1. Astronomy 1020-H Stellar Astronomy Spring_2016 Day-32

2. Course Announcements Remaining Observing nights: 1 st Q – Wed. 4/13, 7:30pm on campus Observing Reports are due: Mon. 4/18 at class time.

4.  High-mass stars live different, faster lives.  On the main sequence, energy is generated from the carbon-nitrogen-oxygen (CNO) cycle, with carbon as a catalyst: 12C + 4  1H + 2  e  = 12C + 4He + gamma rays + neutrinos.

5.  High-mass stars have convection to mix H in the core.  Increases the mass available for fusion.  Once H is exhausted from the core, the star leaves the main sequence and expands and cools.

6.  Move right on the H-R diagram: supergiants.  Ignite He in a nondegenerate core, unlike low- mass stars.  With rising central temperatures, heavier elements (C, Ne, etc.) fuse, generating energy.

7. After the carbon runs out, heavier elements start fusing

8.  The more massive the star, the heavier the elements that can fuse.  As temperature rises and core fuel is used up, heavier and heavier elements will fuse, up until iron.  The fusion shells build up like the layers of an onion.

9.  As high-mass stars expand and cool, they can pass through the instability strip on the HR diagram.  Here, the combination of temperature and luminosity results in the stars’ pulsation.

10.  These pulsating variable stars are extremely important for determining distances.  Specifically, they have a period- luminosity relationship.

11.  Cepheid variables: High-mass stars becoming supergiants. Periods from one to 100 days. More luminous stars have longer periods.  RR Lyrae variables: Low-mass stars on the horizontal branch. Less luminous than Cepheid variables.

12.  Intermediate-mass stars have masses between 3 and 8 M .  Start off evolving as high-mass stars, but finish as low-mass stars do, as white dwarfs.  Very massive stars may shed some mass due to instabilities and go through a luminous blue variable (LBV) phase. CONNECTIONS 17.1

13. Concept Quiz—Cepheid Variables Why are Cepheid variable stars so important? A.We can know their luminosities. B.They produce pulsars. C.They are about to explode as supernovae. D.They generate most heavy elements.

14. Once an iron core starts to form, the end comes quickly

15.  Fusion of iron or more massive elements requires energy—the star cannot use them for fuel.  Once the star has an iron core, it cannot generate more energy.  Fusion stops, and the core collapses.

16. Each type of fusion takes higher temperatures and last less time

17.  The net energy released by a nuclear reaction is the difference between the binding energy of the products and the binding energy of the reactants.  For the triple-alpha process:  For the fusion of iron, the binding energy of the products is less than that of the reactants, so the net energy is negative. MATH TOOLS 17.1

18. Star Formation & Lifetimes Lecture Tutorial pg. 119 Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now!!!! Come to a consensus answer you both agree on and write complete thoughts into your LT. If you get stuck or are not sure of your answer, ask another group.

19. Photodisintegration doesn’t relieve the electron degeneracy pressure Things get so crowded the electrons are squeezed into the nucleus where they combine with protons to make neutrons in a process called Reverse Beta Decay

20.  Each stage of burning is progressively shorter.  Example: Si burning only lasts for a few days.  Why? Huge production of neutrinos, which carry away energy  neutrino cooling.  The star cannot access the huge amount of energy produced in neutrinos.

21.  Core collapses, central temperature rises.  Photodisintegration, neutrino cooling reduces pressure, collapse accelerates.  Electron degeneracy cannot help.

22.  Collapses until it reaches nuclear densities.  At these high densities, nuclear forces repel atoms.  Core stops, bounces back, driving a shock wave through star.

23.  Shock wave takes a mere few hours to rip through the star.  Outer layers blow off in tremendous explosion (Type II supernova).  Dense core remains.

24.  Light energy emitted is about 1 billion Suns.  Kinetic energy of blown-off outer parts: 100x.  This kinetic energy is transferred to the interstellar medium (ISM), heating it.  Neutrinos carry off an energy of 100 times larger still!

25.  Shock wave heats and pushes the ISM.  New elements created in the explosion (nucleosynthesis).  Most atoms heavier than iron are made in supernova explosions.