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Incredible Shrinking Stars

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Incredible Shrinking Stars. The End States of Stars. White Dwarfs. Earth. White Dwarf. White Dwarf Facts. * Composition: carbon, oxygen * Mass limit < 1.4 M  ( Chandrasekhar Limit ) * Avg mass  0.5 M . Mass, radii  density  1 ton/cm 3. Sirius B (white dwarf). Sirius.

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Presentation Transcript
slide1
Incredible

Shrinking

Stars

The End States of Stars

slide3
Earth

White Dwarf

White Dwarf Facts

* Composition: carbon, oxygen

* Mass limit < 1.4 M (Chandrasekhar Limit)

* Avg mass  0.5 M

Mass, radii  density  1 ton/cm3

slide4
Sirius B (white dwarf)

Sirius

Ultimate fate:

continuous cooling . . .

slide6
~ 10 km

Neutron Star: Final state of stellar core in some

supernova explosions.

  • Composition: neutrons
  • Mass: 1 – 2 M

Density ~ 108 ton/cm3

slide7
Living on a Neutron Star

1. Your weight is enormous

m = your mass

M = neutron star mass

R = neutrons star radius

150 lbs on Earth  ~ 1013 lbs on a neutron star

slide8
1 meter

Splat!

2. Marshmallows become dangerously explosive

Energy released equivalent to

detonating ~ 10 tons of TNT.

Drop marshmallow on

neutron star.

slide9
* Neutron stars should be rapidly spinning . . .

Neutron Star

(small diameter,

rapid spin)

Core, before collapse

(large diameter, slow spin)

slide10
Slow

Fast

slide12
very

high

small

tiny

Finding neutron stars

1. They should be hot: T  106 K

L = surface area  T4

slide14
Conversion of energy of rotation into
  • em radiation

* Rotating magnetic

field creates electric

field.

* Electric field drives

electrons along lines

of magnetic field.

* Electrons radiate –

mainly where magnetic field strong.

slide17
Crab pulses

X-ray

Period =

33 milliseconds

(~ 30 rotations per

second)

Visual

Radio

slide18
Vela Pulsar: 11.2 pulses/second

PSR 1937+21: 640 pulses/second

Pulsar Sounds

slide20
Mass Limits

* White Dwarf: M  1.4 M

* Neutron Star: M  2 - 3 M

If stellar core should become iron with

mass  2 - 3 M . . .

Black Hole results!

slide21
What is Gravity?

Einstein

Newton

Equivalence Principle

slide23
Light is affected by gravity . . .

Light from distant star follows

curvature of space near Sun.

slide24
Gravity causes time to slow down . . .

Observes

ground-floor

clock to be

slow.

Photon loses energy

as it moves upward –

gravitational redshift.

slide25
Top of tower

Bottom of tower

  • e.g.,
  • 2 cycles/sec
  • (a) 1 cycle/sec
  • more ticks of clock
  • (a) fewer ticks of clock
slide27
Singularity: zero volume,

infinite density!

Schwarzchild Radius (RS)

Event Horizon

Schwarzchild Black Hole

At RS, escape velocity =

speed of light

slide28
RS (km) = 3M M = mass in units of Sun’s mass

e.g., M = 1 M RS = 3  1 = 3 km

6 km

The Sun as a black hole

slide29
Escape velocity

(mi/sec)

7

300

3000

100,000

From surface of:

Earth

Sun

white dwarf

neutron star

slide30
Tide

B. H.

Falling into a Black Hole

1. Stretching

Less gravity

2. Squeezing

More gravity

Result: Heating

slide31
B. H.

As seen by outside observer,

time runs slower & slower

as person approaches B.H.

slide32
Detecting Black Holes

* By their effects on other objects.

  • Strong x-ray source
  • Only one star detected
  • Motion of that star  unseen star is very
  • massive
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