Published on October 7, 2007
Ch11 Neutron Stars and Black Holes: Ch11 Neutron Stars and Black Holes Strange States of Matter Goals: Goals Describe the properties of neutron stars and explain how these strange objects are formed. Explain the nature and origin of pulsars and account for their characteristic radiation. List and explain some of the observable properties of neutron-star binary systems Discuss the basic characteristics of gamma-ray bursts, and some theoretical attempts to explain them Describe how black holes are formed and discuss their effects on matter and radiation in their vicinity. More Goals: More Goals Relate the phenomena that occur near black holes due to the warping of space around them. Discuss the difficulties in observing black holes and explain some of the ways in which the presence of a black hole might be detected. Neutron Star: Neutron Star Highly dense: 1017 to 1018 kg/m3 similar to nucleus Rapid rotation due to conservation of angular momentum. Intense magnetic fields. Original field concentrated with the matter as it collapses. Relative Size of Neutron Star: Fig. 11-1, p. 208 Figure 11.1: A tennis ball and a road map illustrate the relative size of a neutron star. Such an object, containing slightly more than the mass of the sun, would fit with room to spare inside the beltway around Washington, D.C. (Photo by author) Relative Size of Neutron Star Pulsar Radiation: Pulsar Radiation Spin rate will decrease with time as it radiates energy into space. This will take millions of years. Not all neutron stars are pulsars. Pulsar history: Pulsar history Jocelyn Bell 1967 grad student, found precisely timed radio signals.CP 1919 had a period of 1.33730119 s, First thought to be earth based signals then others found celestial in origin, … LGM ? Anthony Hewish, project leader gets Nobel Prize for figuring out explanation LGMs: Fig. 11-2, p. 209 Figure 11.2: The 1967 detection of regularly spaced pulses in the output of a radio telescope led to the discovery of pulsars. This record of the radio signal from the first pulsar, CP1919, contains regularly spaced pulses (marked by ticks). The period is 1.33730119 seconds. LGMs Pulsar: Fig. 11-3, p. 210 Figure 11.3: The pulsar at the center of the Crab Nebula (arrow) is detectable in visual-wavelength photographs. The star just to the right of the pulsar lies much closer to Earth and is not in the Crab Nebula. The white box outlines the area imaged on page 213. (Caltech) Pulsar Slide10: Fig. 11-4, p. 211 Figure 11.4: High-speed images of the Crab Nebula pulsar show it pulsing at visual wavelengths and at X-ray wavelengths. (© AURA, Inc., NOAO, KPNO) The period of pulsation is 33 milliseconds, and each cycle includes two pulses as its two beams of unequal intensity sweep over Earth. (Courtesy F. R. Harnden, Jr., from The Astrophysical Journal, published by the University of Chicago Press; © 1984 The American Astronomical Society) Slide11: Fig. 11-4a, p. 211 Figure 11.4: High-speed images of the Crab Nebula pulsar show it pulsing at visual wavelengths and at X-ray wavelengths. (© AURA, Inc., NOAO, KPNO) The period of pulsation is 33 milliseconds, and each cycle includes two pulses as its two beams of unequal intensity sweep over Earth. (Courtesy F. R. Harnden, Jr., from The Astrophysical Pulsar: Fig. 11-4b, p. 211 Figure 11.4: High-speed images of the Crab Nebula pulsar show it pulsing at visual wavelengths and at X-ray wavelengths. (© AURA, Inc., NOAO, KPNO) The period of pulsation is 33 milliseconds, and each cycle includes two pulses as its two beams of unequal intensity sweep over Earth. (Courtesy F. R. Harnden, Jr., from The Astrophysical Journal, published by the University of Chicago Press; © 1984 The American Astronomical Society) Pulsar Pulse rate and size: Pulse rate and size Objects can not be larger in size than the variation in the signal from the object. A 33 millisecond pulse implies the object is no larger than 33 milli-light seconds across. Pulsar Winds : Fig. 11-5, p. 211 Figure 11.5: The effects of pulsar winds can be seen at X-ray wavelengths. The high energy gas of the winds is sometimes detectable, as is the interaction of the winds with surrounding gas. Not all pulsars have detectable winds. (NASA/CXC/SAO/U. Mass: F. Lu/McGill: V. Kaspi) Pulsar Winds Slide15: Fig. 11-5, p. 211 Figure 11.5: The effects of pulsar winds can be seen at X-ray wavelengths. The high energy gas of the winds is sometimes detectable, as is the interaction of the winds with surrounding gas. Not all pulsars have detectable winds. (NASA/CXC/SAO/U. Mass: F. Lu/McGill: V. Kaspi) Slide16: Fig. 11-5, p. 211 Slide17: Fig. 11-5, p. 211 Slide18: Fig. 11-5, p. 211 Slide19: Fig. 11-5, p. 211 Lighthouse Effect: p. 211 Lighthouse Effect Lighthouse model: Lighthouse model Pulses are explained as a beam of radiation from the magnetic pole. Intense magnetic fields focus charged particles. Magnetic poles and rotational poles do not have to coincide. Slide22: p. 211 Slide23: p. 212 Crab Pulsar: Crab Pulsar Visual image X-ray blink Chandra x_ray image shows disk and jets Light curve Slide25: p. 212 Slide26: p. 212 Slide27: p. 212 Slide28: p. 212 Isolated neutron star with 700,000 K temperature Figure 11.6 Isolated Neutron Star: Figure 11.6 Isolated Neutron Star Asymmetry in supernova explosion can give a recoil velocity to the remnant neutron star. Most have high velocities. Slide30: Fig. 11-7b, p.215 Figure 11.7: The radial velocity of pulsar PSR1913+16 can be found from the Doppler shifts in its pulsation. Analysis of the radial velocity curve allows astronomers to determine the pulsar’s orbit. Here the center of mass does not appear to be at a focus of the elliptical orbit because the orbit is inclined. (Adapted from data by Joseph Taylor and Russell Hulse) Gamma Ray Pulsars: Gamma Ray Pulsars Top: Geminga’s 0.24 s period via xray detecor on Compton Crab 33ms period too fast for the detector response time. Hercules X-1 : Fig. 11-8b, p.215 Figure 11.8: Sometimes the X-ray pulses from Hercules X-1 are on, and sometimes they are off. A graph of X-ray intensity versus time looks like the light curve of an eclipsing binary. (Insets: J. Trümper, Max-Planck Institute) (b) In Hercules X-1 matter flows from a star into an accretion disk around a neutron star producing X rays, which heat the near side of the star to 20,000 K compared with only 7000 K on the far side. X rays turn off when the neutron star is eclipsed behind the star. Hercules X-1 F11.9a: Fig. 11-9a, p.215 Figure 11.9: (a) At visible wavelengths, the center of star cluster NGC6624 is crowded with stars. F11.9a F11.9b: Fig. 11-9b, p.215 Figure 11.9: (b) In the ultraviolet, one object stands out, an X-ray source consisting of a neutron star orbiting a white dwarf. F11.9b F11.9 c X-ray burster: Fig. 11-9c, p.215 Figure 11.9: (c) An artist’s conception shows matter flowing from the white dwarf into an accretion disk around the neutron star. (a and b, Ivan King and NASA/ESA) F11.9 c X-ray burster Slide36: Fig. 11-10a, p.218 Figure 11.10: (a) The dots in this graph are observations showing that the period of pulsar PSR1257+12 varies from its average value by a fraction of a billionth of a second. The blue line shows the variation that would be produced by planets orbiting the pulsar. Pulsar wobble: Fig. 11-10b, p.218 Figure 11.10: (b) As the planets orbit the pulsar, they cause it to wobble by less than 800 km, a distance that is invisibly small in this diagram. (Adapted from data by Alexander Wolszczan) Pulsar wobble X-ray Burster: X-ray Burster Similar t o the model for nova x-ray bursters are though to result from explosion of nuclear fuel from surface of a neutron star. X-ray Emission: X-ray Emission Jets are often found. Infalling matter heats up emits xrays b) SS 433 radio images show jets and rotation of central source about companion Millisecond Pulsar: Millisecond Pulsar Rotation rate of pulsar can be increased by infalling matter. Explains rapid rotation of what should be older pulsars in globular clusters. Binary Exchange: Binary Exchange Supernova should destroy nearby star. How are there binary systems including one neutron star ? Gamma-Ray Bursts: Gamma-Ray Bursts Intense bursts of gamma rays detected by satellites. First thought to be scaled up versions of xray bursters.( in our galaxy) Isotropic distribution doesn’t support this Gamma-Ray Counterparts: Gamma-Ray Counterparts Optical images settle debate. These are extragalactic sources. Energy involved must be huge. Gamma Ray Burst Models: Gamma Ray Burst Models Blackholes: Fig. 11-12, p.220 Figure 11.12: Some supernovae remnants contain no neutron star, perhaps because the supernova formed a black hole instead. However, this supernova remnant, formed by Tycho Brahe’s supernova of 1572, has the chemical composition typical of the remains of a Type Ia supernova. Such supernovae are believed to destroy the white dwarf entirely and do not leave behind a neutron star or a black hole. (John P. Hughes, Rutgers University) Blackholes 22.5 Black Holes: 22.5 Black Holes When an objects gravity is too great for even neutron degeneracy pressure to hold it up it shrinks to a singularity. Escape velocity is greater than speed of light. Schwartzchild radius is where c is the escape velocity, defines the “size” of a black hole. Slide47: Fig. 11-13, p.220 Figure 11.13: A black hole forms when an object collapses to a small size (perhaps to a singularity) and the escape velocity becomes so great light cannot escape. The boundary of the black hole is called the event horizon because any event that occurs inside is invisible to outside observers. The radius of the black hole RS is the Schwarzschild radius. Schwartzchild Radius: Table 11-1, p.221 Schwartzchild Radius Everything has a Schwartzchild radius. It’s just not a blackhole unless its mass is concentrated inside the Schwartzchild radius. Escape Velocity: Fig. 11-11, p.219 Figure 11.11: Escape velocity, the velocity needed to escape from a celestial body, depends on mass. The escape velocity at the surface of a very small body would be so low we could jump into space. Earth’s escape velocity is much larger, about 11 km/s (25,000 mph). Escape Velocity Figure 22.14 Curved Space : Figure 22.14 Curved Space Figure 22.15 Space Warping: Figure 22.15 Space Warping Figure 22.16 Black-Hole Heating: Figure 22.16 Black-Hole Heating Intense gravitational fields will have tidal forces that rip objects apart. Frictional heating is intense . Objects superheat and emit radiation even in the x-ray region. Slide53: Fig. 11-14, p.222 Figure 11.14: Leaping feet first into a black hole. A person of normal proportions (left) would be distorted by tidal forces (right) long before reaching the event horizon around a typical black hole of stellar mass. Tidal forces would stretch the body lengthwise while compressing it laterally. Friction from this distortion would heat the body to high temperatures. Gravitational Redshift: Gravitational Redshift 22.8 Observational Evidence: 22.8 Observational Evidence Gravitational lensing Cygnus X-1: Cygnus X-1 Possible blackhole Slide57: Fig. 11-15, p.222 Figure 11.15: The X-ray source Cygnus X-1 is a supergiant O star and a compact object orbiting each other. Gas from the O star’s stellar wind flows into the hot accretion disk, and the X rays we detect come from the disk. Slide58: Table 11-2, p.223 Black-Hole Binary Model: Black-Hole Binary Model We don’t see the black hole. We see the radiation emitted away from the hole by infalling superheated matter. Slide60: Fig. 11-16, p.224 Figure 11.16: In this artist’s impression, matter from a normal star at left flows into an accretion disk around a compact object at right. Processes in the spinning disk eject gas and radiation in jets perpendicular to the disk. The jet inclined slightly toward us has a blue shift, and the jet inclined slightly away from us has a red shift. (NASA) Intermediate-Mass Black-Holes: Intermediate-Mass Black-Holes M82 Starburst galaxy X-ray observations show collection of objects thought to be black holes 100 to 1000 solar masses. Figure 22.23 Infalling Matter Observed ?: Figure 22.23 Infalling Matter Observed ? Explanation for Hubble uv observations UV Intensity plot has no flash at the end. The matter just disappears. Slide63: Gravity waves should result from accelerating masses. Like em waves result from accelerating charges. Binary systems are masses changing directions ..ie accelerating. No gravity waves have been detected (yet). Slide64: General Theory of Relativity predicts the observed changes in Mercury’s orbit.