After the Main Sequence © Sierra College Astronomy Department

2. Stellar Evolution - Life After the Main Sequence Why Do Stars Evolve? The Changing of Chemical Composition • As a star uses fuel, it transforms light elements into heavy elements. • There are three processes in a star which depend on chemical composition: • The rate at which a star produces energy • The rate at which energy flows to the surface • The generation of pressure which resists gravitational collapse • These processes are driven by: • Primarily mass • Internal structure a factor as well © Sierra College Astronomy Department

3. Stellar Evolution - Life After the Main Sequence Why Do Stars Evolve? Stellar Nuclear Fusion • The core temperature rises to about 10 million K after the star’s contraction from a molecular core fragment. • Stars of low mass like the Sun (<1.5 M) use the proton-proton chain to generate energy. • Stars of mass greater than 1.5 M have higher core temperatures that allow the CNO cycle to fuse of hydrogen into helium (4H  He). • The CNO cycle is more efficient at the higher core temperatures of these stars • This series of reactions involves hydrogen with carbon, nitrogen, and oxygen as catalysts. • Hydrostatic equilibrium (pressure balances gravity) maintains fusion at a uniform rate. © Sierra College Astronomy Department

4. Stellar Evolution - Life After the Main SequenceStellar Maturity Main Sequence Life (MSL) • Recall: TMSL = 1010 M/L years • An aging main sequence star becomes more luminous and cooler as it attempts to maintain hydrostatic equilibrium. • Stars start their main sequence lives on the left side of the main sequence “strip”, then move up and to the right as they age. • The more massive stars do this the fastest. © Sierra College Astronomy Department

5. Stellar Evolution - Life After the Main SequenceStellar Maturity Towards Star Death • Until their lives end on the main sequence, the main difference between the evolution of stars of various masses is the amount of time they spend as protostars and main sequence stars. • Stars can be grouped by mass as low-mass,intermediate-mass stars, or high-mass depending on how they reach their end state. [However, other groupings are possible.] © Sierra College Astronomy Department

6. Stellar Evolution - Life After the Main SequenceLow-Mass Stars Low-Mass Stars • In stars with a mass of less than about 0.4 M, convection occurs throughout most or all of the volume of the star. • Hydrogen from throughout the star is cycled through the core, and the entire star runs low on hydrogen at the same time. • A low-mass star will take at least 100 billion years to completely burn its hydrogen. © Sierra College Astronomy Department

7. Stellar Evolution - Life After the Main SequenceLow-Mass Stars • Ultimately, low-mass stars will become white dwarfs through gravitational shrinkage. • The hypothetical lifetime of a low-mass star is more than the assumed age of the universe. • Consequently, white dwarfs currently observed must have originated in a different manner. • Eventually, however, low-mass stars will follow the evolutionary sequence of intermediate-mass stars. © Sierra College Astronomy Department

8. Stellar Evolution - Life After the Main SequenceLeaving the Main Sequence Hydrogen Shell Burning • After the core is depleted of hydrogen, the star has no source of nuclear energy and must turn to gravitation energy as a source • Soon a region around the core, still rich in hydrogen begins to ignite and fuse into helium • For stars like the Sun (an intermediate-mass star) this happens at almost the same time the core runs out of hydrogen • For high-mass stars the core must contract for 100,000s to millions of years before shell burning can occur. © Sierra College Astronomy Department

9. Stellar Evolution - Life After the Main SequenceLeaving the Main Sequence Movement of the Hydrogen shell • As time goes on the shell steadily flows outward, leaving behind helium which migrates towards the core. • While the star’s interior continues to contract the surface layers expand and cool turning the star into a red giant. • For stars as massive as the Sun this process may go for 1 billion years. • For a star which has 9 times the mass of the Sun, this process lasts only a million years. • This causes the star to steadily move the right and upward on the H-R diagram. © Sierra College Astronomy Department

10. Stellar Evolution - Life After the Main SequenceDegenerate Matter Pressure Under Extreme Conditions • Under extremely high density conditions, pressure no longer follows the ideal gas law and is produced by electron degeneracy. • Electron degeneracy is a quantum state of a gas in which its electrons are packed as densely as nature permits. • Pressure of such a high-density gas is not dependent on temperature as it is in a “normal” gas. © Sierra College Astronomy Department

11. Stellar Evolution - Life After the Main SequenceHelium as a Fuel Helium Fusion • For stars with a mass less than about 2 Mthe ever growing core will become degenerate. • This prevents further contraction of the core, though the temperature will continue to rise as more helium is dumped onto the core from the hydrogen shell burning. • Eventually, the core reaches 100 million K and the triple alpha process begins to form carbon (and some oxygen) . © Sierra College Astronomy Department

12. Stellar Evolution - Life After the Main SequenceHelium as a Fuel The Helium Flash • At this stage the star in nearly as big a 1 AU and 1000 times more luminous as the Sun • As the helium fuses into carbon the temperature increases. Since the core is degenerate, it does not expand which would normally regulate the fusion rate • Since the temperature continues to rise, the rate of the triple alpha process increases rapidly, and the helium in the core ignites in what is termed thehelium flash, more energy released than needed to hold off core contraction. • The core at this stage is 300 million K. © Sierra College Astronomy Department

13. Stellar Evolution - Life After the Main SequenceHelium as a Fuel More Massive Red Giants • Stars with masses larger than 2 Mare spared the helium flash. • Instead, before the core becomes degenerate, it starts the triple alpha process and steadily burns He into carbon. • Regardless of a star’s mass, stars with helium fusion in the core and hydrogen fusion in an outer shell are referred to as helium-burning stars and they occupy the horizontal branch of the H-R diagram. © Sierra College Astronomy Department

14. Stellar Evolution - Life After the Main SequenceThe Variable Star Phase Horizontal Branch Stars • After helium core burning begins, a stars core returns to an ideal gas and its surface becomes hotter and smaller - settling into moving across the H-R diagram with roughly a constant luminosity passing into a yellow giant stage. • Many of these yellow giants (whether an aging high-mass or low-mass star) swell and shrink rhythmically: they pulsate. • These pulsating yellow giants are located in the instability strip of the H-R diagram. • High-mass pulsating giants are Cepheid variables (periods of about 1-70 days). • Low-mass pulsating giants are RR Lyrae variables(periods of about 12 hours). © Sierra College Astronomy Department

15. Stellar Evolution - Life After the Main SequenceThe Variable Star Phase Pulsation Physics • The cause for the pulsation is a special situation where the yellow giant’s atmosphere can trap some of its radiated energy. • This heats the atmosphere which then expands to a point that the star’s trapped radiation can escape. • This causes the atmosphere to cool, shrink to its original size, and start the process all over again. • The regular pulsation process of variable stars has led to the period-luminosity relation: higher average luminosity leads to longer periods. © Sierra College Astronomy Department

16. Stellar Evolution - Life After the Main SequenceBeyond Helium Burning Exhausting the Helium Fuel in the Core • Eventually, the star will consume all its helium in the the core, leaving behind carbon and oxygen. • Just like in a red giant, the core contracts and becomes degenerate again, but now helium shell burning begins and the star is referred to as a double-shell burning star. • The evolutionary track of a star at this stage is called the Asymptotic Giant Branch (AGB) and the star returns to being a red giant (or a red supergiant in the case of a high-mass star). © Sierra College Astronomy Department

17. Stellar Evolution - Life After the Main SequenceBeyond Helium Burning More Exhaustion on the AGB • Hydrogen shell burning is present and provides most of the energy of the star. • However, once enough helium has built between the shells, it is consumed in a nearly explosive event called a thermal pulse. • The thermal pulse is repetitive and increases the mass of the degenerate carbon/oxygen core. • The pulses dredge up carbon from the core, enriching the surface with carbon and creating a red giant called a carbon star, which creates “carbon smog” that is easily blown into space by the star’s wind providing the largest source of interstellar dust © Sierra College Astronomy Department

18. Stellar Evolution - Life After the Main SequenceApproaching the End Mass Loss In AGB Stars • The solar wind carries away about 10–14 of the Sun’s mass each year. Over the course of 10 billion years, the Sun will lose only 0.01% of its mass this way. • In AGB stars, it is thought that core instabilities and pulsations are responsible for large mass losses. • A typical AGB loses from 10–7 to 10-4 solar masses a year and hence can last at most 10 million to just 10,000 years. © Sierra College Astronomy Department

19. Stellar Evolution - Life After the Main SequenceIntermediate-Mass Stars After the Main Sequence Final Stage of an Intermediate-Mass Star • A Planetary nebula is a spherical shell of gas that is expelled by a low-mass red giant near the end of its life. • The material in the shell glows because UV radiation from the central hot star causes it to fluoresce. • Pulsations and/or stellar winds are thought to cause planetary nebulae. © Sierra College Astronomy Department

20. Stellar Evolution - Life After the Main SequenceMore Quantum Physics The Chandrasekhar Limit • Degenerate electrons can withstand pressures created by up to 1.4 solar masses. • Beyond that point - the Chandrasekhar limit (or white dwarf limit) - white dwarf stars cannot exist. • Main sequence stars with masses up to 6 solar masses (and perhaps higher) can end up as white dwarfs only if they lose enough mass during the red giant and planetary nebula phases. © Sierra College Astronomy Department

21. Stellar Evolution - Life After the Main SequenceFinal Days for the High-Mass Stars High-Mass Stars on the AGB • Massive stars eventually become supergiants. • Typical supergiants have luminosities a million times that of the Sun and absolute magnitudes of -10. • The greater core temperatures and pressures, fuse elements beyond helium and up to iron on the periodic table. • The creation of these elements is known as nucleosynthesis. © Sierra College Astronomy Department

22. Stellar Evolution - Life After the Main SequenceSupernova – Type II Core Collapse • Type II supernovae (or massive star supernova) begin with the conversion of silicon to iron. The fusing of silicon to iron in a supergiant star will take only a few days. • Because the iron fusion reaction absorbs more energy than it releases, the core shrinks, heats up, but produces no new more massive elements. • At the Chandrasekar limit, the core collapses violently. After reaching its minimum size, the core rebounds, colliding violently with infalling material. © Sierra College Astronomy Department

23. Stellar Evolution - Life After the Main SequenceSupernova Remnants On the Rebound • This collision between the infalling material and the rebounding core produces two effects: • Enough energy is produced to fuse iron into heavier elements. • Shock waves are sent outward that throw off the outer layers of the supergiant. These shock waves may be further heated by the neutrinos escaping the collapsed core. • The thrown-off outer layers create a supernova remnant – an expanding cloud of debris. • Years of last five supernova observed without the need of a telescope: 1006, 1054, 1572, 1604, 1987. © Sierra College Astronomy Department

24. Stellar Evolution - Life After the Main SequenceEvolution in Stellar Systems The Lives of Close Binaries • Most stars in binary systems proceed in their evolution as if they were alone. • The Algol paradox • Close binary pair of which one star is a 3.7 M main-sequence star and the other a 0.8 M subgiant. • If they were born at about the same time, how can the more massive star be a main-sequence star? • Answer: Mass Transfer © Sierra College Astronomy Department