Cosmic Catastrophes Wheeler 309N SPRING 2010 February 1, 2010 (49031) Review for Test #1 BACKGROUND AND INTRODUCTION TO SUPERNOVAE Gravity – acts as if all the mass beneath were concentrated in the center. Gravity at surface depends on the mass and the radius of the object. Gravity is stronger inside massive, compact stars. Gravity gets weaker further away from any object. Stellar balancing act — dynamic equilibrium. A star spends most of its lifetime at a relatively constant size, temperature, luminosity, etc. while it fuses some fraction of its hydrogen into helium. During this time there is a balance between the forces inward and the forces outward. Forces inward — due to gravity, without the forces acting outwards the star would collapse. Forces outward — pressure Thermal pressure — For most of the lifetime of the star this is the dominant source of outward pressure. With this pressure a star can regulate its temperature. Regulated temperature – the nuclear burning is regulated on the main sequence. If too much energy is temporarily lost, the star contracts and heats, increasing nuclear input. If too much energy is temporarily gained, the star expands and cools, and nuclear input declines. Quantum pressure — Electrons cannot occupy the same region of space if they have the same energy. As matter is squeezed down, electrons develop more energy depending only on the density and independent of the temperature. The electrons’ resistance to being squeezed any closer together provides pressure independent of temperature. With this pressure a star cannot regulate its temperature. Unregulated temperature — loss of energy, contraction, and heating is stopped when the core becomes so compact and dense that electrons are squeezed together and the quantum pressure dominates. Broken thermostat. If such a star (or core) loses energy, it cools since pressure does not depend on the temperature, so there is no loss of pressure and the star does not contract and heat. If the star gains energy, it heats up, more nuclear reactions, more heat, explosion! Main Sequence — stars fuse hydrogen into helium and thus supply energy. Red Giant — when hydrogen burns out in the center, excess heat flowing from the contracting core causes the outer layers to gain energy. They then expand and cool. The outside becomes larger, cooler, redder, and more luminous. Planetary nebulae —Most stars less than 8 M eject their envelopes as planetary nebulae. Core of C and O cools to become white dwarf. White Dwarfs. Size of Earth, mass of Sun. Supported by the quantum pressure. Most are single stars, mass about 0.6 solar mass, cooling time longer than the age of the Universe. Maximum mass of white dwarf, Chandrasekhar Mass ~ 1.4 M, supported by quantum pressure of electrons. Historical Supernovae in the Milky Way - several seen and recorded with naked eye in last 2000 years. SN 386 earliest on record, SN 1006 brightest, SN 1054, now the Crab Nebula, contains a rapidly rotating pulsar and suggestions of a jet. Tycho 1572, Kepler 1604. Cas A not clearly seen about 1680, shows evidence for jets, and a dim compact object in the center. The events that show compact objects also seem to show evidence of “elongated” explosions or “jets.” SN 1006, and SN 1604 were probably Type Ia, exploding white dwarfs, SN1572 definitely was. SN 1987A in a very nearby galaxy came from a massive star, about 20 solar masses, shows elongated ejecta, produced neutrinos so we know it was powered by core collapse.Extragalactic Supernovae - many, but dimmer, more difficult to study. Type I supernovae - no evidence for hydrogen in spectrum. Type II supernovae - definite evidence for hydrogen in spectrum. Type Ia supernovae - brightest, no hydrogen or helium, avoid spiral arms, occur in elliptical galaxies, origin in lower mass stars. Observe silicon early on, iron later. Unregulated burning, explosion in quantum pressure supported white dwarf of nearly Chandrasekhar mass. Star is completely disrupted, no neutron star or black hole. Light curve shows peak lasting about a week. Type II Supernovae - explode in spiral arms, never occur in elliptical galaxies, normal hydrogen, massive stars, recently born, short lived. Observe H early on, O, Mg, Ca later. Probably core collapse in iron core of massive star. Light curve often shows month’s-long “plateau.” Characteristic of explosion in a red giant. Common elements formed from “building blocks” of helium in stars: carbon, oxygen, neon, magnesium, silicon, calcium. Type Ib Supernovae - no hydrogen, but observe helium early on, O, Mg, Ca later. Occur in spiral arms, never in elliptical galaxies. Massive star core collapse. Type Ic Supernovae - no hydrogen, little or no helium early on, O, Mg, Ca later. Occur in spiral arms, never in elliptical galaxies. Massive star core collapse. Light curves of Type Ib and Ic are similar to Type Ia, but dimmer at maximum brightness. Betelgeuse – red giant 427 light years away, 15 to 20M, is expected to explode within 10,000 years, maybe tonight, as core collapse Type II supernova. Keep an eye on