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Lecture Slides CHAPTER 13: Evolution of High-Mass Stars

Learn how high-mass stars evolve differently, the role of stellar mass, supernova stages, variable stars, and the formation of neutron stars. Explore the significance of Cepheid variables and the impact of supernovae on the interstellar medium.

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Lecture Slides CHAPTER 13: Evolution of High-Mass Stars

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  1. Lecture Slides CHAPTER 13: Evolution of High-Mass Stars Understanding Our Universe SECOND EDITION Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal Prepared by Lisa M. Will, San Diego City College

  2. Evolution of High-Mass Stars Describe how high-mass stars evolve differently than low-mass stars. Understand the end stages of high-mass stars: supernovae, neutron stars, black holes.

  3. Role of Stellar Mass • High-mass stars are defined as having at least 8 M. • More mass => more pressure => higher temperatures => faster nuclear reaction rates. • High-mass stars live different, faster lives.

  4. Role of Stellar Mass: Main-Sequence Lifetimes Similar to the evolution of low-mass stars, once hydrogen is exhausted from the core, the star leaves the main sequence and expands and cools.

  5. Role of Stellar Mass: Helium Fusion The transition to helium fusion in core is smooth – no flash! Central temperature continues to rise. Key difference: Heavier elements (C, Ne, etc.) can undergo fusion in high-mass stars!

  6. Role of Stellar Mass: Main-sequence Star The more massive the star, the heavier the elements that can fuse. High-mass stars will fuse elements up until iron. The shells build up like the layers of an onion.

  7. Class Question Here are the temperatures of stars at the main-sequence turnoff in four clusters. Which cluster is the youngest? • 47 Tuc • M67 • NGC 188 • Orion Cluster Temperature (K) 47 Tuc 5,000 M67 4,000 NGC 188 9,000 Orion 22,000

  8. Class Question Here are the temperatures of stars at the main-sequence turnoff in four clusters. Are older clusters more likely to appear blue or red? Blue Red Cluster Temperature (K) 47 Tuc 5,000 M67 4,000 NGC 188 9,000 Orion 22,000

  9. Variable Stars • As stars expand and cool, they can pass through a portion of the H-R diagram called the instability strip. • Here, the combination of temperature and luminosity results in stellar pulsations.

  10. Variable Stars: Cepheid Variables Vs. RR Lyrae Variables • These pulsating variable stars are extremely important for determining distances. • Specifically, they have a relationship between their pulsation period and luminosity. • Cepheid variables: • High-mass stars becoming supergiants. • Periods from 1 to 100 days. • More luminous stars have longer periods. • RR Lyrae variables: • Low-mass stars on the horizontal branch. • Less luminous than Cepheid variables.

  11. High-mass stars have strong stellar winds. Mass loss rates are large, resulting in the stars losing significant amounts of their mass over their lifetimes. Variable Stars: Strong Stellar Winds

  12. Class Question Why are Cepheid variable stars important? We know their luminosities and therefore we can determine their distances. They make black holes. They are about to explode as supernovae. They generate most heavy elements.

  13. Supernova Recall that high-mass stars can fuse elements heavier than helium. Each stageis progressively shorter. Carbon burning lasts for 1000 years. Silicon burning only lasts for a few days.

  14. Supernova: Fusion of Iron Fusion of iron or more massive elements requires an external energy source. Once the star has an iron core, it cannot generate more energy by fusion. Fusion stops, and the core collapses.

  15. Supernova: Fusion of Iron (Cont.)

  16. Supernova: Fusion of Iron (Cont.)

  17. Supernova

  18. Supernova: Core Collapses • Core collapses and its temperature rises. • Photodisintegration and neutrino cooling reduce core pressure =>collapse accelerates. • Core collapses until it reaches nuclear densities. • Core collapse stops, bounces back, driving a shock wave through star. • Shock wave takes a mere few hours to rip through the star.

  19. Supernova: Type II Supernova • Outer layers blow off in tremendous explosion called a Type II supernova. • Light emitted is equivalent to a billion Suns!

  20. Supernova: Impacts • Supernova impact their surroundings. • Shock wave heats interstellar medium. • Most atoms heavier than iron are made in the explosion. • Supernova explosions inject these elements into the interstellar medium.

  21. Supernova: Impacts (Cont.)

  22. Supernova: Impacts (Cont.)

  23. Supernova: Impacts (Cont.)

  24. Class Question What type of stars make Type II supernovae? A white dwarf in a mass-transfer binary. A low-mass star. A high-mass star.

  25. Neutron Stars Type II supernova can leave behind a neutron-degenerate core: neutron star. Mass is between 1.4 M and 3 M. Size is about 10 km. If the neutron star is in a close binary, large amounts of x-rays can be emitted.

  26. Neutron Stars: Magnetic Fields and Rapid Rotation Others are found as pulsars (rapidly rotating neutron stars). Combination of strong magnetic fields and rapid rotation => beam of radiation that sweeps around like a lighthouse beam.

  27. Neutron Stars: The Crab Nebula The Crab Nebula is the result of a supernova observed in 1054 AD. The Crab Nebula pulsar is evidence that our models of the evolution of high-mass stars are correct.

  28. Neutron Stars: The Crab Nebula (Cont.)

  29. Neutron Stars: The Crab Nebula (Cont.)

  30. Neutron Stars: The Crab Nebula (Cont.)

  31. Black Holes • If the mass of a core of a high-mass star is greater than 3 M, it will collapse to a black hole. • Can form directly from Type II supernova (if massive enough) or from accretion by a neutron star in a binary system. • To understand black holes, we must first learn someconcepts from special relativity and general relativity.

  32. Usually, perceived speeds depend on the relative motion between objects. Special relativity dictates light moves at a speed c, regardless of their motion => speed of light is same for all observers. Black Holes: Speeds Depend on Motion

  33. Other results from special relativity: The speed of light is the ultimate speed limit. Time passes more slowly in a moving reference frame. An object is shorter in motion than it is at rest. Black Holes: From Special Relativity

  34. Results from general relativity: Mass distorts spacetime. We can think of mass as warping space like a rubber sheet. The mass changes how objects in its vicinity move on the rubber sheet. Black Holes: From General Relativity

  35. Black Holes: From General Relativity (Cont.)

  36. Black Holes: From General Relativity (Cont.)

  37. Black Holes: From General Relativity (Cont.)

  38. Black Holes: Gravitational Lensing and Waves • Gravitational lensing: the effect of gravity on the path of light. • Gravitational waves: ripples that travel through spacetime at the speed of light.

  39. Black holes: singularities in spacetime. The warping of spacetime causes extreme tidal forces and gravitational redshift. Black Holes: Singularity

  40. The tidal forces cause objects falling into the black hole to stretch greatly – spaghettification! Black Holes: Spaghettification

  41. The event horizon is a sphere with a radius known as the Schwarzchild radius, where the escape velocity equals the speed of light. Black holes: gravity so strong that even light cannot escape. Black Holes: The Event Horizon

  42. Gamma-ray bursts are still not completely understood Black holes can be detected by effects of their gravity on nearby objects. Gamma-ray bursts can be caused by rapidly varying X-ray binary systems. Black Holes: Gamma-ray Bursts

  43. Black Holes: Hawking Radiation Black holes should lose energy via Hawking radiation. Implications of Hawking radiation: Black holes do emit, but intensity is too low to observe. Black holes will not last forever.

  44. Chapter Summary • High-mass stars evolve more quickly than low-mass stars. • High-mass stars explode as Type II supernovae. • Supernovae spread heavier elements throughout interstellar space. • The cores of high-mass stars will result in neutron stars or black holes.

  45. Astronomy in Action Type II Supernova • Click the image to launch the Astronomy in Action Video(Requires an active Internet connection)

  46. Astronomy in Action Click the image to launch the Astronomy in Action Video(Requires an active Internet connection) Pulsar Rotation

  47. Nebraska Applet Click the image to launch the Nebraska Applet (Requires an active Internet connection) H-R Diagram Cluster Fitting Explorer

  48. Click the image to launch the Nebraska Applet (Requires an active Internet connection) Nebraska Applet CNO Cycle Animation

  49. Click the image to launch the Nebraska Applet (Requires an active Internet connection) Nebraska Applet Supernova Light Curve Fitting Explorer

  50. Click the image to launch the Nebraska Applet (Requires an active Internet connection) Nebraska Applet H-R Diagram Explorer

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