November 20, 2016, jeb
Astronomy 122
Prof. J. Brau

Outline

• Neutron Stars
• Pulsars
• Neutron-Star Binaries
• Black Holes
• Black Hole Properties
• Space Travel Near Black Holes
• Black-Hole Evaporation
• Observational Evidence for Black Holes

Neutron Stars

Supernova remnants Following a supernova there may remain a central remnant compressed to the density of nuclear matter, a neutron star This probably only occurs for Type-II supernovae, and only for some at that. Properties of neutron stars Neutron stars are small and massive with a mass greater than the Sun, the neutron star is not much bigger than a city Comparative densities (given in two units, g/cm3 and kg/m3) WATER 1 g/cm3 103 kg/m3 LEAD 11 g/cm3 104 kg/m3 CORE OF SUN 150 g/cm3 1.5 x 105 kg/m3 WHITE DWARF 106 g/cm3 109 kg/m3 NEUTRON STAR 1015 g/cm3 1018 kg/m3 A single thimbleful of neutron-star material would weigh 100 million tons or more, as much as a good-sized mountain on Earth Neutron stars rotate extremely rapidly period of rotation is typically a fraction of a second Some pulsars have periods that are so stable, they are more reliable than the best atomic clocks on Earth May only lose a few seconds in a million years Neutron stars have very strong magnetic fields the field will be trillions (1012) of times the Earth's magnetic field

Pulsars

Can we actually "observe" these very small objects?
YES!


• First observed by Jocelyn Bell in 1967
  • graduate student at Cambridge University
  • she was unaware of the possibility of pulsars
  • studying radio emissions from outer space
  • discovered an object which emitted a regular pattern of "pulses"
    
    



• First thought:
  • Could these be interstellar beacon sent out by extraterrestrial life on another star?
  • Called this source LGM (for little green men)



Bell's research advisor (Anthony Hewish) recognized that these could be the pulsing evidence for neutron stars Hewish developed lighthouse model for rotating neutron star and the observed pulsars Hewish was awarded Nobel Prize in physics for this discovery in 1974 More than 2500 of these objects have been found in the our Milky Way Galaxy Pulsar Catalogue

• The pulsar in the core of the Crab Nebula blinks on and off 30 times a second
  • also in visible light
  • and in X-rays
  • as well as in gamma rays shown along with nearby Geminga pulsar
    • Geminga pulsar is barely visible optically, and undetectable in radio
  • and other wavelengths of EM radiation
  • remnant of supernova that was observed widely in 1054


• An energetic pulsar wind flows outward, primarily in pulsar's equatorial plane
  • Driven by neutron star's strong magnetic field, and rapid rotation
  • Image by Chandra of the pulsar wind of the Crab Nebula


• Not all neutron stars can be detected as pulsars

  The alignment of the "lighthouse" beam must be aimed toward Earth Over time, the rotation and magnetic field will diminish, reducing signal It is estimated there are hundreds of thousands of neutron stars for every one detected Example: an isolated neutron star imaged by the Hubble Space Telescope following X-ray detection (not as a pulsar) 60 pc from Earth, about 1 million years old 30 km diameter 700,000 K surface temperature 25th magnitude object in visible light



• Some supernova remnants have associated pulsars
  • Not all remnants do
  • Typically pulsars have large speeds, thought to have resulted from explosive production

Neutron-Star Binaries

We know most stars are members of binary star systems, so neutron stars might often also be

• This leads to several types of special binaries:
  • X-ray sources
  • millisecond pulsars
  • pulsar planets
  • gamma-ray bursts
  • gravity wave sources

X-ray Sources An X-ray burster produces sudden, intense flash of X-rays (lasting seconds), followed by hours of inactivity, followed by more sudden bursts visible image of the location of an X-ray source, Terzan 2 and X-ray images before and during the X-ray outburst result from accreting neutron star matter is torn from the companion star to the neutron star and a falls toward the surface in an accretion disk with build up of hydrogen on surface, that suddenly ignites into explosive nuclear burning jets of material may also be ejected jets have been observed, as shown here for SS433

• Millisecond pulsars
  A very rapidly rotating neutron star
  • several are observed in globular clusters -> very old
  • these must have spun-up
    • inspiraling matter from the companion star adds angular momentum (spin) to the neutron star
    • Chandra image of core of globular cluster, where more than 10 X-ray sources are found, and more than 1/2 are thought to be millisecond pulsars
    • how does a neutron star find itself in a binary system?
      perhaps long after the supernova explosion


• Pulsar Planets
  Pulse periods are found to vary in quite regular pattern, revealing additional bodies in orbit around the neutron star



Gamma-ray Bursts Discovered in late 1960s by military satellites monitoring nuclear weapons tests Today these bursts are one of astronomy's deepest mysteries Now detected at rate of about 1 per day Uniformly distributed in sky Therefore, not distributed within the Milky Way Galaxy, or would appear as a band across the sky, as the distribution of stars Must be at very great distances from us (say billions of parsecs), which makes their luminosity enormous (this is a conclusion of studying the optical counterparts, such as for GRB080319B) What causes the bursts? Two leading models Hypernova model favored for long duration (more than 2 seconds) bursts. GRB030329 at time of burst and one month later slow fade of afterglow for long duration burst in 2003 explained by hypernova model. Neutron star merger model favored for short duration (less than 2 seconds) bursts.

Binary Pulsars One leading mechanism for explaining gamma-ray bursts is the true end point of a binary-star system Suppose that both members of the binary evolve to become neutron stars As the system continues to evolve, gravitational radiation is released and the two ultra dense stars will spiral in toward each other Once they are within a few kilometers of each another, coalescence is inevitable Such a merger will likely produce a violent explosion, comparable in energy to a supernova, and this could conceivably explain the flashes of gamma rays we observe

Gravity Waves A binary pulsar was discovered by Taylor and Hulse in 1974 Measurements of the periodic Doppler shift of the pulsar's radiation prove that its orbit is slowly shrinking The rate at which the orbit is shrinking is exactly what would be predicted by relativity theory if the energy were being carried off by gravity waves Two large observatories which use laser interferometry (one in Hanford, Washington and one in Louisiana) have been built to look for gravity waves This project is called LIGO (for Laser Interferometer Gravitational Wave Observatory) Our UO experimental gravity group collaborates on the LIGO effort to "observe" gravity waves. Three events have been observed. These events each resulted from the merger of two large black holes. Public lecture I gave earlier this year describing our discovery.

Black Holes




If enough material is left behind in the core after a supernova (more than 3 times M_SUN)
• gravitational pull of the stars matter will crush it inward beyond the neutron star phase
• neutron degeneracy is overcome in the neutron star equivalent of the Chandrasekhar Limit
• nothing can stop the star from collapsing toward a point.
  • resulting object emits no light
  • emits no radiation
  • emits nothing, no information can come out again


Understanding this strange phase of matter and space requires Einstein's theories of relativity
• nothing travels faster than the speed of light
  • speed of light is 300,000 kilometers/second
  • or 186,000 miles/second
• everything (including light) is attracted by gravity


Escape Velocity
Velocity needed to escape the gravitational pull of a body
• example, escape velocity at surface of Earth is 11 km/second
• if Earth were squeezed to a cubic centimeter, escape velocity would reach 300,000 km/second (the speed of light) and it would disappear


Event Horizon
• Schwarzschild radius
critical radius of an object at which the escape velocity is the speed of light, and within which the object can no longer be seen




Object	RSchwarzschild
Earth	1 cm
Jupiter	3 meters
Sun	3 kilometers
3 solar-mass star	9 kilometers



Photon Orbits
• Far from a black hole, all the radiation from a source escapes into space
• At 1.5 Schwarzschild radii, exactly half the radiation emitted by a source escapes into space
  • photons can go into circular orbit around the black hole
• As the source gets closer to the event horizon, most of the radiation gets sucked into the black hole
• At the event horizon, all light rays enter the black hole

Black Hole Properties

This is totally theoretical, based on Einstein's Theory of Relativity

Curved Space
• Close to matter, space is warped
• the more matter, the more extreme the warpage
• close to a black hole, the curvature of space becomes extreme

Consequences of warpage (curvature):
• Black Holes are not cosmic vacuum cleaners
  • That is, far from the black hole there is no influence
  • In fact, only within a few Schwarzschild radii is there a significant effect
  • This means, the black hole does not suck everything into it
• Black Holes are cosmic heaters
  • Matter falling into a black hole is stretched and squeezed
    • Tears matter apart
    • Heats the matter
  • Since matter is heated as it falls into a black hole, it emits radiation
  • For a black hole with the Sun's mass, this radiation will be in the form of X-rays

Space Travel Near Black Holes

Near a black hole the pull of gravity would be extreme
• Closer than 3000 km to a 10-solar mass black hole (100 Schwarzschild radii) the tidal effect of gravity would tear the human body apart (10 g's)

Gravitational Red Shift
• Light emitted from near the black hole is red shifted (this is not the same as the Doppler shift, but shifts the lights wavelength in a similar way)
• The photon is giving up energy as it escapes from the pull of the black hole
• Light from the event horizon is infinitely redshited (beyond perception to infinite wavelength)

Time Dilation
• Clocks near the black hole appear to slow down to an external observer
• A clock will appear to stop altogether at the event horizon

No Theory of the region beyond the event horizon
• Singularity?
• Cosmic Censorship?

Black-Hole Evaporation

Hawking Radiation
• Stephen Hawking realized that Black Holes are not absolutely black
• Particles and anti-particles pairs are continuously created and annihilated in free space
  • this is an understanding of Quantum Physics
• When this pair creation happens near a black hole, it is possible for one of the two particles to cross the event horizon before it meets and annihilates its partner.
  • The other particle would then be free to leave the scene, making the black hole appear to the outside world as a source of matter or radiation.
  • The energy required to create the new particle ultimately comes from the black hole
  • the hole must decrease in mass as it radiates
• Thus black holes will slowly evaporation
• Lifetime:
  • for 1 solar mass, 10⁷⁰ years
  • 10¹² kg, lifetime is the age of the universe
    • this is the mass of a mountain
• so blackholes with masses about the mass of a mountain could have survived since the Big Bang
• it is conceivable that conditions in the very earliest epochs of the universe might have been just right to compress pockets of matter into miniature black holes
  • Such black holes would have Schwarzschild radii of about 10^-15 m
    • comparable to the size of a subatomic particle
  • If they exist, they should be exploding right now
  • Attempts have been made to observe the resultant gamma rays
    • so far without success

Observational Evidence for Black Holes

Stellar Transits
• One approach to search for black holes

Binary Systems
• Infer existence of unseen black hole from motion of companion
  • But the unseen companion is likely a small, dim star; How can we conclude it is a black hole?
    • Mass of several solar masses
    • large amounts of X-ray emission
  • Cygnus X-1 and X-rays from companion
  • Artist's conception of the Cygnus X-1 binary system
  
  
• Gravity waves have been observed from the merging of two large black holes.
  • This evidence shows the merger of two black holes.

Black Holes in Galaxies
• Strong evidence for black holes also comes from observations of the centers of galaxies
  • rapid motion, as if orbiting unseen massive object
  • masses seem to be millions to billions of times the mass of the Sun
  • Evidence in M82 based on X-ray emission
  • Active galaxies, such as 3C296 suggest massive black holes at their centers.

Can we conclude black holes have been "detected"?
• Several black hole candidates have been identified
• Some of these are undoubtably black holes, but we cannot be certain of them all