stellar deaths ii neutron stars and black holes 17
TRANSCRIPT
Stellar Deaths II
Neutron Stars and Black Holes 17
CPS Question
● The pressure that prevents the gravitational collapse of white dwarfs is a result of ______ .
– A) Conservation of energy
– B) Conservation of angular momentum
– C) Einstein's principle of equivalence
– D) The Pauli exclusion principle
CPS Question
● A nova is a result of _____ .
– A) a supergiant star shedding its envelope
– B) the explosion of an extremely high-mass star
– C) fusion on the surface of an accreting white dwarf
– D) pulsation of an isolated brown dwarf
CPS Question
• A low mass object that failed to start nuclear fusion is known as a ______ dwarf.
– A) brown– B) red– C) white– D) black
1. White Dwarf If initial star mass < 8 M
Sun or so.
2. Neutron Star If initial mass > 8 M
Sun and < 25 M
Sun .
3. Black Hole If initial mass > 25 M
Sun .
Final States of a Star
Supernova remnants
• What type of remnant does a carbon-detonation supernova leave behind?
• What about a core-collapse supernova?
Neutron StarsType I supernova (Carbon-detonation): no remnant
Remnant of core-collapse supernova - a tightly packed ball of neutrons.
Diameter: only ~20 km!
Mass: 1.4 - 3 MSun
Incredible densities!
Conservation of Ang. Mom.=> what?
A neutron star over the Sandias?
Neutron StarsType I supernova (Carbon-detonation): no remnant
Remnant of core-collapse supernova - a tightly packed ball of neutrons.
Diameter: only ~20 km!
Mass: 1.4 - 3 MSun
Incredible densities!
Conservation of Ang. Mom.=> Rapid rotation rate
Magnetic field: 1012 x Earth's!
A neutron star over the Sandias?
Pulsars
Objects that give off periodic pulses of radiation Frequencies: ~one to several hundred pulses/sec
What are they?
Pulsars
• What produces the beam of radiation received from a pulsar?
• Do we see all neutron stars as pulsars? Why or why not?
The Lighthouse Model of a Pulsar• Rapid rotation =>
huge magnetic field• Accelerates
charges• Narrow “beam” of
radiation• Not all neutron
stars are pulsars• Can be too old • Orientation may
be wrong
Pulsars are incredibly accurate clocks!
Accurate to within a few seconds in a million years!
Better than best atomic clocks on Earth!
Observed fluctuations in period can be used to detect planets orbiting a pulsar.
1. White Dwarf If initial star mass < 8 M
Sun or so.
2. Neutron Star If initial mass > 8 M
Sun and < 25 M
Sun .
3. Black Hole If initial mass > 25 M
Sun .
Final States of a Star
Concept Review
• What determines whether or not an object is capable of retaining an atmosphere?
• How might this concept be related to black holes?
Review of Escape Speed
Speed needed to escape the gravitational pull of an object.
vesc
= 2GM R
Escape speed from Earth's surface is 11 km/sec.
If Earth were crushed down to 1 cm size, escape speed would be the speed of light.
Schwarzschild Radius = if an object is crushed to within the Sr, not even light can escape its gravitational pull
Black Holes
If core with > 3 MSun
collapses, not even neutron pressure (Pauli Exclusion Principle) can stop the collapse (initial mass of star > 25 M
Sun).
Collapses to a point, a "singularity". Still searching for a theory of quantum gravity.
Gravity is so strong that nothing can escape, not even light (so no information) => black hole.
So, how might we detect them?
Event horizon: “Surface” of black hole. Point of no return.
Event horizon
Schwarzschild Radius
Black Holes
• What are some of the strange phenomena we might encounter if we fell into a black hole?
Effects around Black Holes
Near event horizon:
1) Enormous tidal forces.
2) Bending of light:
2) Gravitational redshift.
3) Time dilation.
Examples:
1. Bending of light. If light travels in straight lines in free space, then gravity causes light to follow curved paths.
First observed in 1919 eclipse.
Gravitational lensing. The gravity of a foreground cluster of galaxies distorts the images of background galaxies into arc shapes.
• Gravitational Red shift• Light emitted from a large gravity
well object (large star) will appear red shifted on smaller object
• Photons need energy to leave gravity well so photons lowers in frequency to provide that energy and become red shifted
Einstein's Principle of Equivalence
• What phenomenon produces all of the same physical effects as being in gravity?– (Hint: Think about riding in an elevator.)
Einstein's Principle of Equivalence
• According to Einstein, the effects of gravity and acceleration are indistinguishable from one another!
• The laws of physics are identical in either case.– => acceleration should be able to
reproduce effects of a black hole!
Do Black Holes Really Exist? Good Candidate: Cygnus X-1
- Binary system: 30 MSun
star with unseen companion.
- Binary orbit => companion ~10 MSun
.
- X-rays => million degree gas falling into black hole.