Mar 24 neutron stars

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Published on November 13, 2007

Author: fazil

Source: authorstream.com

Slide1:  Neutron Star and Pulsars Chapter 18.2 Chapter 18.2: Neutron Stars (NS) Road Map:  Chapter 18.2: Neutron Stars (NS) Road Map History Neutron stars were predicted in 1933 (Zwicky and Baade) Discovered accidentally in 1967 (Bell and Hewish) as radio pulsars Origin Neutron stars are the remnants of massive stars after they explode as supernovae (pre-SN mass M >3 Msun) Final NS remnant has a mass between 1.4 Msun and ~3Msun. Composition NS extremely small (~30km diameter) and dense (100 million tons per cm3 !) Interior is mostly degenerate neutrons in a superconducting superfluid Intense gravitational force balanced by degenerate neutron gas pressure There is a thin (~500 m) solid crust on surface Intense magnetic field implies currents, so must have protons and electrons All NS are rapid rotators (P<10sec) due to angular momentum conservaton Some NS are in binary systems Mass transfer can produce an accretion disk, observed in x-rays When helium on surface reaches 3·107 K, fusion produces x-ray burst Road Map for Pulsars:  Road Map for Pulsars All pulsars are neutron stars, but are all NS pulsars? At least 1,000 known (but only 3 in SN remnants) Lighthouse (oblique rotator) model for emission Spin axis, magnetic field axes are misaligned Particles (produced by pair production) radiate in narrow cone along magnetic field axis (beamed radiation) Radiation is most commonly detected as radio emission , but sometimes optical, x-ray [youngest pulsars] Pulsar rotation periods Range from 0.0015 sec to ~3 sec (most > 0.1 sec) Most are slowing down (rotational energy -> radiation) Hence spin period => age (slower = older) Occasionally observer timing glitch : results from starquake Millisecond pulsars Found mostly in binary systems Old pulsars but fast rotation: paradox? Solution: “spin up” angular momentum transfer from companion HST image of an isolated neutron star:  HST image of an isolated neutron star Structure of a Neutron star:  Structure of a Neutron star Slide7:  For a neutron star to have a magnetic field, it must also have a few protons scattered throughout to create the magnetic field. History: Fritz Zwicky (and W. Baade) propose the existence of neutron stars in 1933.:  The atoms in normal, every day objects are separated by electrons in adjacent atoms. Called degenerate electron pressure. Astronomers Zwicky and Baade proposed that a tiny stellar core crushed by a supernova could be supported by degenerate neutron pressure. Such tiny stars are called neutron stars. A star compacted to neutron density would be very small. A 1 solar mass would be only 30 km across. History: Fritz Zwicky (and W. Baade) propose the existence of neutron stars in 1933. Sidebar: Fritz Zwicky A brilliant, but peculiar astrophysicist and mountain climber:  Sidebar: Fritz Zwicky A brilliant, but peculiar astrophysicist and mountain climber "Non-alpinists again and again ask, why we run up the mountains like mad. Many answers have been given to this question: the greatness of nature, to use the forces of the body, to run away from the daily life or the joy of the adventure (Schiller: "If you don't risk your life, you will never win the life") and so on. But never, I heard the answer which applied to me and my mountain friend Reichstein from the university of Basel, which is: "In daily life, as well as in science, one finds almost never problems, which one can solve alone, complete and in a short time. Also if one tackles them successfully, there are always new aspects popping up, which occupy us for a long time, sometimes for our whole life. We are eager therefore to pursue achievements, which can be closed as a masterpiece, which can be done alone and which nobody will question. The climb of a new mountain or a new difficult path in the mountains is such an achievement". Zwicky (L), Reichstein (R) The discovery of pulsars in the 1960s stimulated interest in neutron stars.:  First detected in 1967 by Cambridge University graduate student Jocelyn Bell. Radio source with an regular on-off-on cycle of exactly 1.3373011 seconds. The discovery of pulsars in the 1960s stimulated interest in neutron stars. Slide11:  Film Excerpt: Discovery of Pulsars by Bell & Hewish The discovery of pulsars in the 1960s stimulated interest in neutron stars.:  First detected in 1967 by Cambridge University graduate student Jocelyn Bell. Radio source with an regular on-off-on cycle of exactly 1.3373011 seconds. Some scientists speculated that this was evidence of an alien civilization’s communication system and dubbed the source LGM: Little Green Men Today, we know pulsars are rapidly spinning neutron stars. The discovery of pulsars in the 1960s stimulated interest in neutron stars. Pulsars are rapidly rotating neutron stars with intense magnetic fields.:  Pulsars are rapidly rotating neutron stars with intense magnetic fields. Early ideas about pulsars were that there were pulsating white dwarfs; however, even a white dwarf (Earth-sized) is too big to oscillate in less than one second. When the Crab Pulsar was detected at the center of the Crab Nebula supernova remnant, astronomers knew pulsars had to be related to supernovae and the stellar core crushed to neutron degeneracy. Pulsar model:  Pulsar model Slide17:  Radio images showing a stellar core (pulsar) ejected during the supernova. Slide18:  PRS EC The physical size of a neutron star is closest to: 1 km 10 km 1,000 km Size of Earth Size of Sun Slide19:  PRS EC Which statement about pulsars is incorrect? All pulsars are neutron stars Pulsars are remnants of supernova Pulsar occasionally have ‘starquakes’ All neutron stars become pulsars Some pulsars spin many time per second Pulsars gradually slow down as they radiate energy into space.:  Pulsars gradually slow down as they radiate energy into space. The Crab Pulsar is slowing 3 x 10-8 seconds per day. Electrons moving in a circular path at enormously high speed release energy in the form of synchrotron radiation. The age of a pulsar can be measured by how fast it is currently spinning. Superfluidity and superconductivity are among the strange properties of neutron stars.:  Superfluidity and superconductivity are among the strange properties of neutron stars. Current models of neutron stars suggest that neutrons stars have a solid crust overlying a sea of neutrons that can flow without any friction whatsoever, called superfluidity. In addition to a general slowing of pulsars over time, they sometimes exhibit a sudden speed-up – called a glitch – caused by an instability in a slowing crust but a still rapidly rotating interior. Slide22:  The fastest pulsars were probably created by mass transfer in close binary systems. Astronomers have cataloged at least 50 super fast pulsars, called millisecond pulsars, that have been “sped up” by mass from a companion star that hits the neutron star and speeds it up. Slide23:  Pulsating X-ray sources are also neutron stars in close binary systems. Far more luminous than typical pulsars, pulsating X-ray sources are close binary systems where the neutron star in the pair steals matter from its companion and quickly heats it at its magnetic pole. Slide24:  PRS EC As a spinning pulsar ages, it: Slows down Speeds up Remains the same Varies periodically Depends on size: larger pulsars slow down, smaller pulsar speed up Explosive thermonuclear processes on white dwarfs and neutron stars produce novae and bursters.:  A NOVA is a sudden brightening and slow dimming of a binary star that has rapidly burned off excess mass obtained from its companion. Explosive thermonuclear processes on white dwarfs and neutron stars produce novae and bursters. This can occur repeatedly as the more mature star alternately burns off stolen matter and then collects more material. Slide28:  PRS EC The person who discovered pulsars was: A graduate student in England A professor at Cambridge University An Astronomer at Caltech An amateur astronomer in New Zealand An engineer at NASA Like a white dwarf, a neutron star has an upper limit on its mass.:  Like a white dwarf, a neutron star has an upper limit on its mass. White dwarfs will collapse if they exceed the Chandrasekhar limit of 1.4 M Neutron star upper mass limits are due to: Degenerate nature of neutrons. Strong nuclear force holding neutrons together. If a neutron star exceeds 3 M then even photons cannot escape the star’s gravity and the object is a bizarre object called a black hole (subject of next lecture!)

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