Transcript Stellar Deaths II - Mid

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
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A nova is a result of _____ .
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A) a supergiant star shedding its envelope
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B) the explosion of an extremely high-mass star
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C) fusion on the surface of an accreting white dwarf
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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
Final States of a Star
1. White Dwarf
If initial star mass < 8 MSun or so.
2. Neutron Star
If initial mass > 8 MSun and < 25 MSun .
3. Black Hole
If initial mass > 25 MSun .
Supernova remnants
• What type of remnant does a carbondetonation supernova leave behind?
• What about a core-collapse supernova?
Neutron Stars
Type 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 Stars
Type 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
Rapid rotation => Pulsar
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.
Final States of a Star
1. White Dwarf
If initial star mass < 8 MSun or so.
2. Neutron Star
If initial mass > 8 MSun and < 25 MSun .
3. Black Hole
If initial mass > 25 MSun .
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
MSun).
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.