Neutron Stars and Black Holes

1
Neutron Stars and Black Holes

• Chapter 11
• Mr. Saks
• Astronomy
• Unit Four

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Almost anything is easier to get into than out of.

• Agnes Allen

3
Review

• Gravity always will win in the end
• However a star lives it must eventually die
• White dwarf
• Neutron Star
• Black Hole
• These compact objects are all monuments of
  gravity
• These objects, due to their nature, are hard
  detect
• Astronomer must look for real objects, find real
  neutron stars and black holes to confirm their
  theory

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Neutron Stars

• Neutron star is a star a little over 1 solar mass
  compressed to a radius of about 10 km
• Density is so high that matter is stable only as
  a fluid of neutrons
• Theory predicts that his object spins a number of
  times per second
• Hot at its surface as the inside of the Sun
• Have a magnetic field a trillion times stronger
  than Earths
• Two questions?
• How could theory predict this?
• Do they really exist?

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Theoretical Prediction of Neutron Star

• Neutron was discovered in 1932
• Found to spin like electron and so then
  hypothesized they could be packed tightly enough
  to become degenerate like electrons
• 1932, Russian Physicist Lev Landau suggested a
  neutron star cold exist supported by degenerate
  neutrons
• Core would have to be much denser than white
  dwarf
• 1934, Walter Baade and Fritz Zwicky
• Suggested some of most luminous novae in recorded
  history were actually collapse of massive star in
  supernova
• What was left was core, small and highly dense
  made of neutrons

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How does it become a Neutron Star?

• We know that a star above the Chandrasekhar limit
  of 1.4 solar mass core cannot settle as white
  dwarf, why?
• How does this become a Neutron star?

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A Neutron Star

• A neutron star is theoretically to be only 10 km
  or so in radius
• Density of 1014g/cm3
• On Earth a sugar-cube-sized lump would weigh
  about 100 million tons
• We can think of a neutron star as matter with all
  empty space squeezed out of it.

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How big can it be?

• Difficult because we do not know the strength of
  pure neutron material
• Most widely accepted is no more massive than 2 to
  3 solar masses in the core
• Bigger than this the degenerate neutrons would
  not be able to support the weight and collapse
• That means collapse into a black hole

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Properties of Neutron Star

• Predictions state that this star will spin
  rapidly, be hot, and have a strong magnetic
  field
• Why does it gain heat?
• Why remain so hot?
• Why would it spin?

Why Magnetic?
10
Observing a Neutron Star

• Being so hot, and from our knowledge of black
  body radiation we can predict this type of star
  will emit most energy in the ____________
  spectrum.
• Could not be observed in 1940s and 1950s
  because telescopes were still Earth bound
• Neutron star being so small meant they were faint
  objects in our sky.
• Thus mid 20th century astronomers were not
  surprised that none of the newly predicted stars
  found
• Neutron stars as this point, mid 20th century,
  were still very much theoretical
• Observation finally proved them to exist
• (HMWK 1)

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Discovery of Pulsars

• Jocelyn Bell, November 1967
• Graduate student at Cambridge University
• Found pattern in chart from radio telescope
• Found to be regular pulses
• Her and project leader, Anthony Hewish, thought
  it was interference
• Found same signal day after day in the same spot
  in sky
• First thought it was other life forms so named it
  LGM, Little Green Men

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Locating Pulsars

• Jocelyn, Anthony, and other team members found
  three more objects within a few weeks
• Dropped LGM for pulsar (pulsating star)
• Bell found the first pulsar
• As more were found their pulsating periods were
  different
• From .033 to 3.75 seconds and they were nearly
  exact
• Period did grow longer by a few billionths of a
  second per day
• SO they were getting slower but still highly
  precise

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Eliminate Possibilities

• Why not a white dwarf?
• Hot spot on a massive star?
• Why could it not be a pulse from small white
  dwarf star?

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Finally Proof of Neutron Star

• Only Neutron star was small enough to be a pulsar
• It can spin as fast as 1000 times a second
  without flying apart
• 1968 a pulsar was found in Crab Nebula where
  supernova remnant was known
• Theory predicted that supernovas leave behind
  neutron stars
• Since 1968 1000 pulsars have been found or which
  only a few are found in supernova remnants
• Short pulses and discovery of pulsars in Crab
  Nebula are strong hints that pulsars are Neutron
  stars

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A Model Pulsar

• Lighthouse Model of a Pulsar
• As a pulsar star rotates it emits a beam of
  radiation that sweeps across the sky
• The beam is extremely high energy radiation
• We only see pulsars whose beams sweep over the
  Earth

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Evolution of Pulsars

• When a pulsar first appears it is spinning fast
  (100 times a second)
• Energy it radiates comes from its rotation and
  strong magnetic field
• Oldest pulsar is only a few million years old
  (about 10 million years old)
• Older neutron stars rotate too slowly to generate
  detectable radio beams
• Crab Nebula pulsar is 950 yrs old
• Powerful and emits photons of radio, infrared,
  visible, X-Ray, and gamma-ray wavelength

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Crab Nebula Pulsar

• Crab nebula pulsar blinks twice every rotation
• One beam sweeps almost directly over us with a
  strong pulse
• Half a rotation later the edge of the other beam
  sweeps past us and gives us a weaker pulse

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Location

• We expect to find pulsars inside supernovas
• Not every supernova have pulsars and not every
  pulsar is located in a supernova
• WHY? Por Que?
• Some pulsars beams never sweep over the Earth
  making it hard to detect
• Some pulsars move through space at a high
  velocity
• Supernova explosion happened asymmetrically
• Some happen in binary system and explosion flings
  the two stars apart
• Many neutrons stars leave their supernova
  remnants behind
• Pulsar can be detected for 10 million year,
  supernova remnants cannot survive more than
  50,000 yrs

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Supernova 1987A

• We assume this formed a neutron star because we
  received the blast of neutrinos on Earth
• Theory predicts a collapsing massive star core
  will produce a burst of neutrinos
• A this point no neutron star detected
• Why?
• HMWK 2

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Theory on Pulsars

• We predict as they get older they should gt
  slower
• Some of the fastest are quite old
• PSR 193721 in the constellation Vulpecula pulses
  642 times/sec
• It is a binary star and has gained mass and
  rotational energy from its partner (?)
• Millisecond pulsars have period as short as a
  millisecond (.001s)
• A 10 km radius neutron star spinning 642 times a
  second means the equator of star is moving 40,000
  km/sec
• Enough energy to flatten neutron star

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Binary Pulsars

• Pulsar Quest
• Black Hole Quest

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Disappearing Matter BLACKHOLES

• The story to date
• White dwarf stars less than 8 solar masses
• Ending core is 1.4 solar masses the core is about
  the size of the Earth
• Neutron star stars less than 25 solar masses
• Ending core is 1.4 to 3 solar masses with final
  size about 10 km in diameter.
• What about the stars with greater than 25 solar
  masses with cores ending in greater than 3 solar
  masses?

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Black holes

• A star with greater than an ending core of
  greater than 3 solar masses has nothing that will
  stops its collapse
• The star will collapse and not even basic atomic
  structures like electrons, protons, and neutrons
  can stop the collapse.
• It will get smaller and denser and smaller and
  denser and smaller and denser and smaller and
  denser and smaller and denser and smaller and
  denser and smaller and denser and smaller and
  denser and smaller and denser and smaller and.
  Get the point!!!!!!
• It will collapse upon itself forever.
• Everything comes to a mathematical point.
• Gravity causes the entire star to collapse on
  itself leaving nothing left but a black
  (spherical) hole in space.

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The Idea

• First thought of by John Mitchell a Cambridge
  professor around 1783.
• If a star was significantly massive and compact
  would have such a strong gravitational pull that
  not even light particles could escape its pull.

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What in tarnation is a BLACK HOLE?

• A blackhole is something with zero radius and
  infinite density which means it is a singularity
• Event Horizon is the boundary of the black hole.
• Anything happening inside this boundary can never
  be seen because the light will not escape to let
  it be seen.
• The radius for this horizon is called the
  Schwarzschild Radius.
• Any object can become a black hole if it shrinks
  enough
• The Earth could become a black hole if it
  collapsed into the size of about a 1 cm radius

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Evidence?

• Cygnus X-1
• The object is an X-ray binary that was one of the
  first X-ray sources discovered when it was
  detected in 1962
• The visible object HDE226868 is a 9th magnitude
  blue supergiant star whose radial velocity curve
  shows an orbital period of 5.6 days
• The fact that the object is a strong X-ray
  emitter and that the optical and X-ray emission
  varies on very short time scales (as short as one
  one-thousandth of a second) suggest that the
  companion might be a black hole
• Analysis of the radial velocity variation of the
  primary under the assumption that it is a normal
  star suggests that the mass of the companion is
  about 6 solar masses.

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• Figure HST Finds Blackhole The black hole and
  an 800 light-year-wide spiral-shaped disk of dust
  fueling it, are slightly offset from the center
  of their host galaxy, NGC 4261, located 100
  million light-years away in the direction of the
  constellation Virgo.
•        This discovery is giving astronomers a
  ringside seat to bizarre, dynamic processes that
  may involve a titanic collision and a runaway
  black hole.  This relatively nearby galaxy could
  shed light on how far more distant active
  galaxies and quasars produce their prodigious
  amounts of energy.
•        The new Hubble observations have moved us
  beyond the question of whether black holes
  exist.  Now we can work on the demographics of
  black holes and address a number of other
  questions does every galaxy have a black hole? 
  How do they work in detail? (Image Courtesy of
  H. Ford, L. Ferrarese, NASA)
•        By measuring the speed of gas swirling
  around the black hole astronomers are able to
  calculate its mass to be 1.2 billion times the
  mass of the Sun, yet concentrated into a region
  of space not much larger than the Solar System
  (Image Caption courtesy of STSci.).