Astro 101 Fall 2013 Lecture 9 Stars (continued) – Stellar evolution T. Howard

1. Astro 101 Fall 2013 Lecture 9 Stars (continued) – Stellar evolution T. Howard

2. Spectral Classes Strange lettering scheme is a historical accident. Spectral Class Surface Temperature Examples 30,000 K 20,000 K 10,000 K 7000 K 6000 K 4000 K 3000 K Rigel Vega, Sirius Sun Betelgeuse O B A F G K M Further subdivision: BO - B9, GO - G9, etc. GO hotter than G9. Sun is a G2.

3. Increasing Mass, Radius on Main Sequence The Hertzsprung-Russell (H-R) Diagram Red Supergiants Red Giants Sun Main Sequence White Dwarfs A star’s position in the H-R diagram depends on its mass and evolutionary state.

4. H-R Diagram of Well-known Stars H-R Diagram of Nearby Stars Note lines of constant radius!

5. 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.

6. 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. Heats up. - H burns in hot, dense shell around core: "H-shell burning phase". - Tremendous energy produced. Star must expand. - Star now a "Red Giant". Diameter ~ 1 AU! - Phase lasts ~ 109 years for 1 MSun star. - Example: Arcturus Red Giant

7. Red Giant Star on H-R Diagram

8. Eventually: Core Helium Fusion - Core shrinks and heats up to 108 K, helium can now burn into carbon. "Triple-alpha process" 4He + 4He -> 8Be + energy 8Be + 4He -> 12C + energy - Core very dense. Fusion 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.

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

10. Helium Runs out in Core • - All He -> C. Not hot enough • for C fusion. • - Core shrinks and heats up, as • does H-burning shell. • - 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

12. "Planetary Nebulae" - Core continues to contract. Never hot enough for C fusion. - He shell dense, fusion becomes unstable => “He 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).

15. 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). • - Composition: C, O. • - White dwarfs slowly cool to oblivion. No fusion.

16. Star Clusters Open Cluster Globular Cluster Comparing with theory, can easily determine cluster age from H-R diagram.

17. Following the evolution of a cluster on the H-R diagram Luminosity LSun LSun Temperature 100 LSun LSun LSun LSun

18. 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.

19. 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

20. Evolution of Stars > 12 MSun Low mass stars never got past this structure: Eventual state of > 12 MSun star Higher mass stars fuse heavier elements. Result is "onion" structure with many shells of fusion-produced elements. Heaviest element made is iron. Strong winds. They evolve more rapidly. Example: 20 MSun star lives "only" ~107 years.

21. 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) ->

23. Endpoints of Massive Stars • Stars with mass >~ 8 Msun Supernovae • Massive stellar explosions • Remnant core collapses (usually much less than 50% mass) • In most cases, core  neutron star • Electrons and protons “crushed together” creating neutrons • Many neutrinos emitted  these escape the star completely • Neutron stars often end up forming pulsars • What about even more massive stars? • Mass >~ 25 Msun  collapse to a Black Hole

24. Supernovae – extremely violent stellar explosions • Several types& subtypes (called type “I”, “Ia”, “II”, etc.) • Different types arise from • pre-explosion stellar • conditions • We can use the change • in brightness • over time to • distinguish them Example supernova remnant— Crabnebula in Taurus

25. Two major classifications of Supernovae

27. Tycho’s • supernova Supernova remnant in Vela 

28. Light Curves of Supernovae Types

32. Neutron Stars If star has mass 12-25 MSun , remnant of supernova expected to be a tightly packed ball of neutrons. Diameter: 10 km only! Mass: 1.4 - 3(?) MSun Density: 1014 g / cm3 ! Rotation rate: few to many times per second!!! Magnetic field: 1010 x typical bar magnet! A neutron star over the Sandias? Please read about observable neutron stars: pulsars.

33. Pulsars discovered 1967 by Jocelyn Bell Burnell & Anthony Hewish.

34. Pulsars – “Lighthouse” model

35. Crab nebula in X-rays (Chandra)  “Blinking” of the Crab pulsar Off

36. A brief digression -- Relativity

37. 1905: Special Theory of Relativity 1915: General Theory of Relativity

38. Relativity stars with assigning “frames of reference” (coordinates) to the Observer and the Event (or Thing) in question z z y y x x Special Relativity  covers situations where velocities may be very high, but frames of reference are not accelerated General Relativity  as it says, the more general case: acceleration between frames of reference is included

39. A fundamental postulate of relativity: The speed of light, c, in free (empty) space is a universal constant. It does not get added to or subtracted from the frame of reference. c = (approx.) 3 x 108 meters/sec Note: the speed of light can be slower in solid, gaseous, or liquid media (glass, water, air), but never faster than c. This effect actually accounts for the bending of light rays in lenses, when entering or leaving a body of water, etc.

41. Black Holes and General Relativity General Relativity: Einstein's (1915) description of gravity (extension of Newton's). It begins with: The Equivalence Principle Here’s a series of thought experiments and arguments: 1) Imagine you are far from any source of gravity, in free space, weightless. If you shine a light or throw a ball, it will move in a straight line.

42. 2. If you are in freefall, you are also weightless. Einstein says these are equivalent. So in freefall, light and ball also travel in straight lines. 3. Now imagine two people in freefall on Earth, passing a ball back and forth. From their perspective, they pass it in a straight line. From a stationary perspective, it follows a curved path. So will a flashlight beam, but curvature of light path small because light is fast (but not infinitely so). The different perspectives are called frames of reference.

43. 4. Gravity and acceleration are equivalent. An apple falling in Earth's gravity is the same as one falling in an elevator accelerating upwards, in free space. 5. All effects you would observe by being in an accelerated frame of reference you would also observe when under the influence of gravity.

45. Some Consequences of General Relativity: Mass “warps” space  i.e., the amount of mass introduces a “curvature” to what would otherwise be perfectly linear (Euclidean) space. This curvature of space is a different way of thinking about gravity. It works, and explains a lot of things. The curvature of space causes things to move in curved lines.  Even rays of light! “Matter tells space how to curve. Space tells matter how to move.” (Misner, Thorne, and Wheeler, 1973)

46. The “rubber sheet” analogy.

48. Curvature of light Observed! In 1919 eclipse. Einstein Sir Arthur Eddington

50. Gravitational lensing. The gravity of a foreground cluster of galaxies distorts the images of background galaxies into arc shapes.