May 20, 2007, jeb
Outline
- 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
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
Can we actually "observe" these very small objects?
- 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 1750 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
- 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
We know most stars are members of binary star systems, so neutron stars
might often also be
- This leads to sveral types of special binaries:
- X-ray sources
- gamma-ray bursts
- millisecond pulsars
- pulsar planets
- 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
- result from accreting 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 follow
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
GRB971214)
- What causes the bursts?
- Two leading models
- Hypernova model favored for long duration (more than 2 seconds) bursts
- Neutron star merger model favored for short duration bursts
- Millisecond pulsars
A very rapidly rotating neutron star
- several are observed in globular clusters -> very old
- these must have spun-up
- Pulsar Planets
Pulse periods are found to vary in quite regular pattern, revealing additional
bodies in orbit around the neutron star
- 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
ultradense 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
If MSTAR is greater than 3 times MSUN
-
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 thatn 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
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
This is totally theoretical, based on Einstein's Theory of Relativity
Curved Space
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
- 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
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?
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
Stellar Transits
Binary Systems
- Infer existence of unseen black hole from motion of companion
Black Holes in Galaxies
- Strongest evidence for black holes may come 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
Can we conclude black holes have been "detected"?
- Several black hole candidates have been identified
- Best guess is at least some of these are black holes, but we cannot be totally certain
