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Chapter 13 Neutron Stars and Black Holes

Chapter 13 Neutron Stars and Black Holes. Neutron Stars. After a Type I supernova , little or nothing remains of the original star. After a Type II supernova , part of the core may survive. It is very dense – as dense as an atomic nucleus – and is called a neutron star. Neutron Stars.

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Chapter 13 Neutron Stars and Black Holes

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  1. Chapter 13Neutron Stars and Black Holes

  2. Neutron Stars After aType I supernova, little or nothing remains of the original star. After aType II supernova, part of the core may survive. It is very dense – as dense as an atomic nucleus – and is called aneutron star.

  3. Neutron Stars Neutron stars, although they have 1-3 solar masses, are sodensethat they are verysmall. This image shows a 1-solar-mass neutron star, about10 kmin diameter, compared to Manhattan.

  4. Neutron Stars Other important properties of neutron stars (beyond mass and size): Rotation– as the parent star collapses, the neutron core spins very rapidly, conserving angular momentum. Typical periods are fractions of a second. Conservation of angular momentum means: Mass x rotation speed x (radius)² is constant, Magnetic field– again as a result of the collapse, the neutron star’s magnetic field becomes enormously strong.

  5. Pulsars The firstpulsarwas discovered in 1967. It emitted extraordinarilyregular pulses; nothing like it had ever been seen before. After some initial confusion, it was realized that this was aneutron star, spinningvery rapidly.

  6. Pulsars But why would a neutron star flash on and off? This figure illustrates thelighthouse effectresponsible: Strongjetsof matter are emitted at the magnetic poles, as that is where they can escape. If therotation axisis not the same as themagnetic axis, the two beams will sweep outcircular paths. If the Earth lies in one of those paths, we will see the star blinking on and off.

  7. Pulsars Pulsarsradiate their energy away quiterapidly; the radiation weakens and stops in a few tens of millions of years, making the neutron star virtually undetectable. Pulsarsalso will not bevisibleon Earth if their jets are not pointing our way.

  8. Pulsars There is apulsarat the center of theCrab Nebula; the images at right show it in the “off” and “on” positions.

  9. 13.2 Pulsars The Crab pulsar also pulses in thegamma ray spectrum, as does the nearby Geminga pulsar.

  10. Location of pulsars (neutron stars) Fig. 13-21, p.272

  11. Binary pulsar perihelion shift due to gravity waves as predicted by Einstein general theory of gravity 4º per year. Fig. 13-26, p.275

  12. LIGO Gravitational Wave detection facility

  13. LIGO Facility

  14. LIGO Interferometer where mirrors are located

  15. Black Holes The massof a neutron star cannot exceed about3 solar masses. If a core remnant is more massive than that, nothing will stop itscollapse, and it will become smaller and smaller and denser and denser. Eventually the gravitational force is so intense that evenlightcannot escape. The remnant has become ablack hole. Gravity is still there and acts on outside objects!!!

  16. Black Holes Einstein General Theory of Relativity Introduction What are black holes Evidence of Black Holes Star like black holes Massive black holes Exotic properties of black holes

  17. Einstein’s General Theory of Relativity • The presence of a massive body warps the space nearby. • Light rays bent by gravity. • Precession of the perihelion of Mercury. Amount predicted by Einstein was already observed. • Solar eclipse observation of starlight.

  18. Einstein’s Theories of Relativity There is noabsoluteframe of reference, and noabsolutestate of rest. Speed of light c is a constant. Space and time are not independent, but areunifiedasspacetime.

  19. AST1622.swf

  20. Fig. 10-24, p. 212

  21. p. 213

  22. Fig. 10-25, p. 214

  23. 13.6 Einstein’s Theories of Relativity Matter tends towarpspacetime, and in doing so redefinesstraight lines(the path a light beam would take): Ablack holeoccurs when the “indentation” caused by the mass of the hole becomesinfinitelydeep.

  24. Mercury orbit precesses slowly Starlight bends near a source of gravity. Fig. 10-23, p. 212

  25. AST1624.swf If gravity is too strong so that the escape velocity > c, then the star becomes a black hole. Not even light can escape.

  26. Introduction Escape velocity v = [2GM/R]½ If v(escape) is more than c = 300,000 km/sec then light cannot escape and we have a black hole. Examples of escape velocities: • Earth = 11 km/sec • Sun = 610 km/sec.

  27. Introduction • Event horizon [Schwarzschild radius] R • The value of Rat v(escape) = c • c = [2GM/R] ½ • R = 2GM/c². • Example: M=M (mass of Sun), R = 3 km. • R(km) = 3 × (M / M) • Density = M/V = M/[(4/3)πR³] α M/M³ =1/M² • So density of black hole decreases as mass increases.

  28. What are Black Holes? • Any object where the mass is inside the event horizon. • Even though the mass can have any value, we only know the existence of black holes with masses greater than the mass of the Sun, all the way up to a several billion solar masses. • Since we cannot “see” inside a black hole, we cannot find out the composition. • In stellar size black holes the density exceeds that of neutron stars. Matter is squeezed beyond neutron star size. • Black holes have mass (has gravitational force outside the event horizon), angular momentum and charge.

  29. Black Holes The radiusat which theescape speedfrom the black hole equals thespeed of lightis called theSchwarzschild radius. The Earth’s Schwarzschild radius is about acentimeter; the Sun’s is about3 km. Once the black hole hascollapsed, the Schwarzschild radius takes on another meaning – it is theevent horizon. Nothing within the event horizon can escape the black hole.

  30. Space Travel Near Black Holes What’s insidea black hole? No one knows, of course; present theory predicts that the mass collapses until its radius iszeroand its densityinfinite; this is unlikely to be what actually happens. Until we learn more about what happens in suchextreme conditions, the interiors of black holes will remain a mystery.

  31. Evidence of Stellar Size Black Holes • Produced during supernova of massive stars. • Look for double star system with only one visible star, but with a massive non visible star. Kepler’s Third Law (1618) M1 + M2 = a3/p2. where M is in units of C, a distance between stars in AU and p is the period of revolution in years. • Cygnus X1 is the first detected black hole and has a mass of at least 6M. • X-ray produced by in-falling matter from outside of event horizon. • So far about 10 stellar size black holes been observed.

  32. Observational Evidence for Black Holes • Cygnus X-1is a very strong black-hole candidate: • Its visible partner is about25solar masses • The system’s total mass is about35solar masses, so the X-ray source must be about10solar masses • Hot gasappears to be flowing from the visible star to an unseen companion • Shorttime-scale variationsindicate that the source must be verysmall

  33. Cygnus X-1 Fig. 14-10a, p.286

  34. 13.8 Observational Evidence for Black Holes Cygnus X-1, in visible light and X-rays:

  35. How Black Holes are formed Two sources • Original. Formed at the beginning of time. • Formed during supernova

  36. NGC 6240-- Two Black Holes revolving around each other, observed by Chandra X-ray observatory p.280

  37. The Cheshire cat From Alice in Wonderland Fig. 14-8, p.285

  38. A few black hole examples Name Distance Period Mass Companion from us M(Sun) • Cygnus X1 8K ly 5.6 d 10-15 Type O • LMC X3 175K 1.7 4-11 B • LMC X1 175K 4.2 4-10 O • V616 3K 0.3 3-4 K • V404 11K 6.5 8-15 K • J0422+32 8K 0.2 4.5 M • J1655-40 10K 2.6 4-5 F • GS2000+25 8K 0.3 5-8 K • Nova Ophiuchi 0.7 4-6 K

  39. Star Like Black Holes • Mass about that of mass of the Sun up to about 20 solar masses. • Formed during supernovae. • Evidence from binary (double) stars, where the mass of the unseen star can be computed. • About ten such objects have been observed.

  40. Massive Black Holes • Center of each galaxy, except irregular galaxies. • Center of globular clusters. M = 109M M = 3×109M

  41. Massive Black Hole examples • Milky Way 2.6 Million Solar mass • Andromeda 30 • M32 3 • NGC4261 400 • M87 3,000 • M104 1,000 • NGC 3377 100

  42. Exotic Properties of Black Holes • Black hole is a singularity. • Black holes with spin collapse into a ring. • If you can pass through the ring of a black hole, information on space and time is lost (discontinuous). I.e. you can end up at a different time and different space. Not too likely. • Black holes that connect two different parts of the universe is called worm hole.

  43. Black Holes are not very BLACK • Hawkins has shown that black holes can radiate energy, called Hawkins radiation. • The decay time of the black hole is related to the mass. The more massive , the longer the decay time. • Very low mass black holes (micro black holes) decay almost instantly. • Stellar or galactic black hole decay times are much longer than the age of the universe.

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