Astronomy 1020 Stellar Astronomy Spring_2015 Day-36

Course Announcements Dark night Alternative exercise is posted. Reports are due Wed. Apr. 22 Final exam (and Exam-4) is (are) scheduled on Wednesday, May 6, 10:30-12:30pm LABS: This Week: Hubble Red Shift “On your own”: Galaxy Zoo Classification Due: Wed. 29 th at class time (NO late labs accepted)

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

 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.

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

 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.

Each type of fusion takes higher temperatures and last less time

 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

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

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

 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.

 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!

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

 If the star is not too massive, the Type II supernova leaves behind a neutron- degenerate core: neutron star.  Mass between 1.4 and 3 M , radius ~ 10 km.  Some neutron stars are found in X-ray binaries, and give off strong X-rays.

 Neutron stars are incredibly dense and therefore have very high surface gravity and escape velocities.  Surface gravity:  Escape velocity: MATH TOOLS 17.2

 Others are found as pulsars (rapidly rotating neutron stars).  Highly magnetized.  Beam of radiation sweeps by Earth like a lighthouse beam.

 The Crab Nebula is the remnant of a Type II supernova first witnessed by the Chinese in 1054 CE and recorded as a “guest star,” lasting in the sky for over three weeks.  Its glow is powered by a pulsar.

Concept Quiz—Supernova What type of star makes a Type II supernova? A.a neutron star in a mass-transfer binary B.a black hole C.a pulsar D.a single massive star

 Star clusters are bound groups of stars, all made at the same time from the same material.  Globular clusters are very dense with up to millions of stars.  Open clusters are looser, with a few dozen to a few thousand stars.

 H-R diagrams of open and globular clusters look very different due to stellar evolution since the stars in them all formed around the same time: snapshots of evolution.

Star Clusters and Stellar Evolution  Young clusters still have massive stars on MS.  In older clusters, massive stars have died.  Location of main-sequence turnoff gives cluster age.

Star Clusters and Stellar Evolution  Star clusters have different colors relating to the abundances of stars in them.  Reflects the idea of stellar populations: groups of stars with similar ages and other shared characteristics.  Young stars have more massive elements in them than older stars, and their clusters are bluer.  Supernovae seed the universe with massive elements.  Earth could not have the elements it has were it not for prior supernovae.

Concept Quiz—Ages Here is a table of the temperatures of stars at the main- sequence turnoff in four clusters. Which cluster is the oldest? ClusterNameTemp (K) AOrion22,000 BNGC 188 9,000 C47 Tuc 5,000 DM 67 7,000

Stellar Evolution Lecture Tutorial pg. 133 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.