Stellar Evolution: Evolution off the Main Sequence

1. Stellar Evolution: Evolution off the Main Sequence Main Sequence Lifetimes Most massive (O and B stars): millions of years Stars like the Sun (G stars): billions of years Low mass stars (K and M stars): a trillion years! While on Main Sequence, stellar core has H -> He fusion, by p-p chain in stars like Sun or less massive. In more massive stars, “CNO cycle” becomes more important.

2. Units of Chapter 12 Leaving the Main Sequence Evolution of a Sun-like Star The Death of a Low-Mass Star Evolution of Stars More Massive than the Sun Supernova Explosions Observing Stellar Evolution in Star Clusters The Cycle of Stellar Evolution Summary of Chapter 12

3. On the Main Sequence During its stay on the main sequence, any fluctuations in a star’s condition are quickly restored; the star is in equilibrium.

4. Leaving the Main Sequence Eventually, as hydrogen in the core is consumed, the star begins to leave the main sequence. Its evolution from then on depends very much on the mass of the star: Low-mass stars go quietly. High-mass stars go out with a bang!

5. Evolution of a Sun-like Star Even while on the main sequence, the composition of a star’s core is changing.

6. Evolution of a Sun-like Star As the fuel in the core is used up, the core contracts; when it is used up the core begins to collapse. Hydrogen begins to fuse outside the core.

7. Evolution of a Low-Mass Star (< 8 Msun , focus on 1 Msun case) - All H converted to He in core. - Core too cool for He burning. Contracts, Collapses, Heats up. - H burns in shell around core: "H-shell burning phase". - Tremendous energy produced. Star must expand. - Star now a "Red Giant". Diameter ~ 1 AU! Sun-like star (1 MSun ) out to Mercury - Phase lasts ~ 109 years for 1 MSun star. - Example: Arcturus Red Giant

8. Red Giant Star on H-R Diagram

9. Eventually: Core Helium Fusion - Core shrinks and heats up to 10,000,000 (108) K, helium can now burn into carbon. "Triple-alpha process" 4He + 4He -> 8Be + energy 8Be + 4He -> 12C + energy - First occurs in a runaway process: "the helium flash". Energy from fusion goes into re-expanding and cooling the core. Takes only a few seconds! This slows fusion, so star gets dimmer again. - Then stable He -> C burning. Still have H -> He shell burning surrounding it. - Now star on "Horizontal Branch" of H-R diagram. Lasts ~108 years for 1 MSun star.

10. More massive less massive Horizontal branch star structure Core fusion He -> C Shell fusion H -> He

11. Evolution of a Sun-like Star Stage 11: Back to the giant branch: As the helium in the core fuses to carbon and oxygen, the core becomes hotter and hotter, and the helium burns faster and faster. The star is now similar to its condition just as it left the main sequence, except now there are two shells.

12. Helium Runs out in Core, Back to the Giant Branch • All He -> C in core. Not hot enough for C fusion. - Core shrinks and heats up. - Get new helium burning shell (inside H burning shell). - High rate of burning, star expands, luminosity way up. - Called ''Red Supergiant'' (or Asymptotic Giant Branch) phase. - Only ~106 years for 1 MSun star. Red Supergiant

14. The Death of a Low-Mass Star This graphic shows the entire evolution of a Sun-like star. Such stars never become hot enough for fusion past oxygen to take place.

15. "Planetary Nebulae" - Core continues to contract. Never gets hot enough for carbon fusion. - Helium shell burning becomes unstable -> "helium shell flashes". - Whole star pulsates more and more violently. - Eventually, shells thrown off star altogether! 0.1 - 0.2 MSun ejected. - Shells appear as a nebula around star, called "Planetary Nebula" (awful, historical name, nothing to do with planets).

16. The Death of a Low-Mass Star There is no more outward fusion pressure being generated in the core, which continues to contract. Stage 12: The outer layers of the star expand to form a planetary nebula.

17. The Death of a Low-Mass Star • The star now has two parts: • A small, extremely dense carbon core • An envelope about the size of our solar system. • The envelope is called a planetary nebula, even though it has nothing to do with planets – early astronomers viewing the fuzzy envelope thought it resembled a planetary system.

20. Bipolar Planetary nebulae

21. What is the Helium Flash? A: Explosive onset of Helium fusing to make Carbon B: A flash of light when Helium fissions to Hydrogren C: Bright emission of light from Helium atoms in the Sun D: Explosive onset of Hydrogen fusing to Helium Clicker Question:

22. What is happening in the interior of a star that is on the main sequence on the Hertzsprung-Russell diagram? A: Stars that have reached the main sequence have ceased nuclear "burning" and are simply cooling down by emitting radiation. B: The star is slowly shrinking as it slides down the main sequence from top left to bottom right. C: The star is generating energy by helium fusion, having stopped hydrogen "burning." D: The star is generating internal energy by hydrogen fusion. Clicker Question:

23. What causes the formation of bipolar planetary nebulae? A: A progenitor star with a rapid rotation B: A progenitor star in a dense environment C: A progenitor star in a binary system D: A progenitor star with strong magnetic fields Clicker Question:

24. The Death of a Low-Mass Star Stages 13 and 14: White and black dwarfs: Once the nebula has gone, the remaining core is extremely dense and extremely hot, but quite small. It is luminous only due to its high temperature.

25. White Dwarfs - Dead core of low-mass star after Planetary Nebula thrown off. - Mass: few tenths of a MSun . -Radius: about REarth . - Density: 106 g/cm3! (a cubic cm of it would weigh a ton on Earth). - White dwarfs slowly cool to oblivion. No fusion.

26. The Death of a Low-Mass Star The small star Sirius B is a white dwarf companion of the much larger and brighter Sirius A.

27. The Death of a Low-Mass Star The Hubble Space Telescope has detected white dwarf stars in globular clusters.

28. Death of a 1 solar mass star

29. Bipolar Planetary nebulae

30. Stellar Explosions: Nova A nova is a star that flares up very suddenly and then returns slowly to its former luminosity.

31. Stellar Explosions Novae White dwarf in close binary system WD's tidal force stretches out companion, until parts of outer envelope spill onto WD. Surface gets hotter and denser. Eventually, a burst of fusion. Binary brightens by 10'000's! Some gas expelled into space. Whole cycle may repeat every few decades => recurrent novae.

32. The Death of a Low-Mass Star Material falls onto the white dwarf from its main-sequence companion. When enough material has accreted, fusion can reignite very suddenly, burning off the new material. Material keeps being transferred to the white dwarf, and the process repeats.

33. Novae

34. Evolution of Stars > 8 MSun Eventual state of > 8 MSun star Higher mass stars evolve more rapidly and fuse heavier elements. Example: 20 MSun star lives "only" ~107 years. Result is "onion" structure with many shells of fusion-produced elements. Heaviest element made is iron.

35. Fusion Reactions and Stellar Mass In stars like the Sun or less massive, H -> He most efficient through proton-proton chain. In higher mass stars, "CNO cycle" more efficient. Same net result: 4 protons -> He nucleus Carbon just a catalyst. Need Tcenter > 16 million K for CNO cycle to be more efficient. Sun (mass) ->

36. In which phase of a star’s life is it converting He to Carbon? A: main sequence B: giant branch C: horizontal branch D: white dwarf Clicker Question:

37. Star Clusters Galactic or Open Cluster Globular Cluster Extremely useful for studying evolution, since all stars formed at same time and are at same distance from us. Comparing with theory, can easily determine cluster age from H-R diagram.

38. Star Clusters Two kinds: 1) Open Clusters -Example: The Pleiades -10's to 100's of stars -Few pc across -Loose grouping of stars -Tend to be young (10's to 100's of millions of years, not billions, but there are exceptions)

39. 2) Globular Clusters - few x 10 5 or 10 6 stars - size about 50 pc - very tightly packed, roughly spherical shape - billions of years old Clusters are crucial for stellar evolution studies because: 1) All stars in a cluster formed at about same time (so all have same age) 2) All stars are at about the same distance 3) All stars have same chemical composition

40. Following the evolution of a cluster on the H-R diagram T

41. Globular Cluster M80 and composite H-R diagram for similar-age clusters. Globular clusters formed 12-14 billion years ago. Useful info for studying the history of the Milky Way Galaxy.

42. Schematic Picture of Cluster Evolution Massive, hot, bright, blue, short-lived stars Time 0. Cluster looks blue Low-mass, cool, red, dim, long-lived stars Time: few million years. Cluster redder Time: 10 billion years. Cluster looks red

43. The age of a cluster can be found by: A: Looking at its velocity through the galaxy. B: Determining the turnoff point from the main sequence. C: Counting the number of stars in the cluster D: Determining how fast it is expanding Clicker Question:

44. Why do globular clusters contain stars with fewer metals (heavy elements) compared to open clusters? A: Open clusters have formed later in the evolution of the universe after considerably more processing B: Metals are gradually destroyed in globular clusters. C: Metals are blown out of globular clusters during supernova explosions D: Metals spontaneously decay to lighter elements during the 10 billion year age of the globular cluster. Clicker Question:

45. Death of a High-Mass Star M > 8 MSun Iron core Iron fusion doesn't produce energy (actually requires energy) => core shrinks and heats up T ~ 1010 K, radiation disrupts nuclei, p + e => n + neutrino Collapses until neutrons come into contact. Rebounds outward, violent shock ejects rest of star => A Core-collapse or Type II Supernova Such supernovae occur roughly every 50 years in Milky Way. Ejection speeds 1000's to 10,000's of km/sec! (see DEMO) Remnant is a “neutron star” or “black hole”.

46. Binding Energy per nucleon

47. Example Supernova: 1998bw

48. Cassiopeia A: Supernova Remnant

49. A Carbon-Detonation or “Type Ia” Supernova Despite novae, mass continues to build up on White Dwarf. If mass grows to 1.4 MSun (the "Chandrasekhar limit"), gravity overwhelms the Pauli exclusion pressure supporting the WD, so it contracts and heats up. This starts carbon fusion everywhere at once. Tremendous energy makes star explode. No core remnant.

50. Supernova 1987A in the Large Magellanic Cloud