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Chapter 13 The Bizarre Stellar Graveyard White Dwarfs... ...are stellar remnants for low-mass stars. ...are found in the centers of planetary nebula. ...have diameters about the same as the Earth’s. ...have masses less than the Chandrasekhar mass. Sirius B is a white dwarf star Sirius A And Sirius B In X-ray Sirius A Sirius B Novas and Supernovas Nova - a stellar explosion Supernova - a stellar explosion that marks the end of a star’s evolution White Dwarf Supernova (Type I supernova)occur in binary systems in which one is a white dwarf Massive Star Supernova (Type II Supernova) occur when a massive star’s iron core collapses Close Binary Systems and Mass Transfer Nova March 1935 Herculis May 1935 Diagram of nova process A nova occurs when hydrogen fusion ignites on the surface of a white dwarf star system Nova T Pyxidis (HST) Light Curve of typical Nova Semidetached Binary System With White Dwarf Star (may result in a white dwarf (type I ) supernova) Type II Supernova The star releases more energy in a just a few minutes than it did during its entire lifetime. » Example: SN 1987A After the explosion of a massive star, a huge glowing cloud of stellar debris - a supernova remnant - steadily expands. » Example: Crab Nebula After a supernova the exposed core is seen as a neutron star - or if the star is more than 3 solar masses the core becomes a black hole. On July 4, 1054 astronomers in China witnessed a supernova within our own galaxy. The remnant of this explosion is The Crab Nebula Supernova 1987a Type I and Type II Supernova Supernova Light Curves Hydrogen and Helium Burning 4(1H )4He energy 2neutrinos 2 positrons 3( He) C energy 4 12 Carbon Burning and Helium Capture C 12 C 24Mg energy 12 C 4He16O energy 12 Still heavier elements are created in the final stages of life of massive stars 28 Si 7( He) Ni energy 4 56 Alpha Process – Helium Capture produces heavier elements up to Fe and Ni. Elements beyond Fe and Ni involve neutron capture. This forms unstable nuclei which then decay into stable nuclei of other elements 57 Fe n Fe 58 Fe n Fe 58 59 Formation of Elements beyond Iron occurs very rapidly as the star approaches supernova. The supernova explosion then distributes the newly formed matter throughout the interstellar space (space between the stars). This new matter goes into the formation of interstellar debris. The remnant core is a dense solid core of neutrons – a neutron star! Neutron Stars ...are stellar remnants for high-mass stars. ...are found in the centers of some type II supernova remnants. ...have diameters of about 6 miles. ...have masses greater than the Chandrasekhar mass. (1.4M) Relative Sizes Earth White Dwarf Neutron Star Pulsars The first pulsar observed was originally thought to be signals from extraterrestrials. (LGM-Little Green Men was their first designation) Period = 1.337301 seconds exact! ~ 20 seconds of Jocelyn Bell’s data- the first pulsar discovered It was later shown to be unlikely that the pulsar signal originated from extraterrestrial intelligence after many other pulsars were found all over the sky. Pulsars The pulsing star inside the Crab Nebula was a pulsar. Pulsars stars. are rotating, magnetized neutron The Crab Nebula The Crab Pulsar Period = 0.033 seconds = 33 milliseconds Light House Model – Beams of radiation emanate from the magnetic poles. – As the neutron star rotates, the beams sweep around the sky. – If the Earth happens to lie in the path of the beams, we see a pulsar. Rotating Neutron Star Light House model of neutron star emission accounts for many properties of observed Pulsars Artistic rendering of the light house model Rotation Rates of Pulsars The neutron stars that appear to us as pulsars rotate about once every second or less. Before a star collapses to a neutron star it probably rotates about once every 25 days. Why is there such a big change in rotation rate? Answer: Conservation of Angular Momentum Neutron –Star Binaries Mass Limits Low mass stars – Less than 8 M on Main Sequence – Become White Dwarf (< 1.4 M) » Electron Degeneracy Pressure High Mass Stars – Less than 100 M on Main Sequence – Become Neutron Stars (1.4M < M < 3M) » Neutron Degeneracy Pressure Black Holes ...are stellar remnants for high-mass stars. – i.e. remnant cores with masses greater than 3 solar masses …have a gravitational attraction that is so strong that light cannot escape from it. …are found in some binary star systems and there may be super-massive black holes in the centers of some galaxies. Supermassive Stars If the stellar core has more than three solar masses after supernova, then no known force can halt the collapse Black Hole Black holes were first predicted by the General Theory of Relativity, which is theory of gravity that corrects for some of the short-falls of Newton’s Theory of Gravity. In general Relativity, space, time and mass are all interconnected Space-Time No mass Distortion caused by mass Predictions of General Relativity Advance of Mercury’s perihelion Bending of starlight Advance of Mercury’s Perihelion 43” per century not due to perturbations from other planets Bending of Starlight 1.75” Apparent position of the star Sun Light from star bent by the gravity of the Sun Schwarzschild Black Hole Event Horizon Rs = 3(Mass) + Rs Singularity Mass Rs 3 M 9 km 5 15 10 30 Near a Black Hole What Can We Know? Mass – gravity Charge – Electric Fields Rotation Rate – Co-rotation How Can We Find Them? Look for X-ray sources – Must come from compact source » White Dwarf » Neutron Star » Black Hole – Differentiate by Mass » WD - < 1.4 M » NS - between 1.4 and 3 M » BH - > 3 M Cygnus X-1 End of Chapters End of Section. Nucleosynthesis Evolutionary Time Scales for a 15 M Star Fused H 4 He 12 C 20 Ne + 16 O 28 Si + 56 Fe Products 4 He 12 C 16 20 O, Ne, 24 Mg, 4He 16 O, 24Mg 28 Si, 32S 56 Fe Neutrons Time 7 Temperature 10 yrs. Few X 106 yrs 1000 yrs. 4 X 106 K 1 X 108 K 6 X 108 K Few yrs. One year Days < 1 second 1 X 109 K 2 X 109 K 3 X 109 K > 3 X 109 K Energy Budget H He C Fusion Stages Fe Anazasi Pictographs Supernova 1998S in NGC 3877 Supernova Remnants Tycho’s SNR - 1572 PSR 0628-28 LGM? Several more found at widely different places in the galaxy Power of a power equals total power potential output of the Earth No Doppler shifts PULSARS Light Time Argument An object which varies its light can be no larger than the distance light can travel in the shortest period of variation. To Darken the Sun Time Delay = Radius/c 500,000 km/300,000 km/s = 1.67 sec Only candidates: White Dwarfs, Neutron Stars Pulse Mechanisms Binary Stars - How quickly can two stars orbit? Two WD about 1m Two NS about 1s. Neutron Stars in orbit should emit gravity waves which should be detectable. Oscillations - Depends only on density WD about ten seconds NS about .001s Little variation permitted. Rotation - Until the object begins to break up. WD about 1s NS about .001s with large variation. SS 433 Synchrotron Radiation Radiation Magnetic lines of force Electron Glitches