DOCSLIB.ORG

Discovery of Pulsars • Discovered by Jocelyn Bell and Tony Hewish in 1967

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Discovery of Pulsars • Discovered by Jocelyn Bell and Tony Hewish in 1967 Discovery of pulsars • Discovered by Jocelyn Bell and Tony Hewish in 1967. • The signal was extremely regular (at about 3 pulses per second). • Little Green Men? No. – While av. very constant, pulses individually irregular – no modulation – broad-band – no planetary motion PSR 1919+21 Pulsar as a rotating neutron star • Neutron stars are the only type of objects known to be capable of such a rapid and strictly periodic signal (1 part in 1012 ). • Beamed radiation from neutron star is modulated by the rotation. • Isolated neutron stars radiate energy via slowing Period down of rapid spinning motion (P usually < 1 sec, dP/dt>0) pulse • For Crab pulsar: P = 0.033 s, dP/dt=3 x 10-11 s-1 ~P/10 interpulse Rotating neutron star • Gravitational force > centrifugal force GMm/r2 > mv2/r where M=4/3πr3ρ and v= 2πr/P Æ ρ > 3π/(GP2) = 1.3 x 1011 g/cm3 for the Crab pulsar This is too high for a white dwarf (which has a density of ~ 106 g/cm3), so it must be a neutron star. • The energy lost by the pulsar dE/dt ~ ½ (2/5MR2)(2π/P)2 – Assuming M~1 solar, R = 10 km for the Crab pulsar dE/dt ~ 3 x 1038 ergs/s – Greater than that inferred from the observed emission (e.g., the 2-20keV range) ~ 1.5 x 1037 ergs/s – Thus, the pulsar can power the nebula. Neutron Star Magnetic Field Magnetic field induction Æ high B of NS • A radius collapse from 7 x 105 km (Sun) to 10 km 4 2 Bns ~ (7 x 10 ) BSun 2 • For the general BSun ~ 10 G, 11 Bns ~ 5 x 10 G • The B force FB~ eVB ~ 0.3 dyn is a factor of ~ 1013 larger than the gravitational 2 -14 force Fg = GmeM/r ~ 10 dyn. Thus FB dominates the particle distribution near a NS. Neutron star magnetosphere Light cylinder • Magnetic axis and rotation axis cannot be co-aligned for a pulsar. ω r=c/ω • Electric field induced Open immediately outside NS surface field 8 (E ~ 2 x 10 B12/P) accelerates particles to very high energy Æ Outer gap cosmic rays • This alters the plasma NS distribution and magnetic field configuration. • charged particles are "guided" and accelerated by magnetic field Æ radiation beams from poles and outer gaps. Radiation Mechanisms Radio beam r=c/ω Light cylinder Closed magnetosphere X-ray beam Irregularities in pulsar emission • Short timescales - pulsar slow-down rate is remarkably uniform. • Longer timescales - irregularities apparent - in particular, ‘glitches’ - discontinuous changes of period. • Glitches are caused by sudden angular momentum transfer from the faster rotating superfluid core to the slowed-down “ normal” surface crust. •For example, ∆P/P ~ 10-10 is the typical observed value for the Crab pulsar. P glitch t PSR J0537-6910 (16 ms pulsar) Timing residuals PSR B1913+16 Hulse & Taylor binary pulsar •The orbital period of the binary pulsar decreases with time • This proves the existence of gravitational waves and gave to Hulse & Taylor the Nobel price in 1993. Taylor J.H. 1993, Pulsar Population • τ ~ P/(dP/dt) is known as “characteristic age” of a pulsar (e.g., τ ~ 2000 years for Crab) • In reality, may be longer than the real age. • Pulsar characteristic lifetime ~ 107 years • Total # of observable pulsars ~ 5 x 104 • To sustain this population then, 1 pulsar must form every 50 years. cf SN rate of 1 every 50 years • only ~10 pulsars associated with visible SNRs (pulsar lifetime 1-10 x 107 yrs, SNRs 1-10 x 104 yrs ... so consistent) • But not all SN may produce pulsars! Pulsars: summary • Pulsar can be thought of as a non-aligned rotating magnet. • Electromagnetic forces dominate over gravitational in magnetosphere. • Field lines which extend beyond the light cylinder are open. • Particles escape along open field lines, accelerated by strong electric fields. • Even if a plasma is absent, a pulsar radiates as a rotating non-aligned magnet (e.g., magnetic dipole model). • The energy loss slows down the pulsar except for glitches. • binary companion => rising from the dead again ! Lab Assignment 2 • Suggested targets: – NGC 4594 (except for the nuclear source; Pellegrini et al. 2002) – NGC 4945 (except for the nuclear source; Schurch et al. 2002) – NGC 1068 (except for the nuclear source; Young et al. (2001) Compact Objects • White Dwarfs, neutron stars, and black holes • White dwarfs are fairly common (~ 6 ×10-3 pc-3, or about 50 within 40 light years of the Sun). • The majority of the 100 brightest objects on the X-ray sky are accreting binaries with white dwarf, neutron star, or black hole primaries • Neutron stars in the form of rotation powered pulsars are among the most conspicuous objects on the radio sky. • They are extreme objects, with densities of the order of 106 gcm-3 (white dwarfs) and 4×1014 gcm-3 (neutron stars), respectively • They serve as a laboratory for physics under extreme conditions. White Dwarf (WD) WD: Formation • End product of Sun-like stars. During their MS phase, helium ``ash'' builds up. • Nursed in low-mass red giant stars. During hydrogen shell burning, the helium core contracts and becomes fully degenerate • electron (exclusion) degeneracy pressure opposes gravity • pressure ∝ density, but NOT ∝ temperature (like for gas) • Normal, steady, nuclear fusion not allowed in degenerate matter. Thus fusion occurs in a runaway (eg, helium flash) WD: planetary nebula stage Shedding the extended atmosphere => the ``planetary nebula'' stage WD: Mass • An O-Ne-Mg degenerate core (leading to a ~ 1Msun WD) • C-O core ( ~ 0.6 Msun WD) • or a He core (lower mass WD, but rare because universe is still too young). •if M > 1.4 MSUN gravity wins (Chandrasekhar Limit) => collapse followed by explosion (supernova) No White Dwarfs with M > 1.4 MSUN ! WD: evolution Young white dwarfs have plenty of internal thermal energy left from the nuclear fusion; they radiate it away and cool over the next billions of years. • initially very hot ~ 105 K (surface), rapidly cool to ~ 104 K (over millions of years) • finally, cool off (and dim) to become BLACK DWARF • If the mass of a white dwarf (in an accreting binary) increases beyond the Chandrasekhar limit, it will cause either: – Complete detonation of the white dwarf = Type Ia Supernova. – Accretion Induced Collapse, forming a neutron star. WD: Structure Gas Properties at “low T” • No two Fermions can occupy the same quantum mechanical states (=Pauli's exclusion principle). • Distribution function of ideal gas is: -1 • f(E) = [exp((E-EF)/kT)±1] – plus sign for Fermions – negative sign for Bosons. • For both types, at high enough temperature (kT >> EF) and low enough pressure, the distribution becomes Maxwellian (f(E) ∝ exp((EF-E)/kT)). • Fermions at the low temperature limit fills all states up to E = EF and none have higher energies (=complete degeneracy). Equation of state of Fermi gas Number of quantum states in a phase space dxdydzdpx dp y dpz dN = 2 3 (2πh) Total number of quantum states (or total number of particles of a Fermi gas at T = 0 K) pF 4πp 2dpV Vp 3 N = 2 = F ∫ 3 2 3 0 (2πh) 3π h 1 Total energy pF 4πp 2dpV E = ε 2 ε ∫ 3 F 0 (2πh) kT ε where ε is the particle energy p 2 DEGENERACY TEMPERATURE ε = for a non - relativistic case 2m kT ≈ ε =p 2/2m = cp for a relativistic case F F F Statistical Physics by Landau & Lifshitz Equation of state of Fermi gas (cont.) Pressure : 5/3 2E (3π 2 ) 2 / 3 2 N P = = h for a non - relativistic case 3V 5m V 4/3 1E (3π 2 )1/ 3 c N = = h for a relativistic case 3V 4 V where N ρ = V µ Therefore,the equation of state P = κρ γ where γ = 5/3 (non - relativistic)or = 4 / 3 (relativistic), corresponding to polytropic index n = 3/2 and = 3, respectively. Chandrasekhar limit These equations allow for the Hydrostatic equilibrium calculation of the density as a dP GMρ function of radius r. = − dr r 2 For example (see Longair Vol 2, section 15.3), Mass consservation 2n/(1-n) ρc ∝ R dM M ∝ρR3 ∝ R(3-n)/(1-n) = 4πr 2 ρ c dr Thus M ∝ R-3, if n=3/2. As M↑, R↓ and ρc↑, until a critical M is research, nÆ 3 (relativisitic), when M is independent of R. The critical mass is the Chandrasekhar Mch = 1.46 solar mass. Basic properties of a neutron star • radius ~ 10 km • mass ~ 1-3 MSUN (cannot be more or Black Hole) • density ~ billion tons/cm3 => solid => low luminosity (dim) • super strong magnetic field (rotation + squeezing) ~ 1012 times Earth • extremely rapid rotation P ~ 0.01 seconds Black Holes in General Relativity • the spherically symmetric solution for a single mass (Schwarzschild metric in natural units; G = c = 1): ds2 = - (1-2M/r) dt2+ (1-2M/r)-1 dr2 + r2 dθ2+ r2 sin2 θdφ2 • The Schwarzschild radius, 2 Rs = 2GM/c ∼ 3 [M/( M\odot )] km. defines the event horizon. – Once inside the event horizon, no light nor particle can escape to the outside: thus, J.C. Wheeler coined the term, black hole. – Objects just outside an event horizon are seen to experience severe time dilation by an observer at infinity. • A singularity at the center of a black hole -- a point of infinite density, where the known laws of physics break down. • Black holes can have only three measurable properties: mass, spin, and charge. • Real black holes are unlikely to accumulate a significant charge, but spinning black holes (described by the Kerr metric) are highly likely.
Load more

Recommended publications
  • Plasma Physics and Pulsars
    Plasma Physics and Pulsars On the evolution of compact o bjects and plasma physics in weak and strong gravitational and electromagnetic fields by Anouk Ehreiser supervised by Axel Jessner, Maria Massi and Li Kejia as part of an internship at the Max Planck Institute for Radioastronomy, Bonn March 2010 2 This composition was written as part of two internships at the Max Planck Institute for Radioastronomy in April 2009 at the Radiotelescope in Effelsberg and in February/March 2010 at the Institute in Bonn. I am very grateful for the support, expertise and patience of Axel Jessner, Maria Massi and Li Kejia, who supervised my internship and introduced me to the basic concepts and the current research in the field. Contents I. Life-cycle of stars 1. Formation and inner structure 2. Gravitational collapse and supernova 3. Star remnants II. Properties of Compact Objects 1. White Dwarfs 2. Neutron Stars 3. Black Holes 4. Hypothetical Quark Stars 5. Relativistic Effects III. Plasma Physics 1. Essentials 2. Single Particle Motion in a magnetic field 3. Interaction of plasma flows with magnetic fields – the aurora as an example IV. Pulsars 1. The Discovery of Pulsars 2. Basic Features of Pulsar Signals 3. Theoretical models for the Pulsar Magnetosphere and Emission Mechanism 4. Towards a Dynamical Model of Pulsar Electrodynamics References 3 Plasma Physics and Pulsars I. The life-cycle of stars 1. Formation and inner structure Stars are formed in molecular clouds in the interstellar medium, which consist mostly of molecular hydrogen (primordial elements made a few minutes after the beginning of the universe) and dust.
    [Show full text]
  • Beyond the Solar System Homework for Geology 8
    DATE DUE: Name: Ms. Terry J. Boroughs Geology 8 Section: Beyond the Solar System Instructions: Read each question carefully before selecting the BEST answer. Use GEOLOGIC VOCABULARY where APPLICABLE! Provide concise, but detailed answers to essay and fill-in questions. Use an 882-e scantron for your multiple choice and true/false answers. Multiple Choice 1. Which one of the objects listed below has the largest size? A. Galactic clusters. B. Galaxies. C. Stars. D. Nebula. E. Planets. 2. Which one of the objects listed below has the smallest size? A. Galactic clusters. B. Galaxies. C. Stars. D. Nebula. E. Planets. 3. The Sun belongs to this class of stars. A. Black hole C. Black dwarf D. Main-sequence star B. Red giant E. White dwarf 4. The distance to nearby stars can be determined from: A. Fluorescence. D. Stellar parallax. B. Stellar mass. E. Emission nebulae. C. Stellar distances cannot be measured directly 5. Hubble's law states that galaxies are receding from us at a speed that is proportional to their: A. Distance. B. Orientation. C. Galactic position. D. Volume. E. Mass. 6. Our galaxy is called the A. Milky Way galaxy. D. Panorama galaxy. B. Orion galaxy. E. Pleiades galaxy. C. Great Galaxy in Andromeda. 7. The discovery that the universe appears to be expanding led to a widely accepted theory called A. The Big Bang Theory. C. Hubble's Law. D. Solar Nebular Theory B. The Doppler Effect. E. The Seyfert Theory. 8. One of the most common units used to express stellar distances is the A.
    [Show full text]
  • SHELL BURNING STARS: Red Giants and Red Supergiants
    SHELL BURNING STARS: Red Giants and Red Supergiants There is a large variety of stellar models which have a distinct core – envelope structure. While any main sequence star, or any white dwarf, may be well approximated with a single polytropic model, the stars with the core – envelope structure may be approximated with a composite polytrope: one for the core, another for the envelope, with a very large difference in the “K” constants between the two. This is a consequence of a very large difference in the specific entropies between the core and the envelope. The original reason for the difference is due to a jump in chemical composition. For example, the core may have no hydrogen, and mostly helium, while the envelope may be hydrogen rich. As a result, there is a nuclear burning shell at the bottom of the envelope; hydrogen burning shell in our example. The heat generated in the shell is diffusing out with radiation, and keeps the entropy very high throughout the envelope. The core – envelope structure is most pronounced when the core is degenerate, and its specific entropy near zero. It is supported against its own gravity with the non-thermal pressure of degenerate electron gas, while all stellar luminosity, and all entropy for the envelope, are provided by the shell source. A common property of stars with well developed core – envelope structure is not only a very large jump in specific entropy but also a very large difference in pressure between the center, Pc, the shell, Psh, and the photosphere, Pph. Of course, the two characteristics are closely related to each other.
    [Show full text]
  • A Star Is Born
    A STAR IS BORN Overview: Students will research the four stages of the life cycle of a star then further research the ramifications of the stage of the sun on Earth. Objectives: The student will: • research, summarize and illustrate the proper sequence in the life cycle of a star; • share findings with peers; and • investigate Earth’s personal star, the sun, and what will eventually come of it. Targeted Alaska Grade Level Expectations: Science [9] SA1.1 The student demonstrates an understanding of the processes of science by asking questions, predicting, observing, describing, measuring, classifying, making generalizations, inferring, and communicating. [9] SD4.1 The student demonstrates an understanding of the theories regarding the origin and evolution of the universe by recognizing that a star changes over time. [10-11] SA1.1 The student demonstrates an understanding of the processes of science by asking questions, predicting, observing, describing, measuring, classifying, making generalizations, analyzing data, developing models, inferring, and communicating. [10] SD4.1 The student demonstrates an understanding of the theories regarding the origin and evolution of the universe by recognizing phenomena in the universe (i.e., black holes, nebula). [11] SD4.1 The student demonstrates an understanding of the theories regarding the origin and evolution of the universe by describing phenomena in the universe (i.e., black holes, nebula). Vocabulary: black dwarf – the celestial object that remains after a white dwarf has used up all of its
    [Show full text]
  • Astro 404 Lecture 30 Nov. 6, 2019 Announcements
    Astro 404 Lecture 30 Nov. 6, 2019 Announcements: • Problem Set 9 due Fri Nov 8 2 3/2 typo corrected in L24 notes: nQ = (2πmkT/h ) • Office Hours: Instructor – after class or by appointment • TA: Thursday noon-1pm or by appointment • Exam: grading elves hard at work Last time: low-mass stars after main sequence Q: burning phases? 1 Q: shell burning “mirror” principle? Low-Mass Stars After Main Sequence unburnt H ⋆ helium core contracts H He H burning in shell around core He outer layers expand → red giant “mirror” effect of shell burning: • core contraction, envelope expansion • total gravitational potential energy Ω roughly conserved core becomes more tightly bound, envelope less bound ⋆ helium ignition degenerate core unburnt H H He runaway burning: helium flash inert He → 12 He C+O 2 then core helium burning 3α C and shell H burning “horizontal branch” star unburnt H H He ⋆ for solar mass stars: after CO core forms inert He He C • helium shell burning begins inert C • hydrogen shell burning continues Q: star path on HR diagram during these phases? 3 Low-Mass Post-Main-Sequence: HR Diagram ⋆ H shell burning ↔ red giant ⋆ He flash marks “tip of the red giant branch” ⋆ core He fusion ↔ horizontal branch ⋆ He + H shell burning ↔ asymptotic giant branch asymptotic giant branch H+He shell burn He flash core He burn L main sequence horizontal branch red giant branch H shell burning Sun Luminosity 4 Temperature T iClicker Poll: AGB Star Intershell Region in AGB phase: burning in two shells, no core fusion unburnt H H He inert He He C Vote your conscience–all
    [Show full text]
  • CHIME and Probing the Origin of Fast Radio Bursts by Liam Dean Connor a Thesis Submitted in Conformity with the Requirements
    CHIME and Probing the Origin of Fast Radio Bursts by Liam Dean Connor A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Graduate Department of Astronomy and Astrophysics University of Toronto c Copyright 2016 by Liam Dean Connor Abstract CHIME and Probing the Origin of Fast Radio Bursts Liam Dean Connor Doctor of Philosophy Graduate Department of Astronomy and Astrophysics University of Toronto 2016 The time-variable long-wavelength sky harbours a number of known but unsolved astro- physical problems, and surely many more undiscovered phenomena. With modern tools such problems will become tractable, and new classes of astronomical objects will be revealed. These tools include digital telescopes made from powerful computing clusters, and improved theoretical methods. In this thesis we employ such devices to understand better several puzzles in the time-domain radio sky. Our primary focus is on the origin of fast radio bursts (FRBs), a new class of transients of which there seem to be thousands per sky per day. We offer a model in which FRBs are extragalactic but non-cosmological pulsars in young supernova remnants. Since this theoretical work was done, observations have corroborated the picture of FRBs as young rotating neutron stars, including the non-Poissonian repetition of FRB 121102. We also present statistical arguments regard- ing the nature and location of FRBs. These include reinstituting the classic V=Vmax-test @ log N to measure the brightness distribution of FRBs, i.e., constraining @ log S . We find consis- tency with a Euclidean distribution. This means current observations cannot distinguish between a cosmological population and a more local uniform population, unless added assumptions are made.
    [Show full text]
  • Equation of State of a Dense and Magnetized Fermion System Israel Portillo Vazquez University of Texas at El Paso, [email protected]
    University of Texas at El Paso DigitalCommons@UTEP Open Access Theses & Dissertations 2011-01-01 Equation Of State Of A Dense And Magnetized Fermion System Israel Portillo Vazquez University of Texas at El Paso, [email protected] Follow this and additional works at: https://digitalcommons.utep.edu/open_etd Part of the Physics Commons Recommended Citation Portillo Vazquez, Israel, "Equation Of State Of A Dense And Magnetized Fermion System" (2011). Open Access Theses & Dissertations. 2365. https://digitalcommons.utep.edu/open_etd/2365 This is brought to you for free and open access by DigitalCommons@UTEP. It has been accepted for inclusion in Open Access Theses & Dissertations by an authorized administrator of DigitalCommons@UTEP. For more information, please contact [email protected]. EQUATION OF STATE OF A DENSE AND MAGNETIZED FERMION SYSTEM ISRAEL PORTILLO VAZQUEZ Department of Physics APPROVED: Efrain J. Ferrer , Ph.D., Chair Vivian Incera, Ph.D. Mohamed A. Khamsi, Ph.D. Benjamin C. Flores, Ph.D. Acting Dean of the Graduate School Copyright © by Israel Portillo Vazquez 2011 EQUATION OF STATE OF A DENSE AND MAGNETIZED FERMION SYSTEM by ISRAEL PORTILLO VAZQUEZ, MS THESIS Presented to the Faculty of the Graduate School of The University of Texas at El Paso in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE Department of Physics THE UNIVERSITY OF TEXAS AT EL PASO August 2011 Acknowledgements First, I would like to acknowledge the advice and guidance of my mentor Dr. Efrain Ferrer. During the time I have been working with him, the way in which I think about nature has changed considerably.
    [Show full text]
  • The Core Helium Flash Revisited
    A&A 520, A114 (2010) Astronomy DOI: 10.1051/0004-6361/201014461 & c ESO 2010 Astrophysics The core helium flash revisited III. From Population I to Population III stars M. Mocák1,S.W.Campbell2,3,E.Müller4, and K. Kifonidis4 1 Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, CP 226, 1050 Brussels, Belgium e-mail: [email protected] 2 Departament de Física i Enginyeria Nuclear, EUETIB, Universitat Politécnica de Catalunya, C./Comte d’Urgell 187, 08036 Barcelona, Spain e-mail: [email protected] 3 Centre for Stellar and Planetary Astrophysics, School of Mathematical Sciences, Monash University, Melbourne 3800, Australia 4 Max-Planck-Institut für Astrophysik, Postfach 1312, 85741 Garching, Germany Received 18 March 2010 / Accepted 13 June 2010 ABSTRACT Context. Degenerate ignition of helium in low-mass stars at the end of the red giant branch phase leads to dynamic convection in their helium cores. One-dimensional (1D) stellar modeling of this intrinsically multi-dimensional dynamic event is likely to be inadequate. Previous hydrodynamic simulations imply that the single convection zone in the helium core of metal-rich Pop I stars grows during the flash on a dynamic timescale. This may lead to hydrogen injection into the core and to a double convection zone structure as known from one-dimensional core helium flash simulations of low-mass Pop III stars. Aims. We perform hydrodynamic simulations of the core helium flash in two and three dimensions to better constrain the nature of these events. To this end we study the hydrodynamics of convection within the helium cores of a 1.25 M metal-rich Pop I star (Z = 0.02), and, for the first time, a 0.85 M metal-free Pop III star (Z = 0) near the peak of the flash.
    [Show full text]
  • Neutron Stars & Black Holes
    Introduction ? Recall that White Dwarfs are the second most common type of star. ? They are the remains of medium-sized stars - hydrogen fused to helium Neutron Stars - failed to ignite carbon - drove away their envelopes to from planetary nebulae & - collapsed and cooled to form White Dwarfs ? The more massive a White Dwarf, the smaller its radius Black Holes ? Stars more massive than the Chandrasekhar limit of 1.4 solar masses cannot be White Dwarfs Formation of Neutron Stars As the core of a massive star (residual mass greater than 1.4 ?Supernova 1987A solar masses) begins to collapse: (arrow) in the Large - density quickly reaches that of a white dwarf Magellanic Cloud was - but weight is too great to be supported by degenerate the first supernova electrons visible to the naked eye - collapse of core continues; atomic nuclei are broken apart since 1604. by gamma rays - Almost instantaneously, the increasing density forces freed electrons to absorb electrons to form neutrons - the star blasts away in a supernova explosion leaving behind a neutron star. Properties of Neutron Stars Crab Nebula ? Neutrons stars predicted to have a radius of about 10 km ? In CE 1054, Chinese and a density of 1014 g/cm3 . astronomers saw a ? This density is about the same as the nucleus supernova ? A sugar-cube-sized lump of this material would weigh 100 ? Pulsar is at center (arrow) million tons ? It is very energetic; pulses ? The mass of a neutron star cannot be more than 2-3 solar are detectable at visual masses wavelengths ? Neutron stars are predicted to rotate very fast, to be very ? Inset image taken by hot, and have a strong magnetic field.
    [Show full text]
  • Extreme Stars White Dwarfs & Neutron Stars
    Extreme Stars White Dwarfs & Neutron Stars White Dwarfs: • Remnants of low-mass stars • Supported by Electron Degeneracy Pressure • Maximum Mass ~1.4 Msun (Chandrasekhar Mass) Neutron Stars: • Remnants of some post-supernova massive stars • Supported by Neutron Degeneracy Pressure • Pulsar = rapidly spinning magnetized neutron star 1 When electron degeneracy becomes nearly relativistic, equation of state is “softer” and star gets smaller rapidly with mass The Stellar Graveyard Question: What happens to the cores of dead stars? Answer: They continue to collapse until either: • A new pressure law takes hold to halt further collapse & they settle into a new hydrostatic equilibrium. • If too massive they collapse to “a singularity” and become a Black Hole. All of these are seen as the remnants of stellar evolution. 7 Degenerate Gas Law At high density, a new gas law takes over: • Pack many electrons into a tiny volume • These electrons fill all low-energy states • Only high-energy = high-pressure states left Result is a "Degenerate Gas": • Pressure is independent of Temperature. • Compression does not lead to heating. This means that the objects could be very cold but still have enough pressure to maintain a state of Hydrostatic Equilibrium (BIG ROCKS!, well almost…) Can such objects exist? 8 White Dwarfs These are the remnant cores of stars with M < 8 Msun. • Supported against gravity by Electron Degeneracy Pressure • M<4 Msun: C-O White Dwarfs • M=4-8 Msun: O-Ne-Mg White Dwarfs Properties: • Mass < 1.4 Msun • Radius ~ Rearth (<0.02 Rsun) • Density ~ 105-6 g/cc • Escape Speed: 0.02c (2% speed of light) No nuclear fusion or gravitational contraction.
    [Show full text]
  • Chapter 1: How the Sun Came to Be: Stellar Evolution
    Chapter 1 SOLAR PHYSICS AND TERRESTRIAL EFFECTS 2+ 4= Chapter 1 How the Sun Came to Be: Stellar Evolution It was not until about 1600 that anyone speculated that the Sun and the stars were the same kind of objects. We now know that the Sun is one of about 100,000,000,000 (1011) stars in our own galaxy, the Milky Way, and that there are probably at least 1011 galaxies in the Universe. The Sun seems to be a very average, middle-aged star some 4.5 billion years old with our nearest neighbor star about 4 light-years away. Our own location in the galaxy is toward the outer edge, about 30,000 light-years from the galactic center. The solar system orbits the center of the galaxy with a period of about 200,000,000 years, an amount of time we may think of as a Sun-year. In its life so far, the Sun has made about 22 trips around the galaxy; like a 22-year old human, it is still in the prime of its life. Section 1.—The Protostar Current theories hold that about 5 billion years ago the Sun began to form from a huge dark cloud of dust and vapor that included the remnants of earlier stars which had exploded. Under the influence of gravity the cloud began to contract and rotate. The contraction rate near the center was greatest, and gradually a dense central core formed. As the rotation rate increased, due to conservation of angular momentum, the outer parts began to flatten.
    [Show full text]
  • Announcements
    Announcements • Next Session – Stellar evolution • Low-mass stars • Binaries • High-mass stars – Supernovae – Synthesis of the elements • Note: Thursday Nov 11 is a campus holiday Red Giant 8 100Ro 10 years L 10 3Ro, 10 years Temperature Red Giant Hydrogen fusion shell Contracting helium core Electron Degeneracy • Pauli Exclusion Principle says that you can only have two electrons per unit 6-D phase- space volume in a gas. DxDyDzDpxDpyDpz † Red Giants • RG Helium core is support against gravity by electron degeneracy • Electron-degenerate gases do not expand with increasing temperature (no thermostat) • As the Temperature gets to 100 x 106K the “triple-alpha” process (Helium fusion to Carbon) can happen. Helium fusion/flash Helium fusion requires two steps: He4 + He4 -> Be8 Be8 + He4 -> C12 The Berylium falls apart in 10-6 seconds so you need not only high enough T to overcome the electric forces, you also need very high density. Helium Flash • The Temp and Density get high enough for the triple-alpha reaction as a star approaches the tip of the RGB. • Because the core is supported by electron degeneracy (with no temperature dependence) when the triple-alpha starts, there is no corresponding expansion of the core. So the temperature skyrockets and the fusion rate grows tremendously in the `helium flash’. Helium Flash • The big increase in the core temperature adds momentum phase space and within a couple of hours of the onset of the helium flash, the electrons gas is no longer degenerate and the core settles down into `normal’ helium fusion. • There is little outward sign of the helium flash, but the rearrangment of the core stops the trip up the RGB and the star settles onto the horizontal branch.
    [Show full text]