Review

Star Remnants

White Dwarfs

Degeneracy

Maximum mass

Speed limit

Neutron stars

Characteristics

Pulsars

Black holes

Bending light

Black Holes

Today’s outline

Homework due Thursday morningTest Friday

Review: Lives of High Mass Stars

Stellar remnants

I White dwarf stars

I Neutron stars

I Black holes

Review

Star Remnants

White Dwarfs

Degeneracy

Maximum mass

Speed limit

Neutron stars

Characteristics

Pulsars

Black holes

Bending light

Black Holes

Question concepts

Question #1: Heavier nuclei have higher electric charge thanlighter nuclei, therefore fusing them together requires highertemperatures in order to overcome the strongerelectromagnetic repulsion.

Question #2: The creation of a new core burning phaseinside a star proceeds through the following events: fueldepletion in the core, formation of a burning shell around thcore, contraction of the (now inert) core raising itstemperature, eventual ignition of burning in the core.

Question #3: When a high mass star dies it will leavebehind a neutron star or black hole. A low mass star willleave behind a white dwarf.

Review

Star Remnants

White Dwarfs

Degeneracy

Maximum mass

Speed limit

Neutron stars

Characteristics

Pulsars

Black holes

Bending light

Black Holes

Stellar remnants

I White dwarf stars

I Neutron stars

I Black holes

Review

Star Remnants

White Dwarfs

Degeneracy

Maximum mass

Speed limit

Neutron stars

Characteristics

Pulsars

Black holes

Bending light

Black Holes

White dwarf stars

White dwarf

I remnant core of low-mass star

I cooling from being the hot core of a star

I millions of times more dense than waterteaspoon of white dwarf core = 100 tons

I held up by electron degeneracy pressure

I more mass is smaller

Review

Star Remnants

White Dwarfs

Degeneracy

Maximum mass

Speed limit

Neutron stars

Characteristics

Pulsars

Black holes

Bending light

Black Holes

Question concepts

Question #4: Higher mass white dwarf stars are smaller insize than lower mass ones.

Review

Star Remnants

White Dwarfs

Degeneracy

Maximum mass

Speed limit

Neutron stars

Characteristics

Pulsars

Black holes

Bending light

Black Holes

Electron degeneracy

high temp

low dens

low temp

low dens

low temp

high dens

hig

her

en

erg

y

Thus Temperature is not necessaryto ”hold up” the starmore like a ”normal” object - doesnot contract as it loses energy

Degeneracy – particles like electronsare not allowed to occupy the sameenergy level

I At high temperature, particlestend to not be in same levelsanyway

I At low temperatures particleswill ”stack up” in lowerenergy levels

I At high densities the ”top” ofthe stack can be quite highenergy

The pressure from electrons on the”top” of the stack holds up a whitedwarf star without thermal pressure(though the interior is still severalmillion K)

Review

Star Remnants

White Dwarfs

Degeneracy

Maximum mass

Speed limit

Neutron stars

Characteristics

Pulsars

Black holes

Bending light

Black Holes

Maximum white dwarf mass

Electrons can’t move fasterthan speed of light

I Upper limit to pressure

I Maximum mass for whitedwarf

I about 1.4MSun

Above this mass, electrons areforced onto protons– form neutronsTake up much less space

Collapses to neutron star

Review

Star Remnants

White Dwarfs

Degeneracy

Maximum mass

Speed limit

Neutron stars

Characteristics

Pulsars

Black holes

Bending light

Black Holes

Speed of light and relativityThe speed of light is always thesame– implies that it is also the highestpossible speed

I normal objects thrown frommoving cars– object thrown from fastercar moves faster

I light emitted from movingcars– always goes the same speed

Thus an object can never go fasterthan light it emits

Has some very counterintuitive results for the nature ofspace and time – time dilation, distance contraction

Hinges on something that we take for granted– Simultaneity - events separated in space can occur at thesame time - not as true as you think

Review

Star Remnants

White Dwarfs

Degeneracy

Maximum mass

Speed limit

Neutron stars

Characteristics

Pulsars

Black holes

Bending light

Black Holes

Neutron Stars

?

neutrons

crust of heavy nucleiatmosphere of light nuclei

Without electrons, neutron starsare as dense as nucleiPaperclip of this density hasmass of mount everest

I about 10km in radius(about 6 miles)

I about as heavy as sun

I We don’t know whatmaterial at center is like(quark-gluon plasma!)

I has a solid crust as well asoceans (of liquid metal)

Already very much like a blackhole - very smallhas an innermost orbit – orbitalradius below which objects can’tstay in orbit

Review

Star Remnants

White Dwarfs

Degeneracy

Maximum mass

Speed limit

Neutron stars

Characteristics

Pulsars

Black holes

Bending light

Black Holes

Question concepts

Question #5: Neutrons have no charge, and therefore noelectric repulsion. This makes it easy for neutrons to getclose to each other and to other nuclei.

Review

Star Remnants

White Dwarfs

Degeneracy

Maximum mass

Speed limit

Neutron stars

Characteristics

Pulsars

Black holes

Bending light

Black Holes

Pulsars

Spinning neutron stars

Extremely regularly pulsedradio signals

I Caused by radiationbeamed from magneticpoles

I Rotation causes”flashing” as beam pointstoward and away from us

I Born rapidly rotating ascore of star collapses insupernova

I spins down over time dueto losses due to magneticfield

Review

Star Remnants

White Dwarfs

Degeneracy

Maximum mass

Speed limit

Neutron stars

Characteristics

Pulsars

Black holes

Bending light

Black Holes

Pulsars power supernova remnants

Rotation of magnetic neutronstar (pulsar) is the energysource that lights up thecentral part of supernovaremnant.

Central blue haze from energyand magnetic field shed byneutron star

Review

Star Remnants

White Dwarfs

Degeneracy

Maximum mass

Speed limit

Neutron stars

Characteristics

Pulsars

Black holes

Bending light

Black Holes

Black Holes

At too high mass, quarks can no longer move at high enoughspeeds to hold up the neutron star. Happens above about

3Msun

Collapse to black hole

Review

Star Remnants

White Dwarfs

Degeneracy

Maximum mass

Speed limit

Neutron stars

Characteristics

Pulsars

Black holes

Bending light

Black Holes

Gravity can bend light around a star

Light bends somewhat likeparticle (planetary) orbits

If light is subject to very highgravity, very close to a smallobject, can get trapped. Thisis a black hole.

The bending of spacetimeinfluences causality– the ability for events to causeother events– information cannot propagatefaster than light

I event horizon is the radiusinside which events cannotcause events at larger radius

I Light cannot move outwardfrom this radius

Black holes are remarkably smallnot remarkably heavy

Can be just a few times our sun’smassFar away, no stronger gravity than asimilar mass star

Review

Star Remnants

White Dwarfs

Degeneracy

Maximum mass

Speed limit

Neutron stars

Characteristics

Pulsars

Black holes

Bending light

Black Holes

Gravity warps space and time

Clocks run more slowly neargravity sourceObserved for the earth too,but very weak.

From outside black hole, younever see an object reach theevent horizon. It appears tobe in freefall forever, with anever-slowing rate.

The object falling in the holedoes not notice passingthrough the event horizon.But once beyond it can neverreturn

Review

Star Remnants

White Dwarfs

Degeneracy

Maximum mass

Speed limit

Neutron stars

Characteristics

Pulsars

Black holes

Bending light

Black Holes

Question concepts

Question #6: From a typical planetary orbit, a black hole’sgravitational field is no different than that of a star with thesame mass. The difference is that a black hole is very small,so that very close to it gravity is quite strong.

Question #7: Events that occur closer to a black hole thanthe event horizon cannot influence events outside the eventhorizon. That is, information cannot propagate from a pointinside the event horizon to a point outside it.