November 27, 2016
Astronomy 122
Professor J. Brau

• Stellar Explosion
• Neutron Stars and Black Holes

Life After Death for White Dwarfs

White dwarfs can spring back to life in a couple of ways: Novae Supernovae (Type I) Novae: Mild Stellar Explosions Star with sudden outburst of energy Brightens hundreds to many thousand-fold Binary star system: degenerate partner collects hydrogen from red giant eventually ignites on the surface Same star may flare up into Nova many times during its lifetime

Supernovae

• Many supernovae observed every year in distant galaxies.
  • Astronomers calculate an observable supernova ought to occur in our Galaxy every 100 years or so.

Two Types of Supernovae

• Type I - binary system
  • Carbon-detonation supernova
  • White dwarf grows from accretion of material from companion, eventually exceeding the Chandrasekhar mass limit (1.4 M_SUN)
  
  
• Type II - single massive star
  • Core-collapse supernova
  • Massive star expends its fuel supply, ending with iron core, and collapses and explodes

Stellar Explosions




Heavyweight stars (more than 3 M_SUN)

Type II supernova - light increased by 10⁷ - 10⁸ or more


1987 - in Large Magellanic Cloud (not in Milky Way Galaxy)

First visible to naked eye since 1604 (last in Milky Way Galaxy)
Occurred 160,000 years ago since LMC is 160,000 light-years away




Many observed in telescopes every year - distant galaxies




Mass at time of collapse exceeds Chandrasekhar limit (1.4 M_SUN)

• Collapses, heats, fuses heavier and heavier elements
• Hydrogen -> Helium -> Carbon -> ... -> Iron
  Iron cannot release energy on fusion
  • Therefore, energy generation in core stops
  • And internal pressure falls
• Core collapse to extremely high density, and rebounds in violent explosion
• Heaviest elements of the universe created in the explosion


A supernova is about 10¹⁰ times as bright as the Sun.

It may even outshine its galaxy
Fades away in months or a few years

leaving behind supernova remnant




Evolutionary stages of a 25 MSUN star

Stage	Temperature (K)	Density (g/cm3)	Duration of stage
Hydrogen burning	4x 107	5	7 x 106 years
Helium burning	2 x 108	700	5 x 105years
Carbon burning	6 x 108	2 x 105	600 years
Neon burning	1.2 x 109	4 x 106	1 year
Oxygen burning	1.5 x 109	107	6 months
Silicon burning	2.7 x 109	3 x 107	1 day
Core collapse	5.4 x 109	3 x 109	1/4 second
Core bounce	2.3 x 1010	4 x 1014	milliseconds
Explosive	about 109	varies	10 seconds



Note: star core gets hotter to start each burning stage. 
Each successive stage lasts for a shorter period 
When core collapses, density reaches astonishing 400,000,000,000,000 g/cm³  



Neutron star often left at center of explosion



Most (99%) of the supernova energy carried off by neutrinos.

• This was confirmed in 1987 when the neutrinos from SN1987A were detected by two underground detectors,
• Short burst of neutrinos arrived at Earth about 20 hours before visible light
  • observed in Japan
  • and in the US (near Lake Erie




Lighter stars (1.4 M_SUN)

Type I supernova (different mechanism from type II)

White dwarf and companion

Companion exchanges matter with the white dwarf, building eventually beyond the Chandrasekhar Limit (1.4 M_SUN)




Supernova remnant

Crab Nebula

Supernova remnant from 1054

Sightings reported in China, SW India, Arabia
Brighter than Venus

Most of the mass was ejected

Some mass left over as a neutron star at the core
....observed as a pulsar today

SN1987A

• Supernova 1987A was a type II supernova that appeared in the Large Magellanic Cloud (LMC).
  • Known to be 160,000 light years from Earth due to location in LMC.
  • First Supernova visible to naked eye since invention of telescope.
  • Its progenitor star had been studied before: Sanduleak -69^o202.
  • Direct evidence of nickel to iron decay was seen in its light curve.
  • Neutrinos from SN1987A were detected.
  • It has been studied by new telescopes, including the Hubble Space Telescope.

Neutron Stars

• Following supernova, a fraction of mass may remain as compressed star.
• Protons and electrons are crushed to produce neutrons and neutrinos (neutronization).
• Neutrinos exit leaving extremely dense stellar matter of just neutrons.
  • Weight of star supported by degenerate-neutron pressure
    if M>3M_SUN, Neutron star collapses into Black Hole.
  • Density of N-star 10¹⁴ g/cm³ (1 thimble full = 100 million tons) 
  • Diameter of l M_SUN N-star 30 km (20 miles) 
    (Radius of white dwarf = 4200 km) 
    
    
  
  
• Properties of neutron stars.
  • Small and massive (greater than Sun's mass).
  • Rotating rapidly (1-1000 times per second).
  • Very strong magnetic field.

Pulsars

• Pulsating radio source produced by rapidly rotating N-star, 1st observed in 1968 by graduate student Jocelyn Bell.
• Explained as signal from neutron star
• Typical pulsar periods: 1/1000 seconds up to few seconds 
• Example: Crab Pulsar -> 1/30 second
Mechanism: Lighthouse Model
N-star rotates with powerful magnetic field 
tilted relative to axis 
Charged particles are accelerated to poles and produce 2 beams of radiation. 
Neutron star rotates and beams sweep past our direction- we see pulses 
Only a very small object can rotate this fast (30/sec, for example) 
• Not all neutron stars will appear as pulsars.
  • The alignment must shine radio signal at Earth.
  • Over time, neutron stars will rotate slower and magnetic field will diminish.

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.
    • An X-ray burster produces sudden, intense flash of X-rays (lasting seconds), followed by hours of inactivity, followed by more sudden bursts
    
    
  • Millisecond pulsars.
    • A very rapidly rotating neutron star that must have spun-up
    • Inspiraling matter from the companion star adds angular momentum (spin) to the neutron star
    
    
  • Pulsar planets.
    • Pulse periods are found to vary in quite regular pattern, revealing additional bodies in orbit around the neutron star
    
    
  • Gamma-ray bursts (GRBs).
    • Bursts of gamma-rays.
    • Detected at rate of about 1 per day
    • Uniformly distributed in sky
    • Must be at very great distances (say billions of parsecs) and enormous luminosity
    • Two leading models of causes of GRBs:
      • Hypernova model favored for long duration (more than 2 seconds) bursts.
      • Neutron star merger model favored for short duration (less than 2 seconds) bursts.
    
    
  • Binary Pulsars
    • Suppose that both members of a binary star system evolve to become neutron stars.
    • We may observe only once pulsar due to the direction of the beam.
    • The second neutron star will appear in the varying pulse period.
    • Gravitational radiation is emitted and the two spiral in toward each other
    • They eventually coalesce in a violent explosion, comparable in energy to a supernova
    • A binary pulsar was discovered in 1974.
      • The periodic Doppler shift of the pulsar's radiation prove that its orbit is slowly shrinking at a rate predicted by general relativity theory with energy carried off by gravity waves.
    
    
  • Gravity wave sources.
    • Two large observatories using laser interferometry (in Hanford, Washington and Louisiana) are looking for gravity waves.
    • This project is called LIGO (for Laser Interferometer Gravitational Wave Observatory).
    • Three events have been observed appearing to result from the merger of two large black holes.

Black Holes

If more than 3 M_SUN remains in the collapsed 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 stops the star from collapsing toward a point.
  • Resulting black hole emits no light, no radiation, nothing.


Einstein's theories of relativity:
• Nothing travels faster than the speed of light.
  • 300,000 kilometers/second or 186,000 miles/second.
• Everything (including light) is attracted by gravity.
  • General Theory of Relativity.


Escape Velocity
Velocity needed to escape the gravitational pull of a body.
• 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 like a black hole.


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


Schwarzschild radius increases linearly with the mass
• 12 solar mass -> 36 km


Photon Orbits
• Far from a black hole, all radiation from a source escapes.
• At 1.5 Schwarzschild radii, exactly half the radiation emitted by a source escapes into space and photons can go into circular orbit.
• At the event horizon, all light rays enter the black hole.

Black Hole Properties

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

Matter curves (warps) space
• 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 and do not suck everything in.
• Black Holes are cosmic heaters since matter is heated as it falls into a black hole, and it emits radiation.

Space Travel Near Black Holes

• Near a black hole the pull of gravity would be extreme.
  
• Light emitted from near the black hole is red shifted (Gravitational Redshift).
  • Light from the event horizon is infinitely redshited (beyond perception to infinite wavelength)
• Clocks near the black hole appear to slow down (Time Dilation) from outside and will appear to stop at the event horizon

Black-Hole Evaporation (Hawking Radiation)

• Stephen Hawking realized that black holes are not absolutely black.
  • Quantum physics tells us particles and anti-particles pairs are continuously created and annihilated in free space.
  • Near a black hole, one of the particles may cross the event horizon and the other particle is then be free to leave as a source of matter or radiation.
  • The hole decreases in mass as it radiates, or slowly evaporates.
  • Lifetime:
    • for 1 solar mass, 10⁷⁰ years
    • 10¹² kg, lifetime is the age of the universe
      • this is the mass of a mountain
  • It is possible that miniature black holes from the early universe could have survived to now, and could be exploding in space.
    • But they have not been seen.

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.
  • Detect unseen mass of several solar masses and large amounts of X-ray emission.
  • Cygnus X-1 and X-rays from companion
  
  
• 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
• Rapid motion of stars near galaxy centers, as if orbiting unseen massive object
• Masses 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.

Several black hole candidates have been identified; some, perhaps not all, are undoubtably black holes.

Neutrinos

Neutrinos are elementary particles



Properties

• No charge
• Very little particle mass
• Hardly interacts with anything (passes through light years of matter)
• Very difficult to detect
• Travel at nearly the speed of light




Neutrinos provide astronomers with information from within the cores of stars

• Emission from SN1987A observed by two underground detectors
  • Short burst of neutrinos arrived at Earth about 20 hours before visible light
  • Kamiokande in Japan
  • Detector near Lake Erie in US
• Solar neutrinos are also routinely detected on Earth
  • Large underground detectors