November 4, 2011 Review Session: Sunday, Nov. 6, here, 6-8pm Midterm #2: Wednesday, Nov. 9 Citizen Astronomy Projects now on d2l Don’t forget to do the Telescope Lab Key Concepts for Midterm 2: now posted on d2l
Final Result of massive star formation: Onion Skin Layers of heavy elements in CORE
These stages are fast. For example, for a 25 Msun star: * Hydrogen fusion lasts 7 million years * Helium fusion lasts 500,000 years * Carbon fusion lasts 600 years * Neon fusion lasts 1 year * Oxygen fusion lasts 6 months * Silicon fusion lasts 1 day The star's core is now pure iron.
. SUPERNOVA * The star hits the IRON wall: Iron is a very stable element, and cannot fuse to form heavier elements. * So when the core becomes IRON, fusion no longer produces enough energy to stop gravitational collapse * The core collapses, until neutron degeneracy pressure stops the collapse of the core. * The outer parts of the star hit the core and bounce off --> a supernova! * What's left is a NEUTRON STAR (if the mass is less than about 8 solar masses) or a BLACK HOLE
Within a massive, evolved star the onion-layered shells of elements undergo fusion, forming an iron core And starts to collapse. The inner part of the core is compressed into neutrons (c), causing infalling material to bounce (d) and form an outward-propagating shock front (red). The shock starts to stall (e), but it is re-invigorated by a process that may include neutrino interaction. The surrounding material is blasted away (f), leaving only a degenerate remnant.
Historic Supernovae: * Supernovae become extremely bright. * Supernovae in our Milky Way can become bright enough to see during the day. * Supernovae in distant galaxies are of intense interest now for cosmology * Famous Historic Supernova: 1054, recorded by Chinese and Native Americans, today is the Crab Supernova remnant 1006: Southern hemisphere supernova 1572: Tycho Brahe's supernova 1604: Kepler's supernova
Since 1604, there have been no supernova explosions in the Milky Way -- we're overdue! In 1987, a supernova in the Large Magellanic Cloud, SN1987A Two neutrino experiments operating at that time detected neutrinos from the explosion Before and after picture
What a star does after the Main Sequence depends on its MASS • Mass < 0.08 M(Sun) = 85x M(Jupiter) Brown dwarf: gravitational collapse stopped by electron degeneracy pressure Never burns H->Helium Gets cooler and dimmer as time goes on
What a star does after the Main Sequence depends on its MASS 2. 0.08 M(Sun) < Mass < 0.23 M(Sun) Star not massive enough to generate a hydrogen Burning shell and so doesn’t become a red giant Becomes a white dwarf, skipping the red giant & planetary nebula phase
What a star does after the Main Sequence depends on its MASS 3. 0.23 M(Sun) < Mass < 4 M(Sun) Star becomes a Red Giant Eventually expells a planetary nebula, leaving a white dwarf core
What a star does after the Main Sequence depends on its MASS 4. 4 M(Sun) < Mass < 20 M(Sun) Star becomes a Red Giant Eventually explodes as a supernova Leaves behind a neutron star: gravitational collapse halted by neutron degeneracy pressure
What a star does after the Main Sequence depends on its MASS 5. M > 20 M(Sun) Star becomes a Red Giant Eventually explodes as a supernova Leaves behind a black hole Even neutron degeneracy pressure is insufficient to stop gravitational collapse, so the star collapses until its radius = 0, density = infinity
Origin of the Elements • All the carbon, oxygen, etc on the Earth, (and in humans) was • produced in the centers of stars. • We are STARDUST! • Most carbon, oxygen comes from low-mass red giant winds • Most of the heavy elements come from supernovae • New stars form out of interstellar gas which has been enriched • with elements by red giant winds, planetary nebulae and • supernovae. • Older stars on the main sequence have relatively fewer • atoms of iron than younger stars, since they were formed out • of gas which had not been polluted by as many generations of stars • We've searched pretty hard, but have never found, • pure hydrogen and helium stars.
White DwarfsNovae, Type 1a Supernovae Main Sequence Stars with M < 4 solar masses end up as WHITE DWARFs The collapse by gravity is halted by electron degeneracy pressure The degenerate core which becomes a white dwarf is mostly carbon
CHANDRASEKHAR limit: the mass of a white dwarf cannot exceed 1.4 solar masses Subrahmanyan Chandrasekhar (1910 – 1995) If the core is more massive electron degeneracy cannot withstand gravity Collapses to a neutron star or black hole 1983 Nobel prize in Physics