Ch 12--Life Death of Stars 9 Nov 2000 ASTR103, GMU, Dr. Correll

What do you think? Will the Sun end its existence? If so, how? What is a nova? What is the origin of the carbon, silicon, oxygen, iron, uranium, and other heavy elements on Earth? What is a cosmic ray? What is a pulsar? 9 Nov 2000 ASTR103, GMU, Dr. Correll

Low-Mass Stars Post Main Sequence hydrogen fusion shell helium flash/helium fusion core helium fusion shell (asymptotic giant branch) 9 Nov 2000 ASTR103, GMU, Dr. Correll

Post Main Sequence Giants Core and interior shells condense giving off more energy causing the outer layers to expand and cool 9 Nov 2000 ASTR103, GMU, Dr. Correll

Planetary Nebula Outer atmosphere of stars is ejected and the surrounding could of gas is called a planetary nebula has nothing to do with planet formation a wide variety of planetary nebula are seen! 9 Nov 2000 ASTR103, GMU, Dr. Correll

Bipolar Planetary Nebula The Hourglass nebula about the star Eta Carinae is an example of a bipolar planetary nebula about 8000 ly away around a massive (~30Msun star) 9 Nov 2000 ASTR103, GMU, Dr. Correll

Remains of the Low-Mass Star After outer layers ejected in planetary nebula and solar wind, all that remains is the hot, dense carbon-oxygen core--a white dwarf! Fusion has ceased, and core cools down over billions of years White dwarf mass < 1.4 Msun due to Chandrasekhar limit Degenerate electrons support only this much mass before combining with protons to form neutrons 9 Nov 2000 ASTR103, GMU, Dr. Correll

Novas Consider what happens to a white dwarf in a binary system As the larger star evolves and expands some of its outer layers fall onto the white dwarf companion What happens when the hydrogen layer becomes dense enough? Hydrogen flash of fusion--a Nova occurs 9 Nov 2000 ASTR103, GMU, Dr. Correll

High-Mass Stars Higher mass enables fusion of heavier elements each successive shell burns in a shorter time carbon for about 600 yrs neon for about 1 yr oxygen for about a month silicon fusion for a day final inert core is iron! A star the size of Jupiter’s orbit is powered by a core the size of the Earth! 9 Nov 2000 ASTR103, GMU, Dr. Correll

High-Mass Stars 9 Nov 2000 ASTR103, GMU, Dr. Correll

Death of Massive Stars As iron core grows, the immense weight is supported by electron degeneracy pressure--ie, electron gas Eventually the electron gas is compressed to the point that the electron degeneracy pressure is overcome by gravity--electrons combine with protons forming neutrons and emitting gamma rays and neutrinos As neutrons form the core collapses to form a neutron core. A rebounding shockwave creates a powerful explosion blowing off outer layers of the star --Supernova! 9 Nov 2000 ASTR103, GMU, Dr. Correll

Death of Massive Stars 9 Nov 2000 ASTR103, GMU, Dr. Correll

Supernova 1987A 9 Nov 2000 ASTR103, GMU, Dr. Correll

Supernova 1987A 9 Nov 2000 ASTR103, GMU, Dr. Correll

Supernova Types Type Ia--begins as a carbon-oxygen rich white dwarf in a close binary system giant companion enlarges, depositing gas onto white dwarf as mass approaches the Chandrasekhar limit, carbon fusion ignites in the core nuclear powered 9 Nov 2000 ASTR103, GMU, Dr. Correll

Supernova Types Type II--core collapse of massive stars mass of core greater than Chandrasekhar limit gravity overwhelms electron degeneracy pressure core falls into the center gravitational energy Outer gas layers give many hydrogen lines in spectrum 9 Nov 2000 ASTR103, GMU, Dr. Correll

Neutron Stars What’s left after a type II supernova for stars of mass of 8-25 Msun, when core collapses and electron combine with protons, we are left with a neutron core--a neutron star! Core about 10-20 km in diameter! Gas layers completely blown away Star now supported by neutron degeneracy pressure 9 Nov 2000 ASTR103, GMU, Dr. Correll

Neutron Stars Stars naturally seem to have magnetic fields. What happens to the magnetic fields when stellar cores collapse? 9 Nov 2000 ASTR103, GMU, Dr. Correll

Pulsars The magnetic field collapses also, creating a very high energy density electric dynamo polar axis of magnetic field not always in line with rotation axis field energizes and beam out charged particle “lighthouse” model 9 Nov 2000 ASTR103, GMU, Dr. Correll

Pulsars We observe pulsars via radio waves 9 Nov 2000 ASTR103, GMU, Dr. Correll

Pulsars--SS433 SS433--a pulsar fed by a close binary companion 9 Nov 2000 ASTR103, GMU, Dr. Correll

X-Ray Bursters Analogous to hydrogen flashes on white dwarves (novas), on a neutron star, the hydrogen slowly fuses into helium and then a helium flash can occur rather than peaking in the optical range, helium flashes are hotter and in the x-ray range x-ray burster 9 Nov 2000 ASTR103, GMU, Dr. Correll

Stellar Evolution Summary 9 Nov 2000 ASTR103, GMU, Dr. Correll

Stellar Evolution Summary 9 Nov 2000 ASTR103, GMU, Dr. Correll

What do you think? Will the Sun end its existence? If so, how? The Sun will shed its outer layers as a planetary nebula in about 7 billions years. Its remnant white dwarf, with fusion ceased, will dim over the next several billion years What is a nova? A relatively gentle explosion of hydrogen gas on the surface of a white dwarf What is the origin of the carbon, silicon, oxygen, iron, uranium, and other heavy elements on Earth? These elements were generated during stellar evolution of massive stars and in supernova explosions What is a cosmic ray? High energy (speed) particles accelerated in supernova explosions What is a pulsar? A rotating neutron star with a magnetic field creating a “lighthouse” of radiation which sweep across space 9 Nov 2000 ASTR103, GMU, Dr. Correll

Questions for Thought Describe the evolution of main sequence star with a mass of 20 Msun. Describe the various phases of energy generation, migration off the main sequence, and the eventual fate of the star to include what forces are at balance throughout. 9 Nov 2000 ASTR103, GMU, Dr. Correll