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Lecture 31

Stellar Remnants

January 14c, 2014

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Neutron Stars • Supernova remnant

• Tightly packed neutron core.

• Size ~ 20 km (small asteroid or medium city)

• Mass ~ 1.4 - 3 M

• Density very high

– 1 tsp. (5 ml) weighs > 100,000,000 tons on Earth!

• Rotates many times per second due to

conservation of angular momentum; any

rotating body spins faster when it shrinks.

Neutron Stars

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• Powerful magnetic field, a trillion times stronger

than the Earth’s

– When star collapses, magnetic field is concentrated.

Artist rendering showing a

neutron star is about the size of

lower Manhattan

Figure 22.1, Chaisson and McMillan, 5th ed.

Astronomy Today, © 2005 Pearson Prentice Hall

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A. Region A

B. Region B

C. Region C

D. Region D

E. Region E

F. Region F

G. Region G

H. Region H

Where would a neutron star be found

on an H-R diagram?

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A. Region A

B. Region B

C. Region C

D. Region D

E. Region E

F. Region F

G. Region G

H. Region H

Where would a neutron star be found

on an H-R diagram?

Neutron stars are hot and very tiny so they’d be found

near region F on an H-R diagram.

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Neutron Star -- HST

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Pulsars

• 1967 Jocelyn Bell

– Observed object emitting pulses of radio waves.

– Pulses repeated every 1.34 seconds

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• Hundreds more have been found.

• Some pulse in optical, X-rays, or gamma rays.

• Periods range from 0.03 to 0.30 sec. Periods

gradually increase as pulsar loses energy and

rotates slower

• Some are associated with supernova remnants,

many apparently not… hurled into space at high

speed by the supernova explosion.

Pulsars

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• Hewitt proposed it

is a rapidly rotating

neutron star

beaming radiation.

– Magnetic pole and

rotational axis not

quite lined up.

– Charged particles at

poles of magnetic

fields emit large

amounts of energy.

“Lighthouse Model”

Pulsars

Figure 22.3, Chaisson and McMillan, 5th ed.

Astronomy Today, © 2005 Pearson Prentice Hall

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• Not all neutron stars are seen to pulse

– Beam may not be pointed at the Earth

– animation

– Older neutron stars have lost energy and no

longer pulse

Earth Earth never sees

beam of energy

Earth Earth sees beam of

energy

http://highered.mcgraw-hill.com/sites/0072509856/student_view0/chapter14/neutron_stars_interactive.html

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Crab Nebula

Figure 21.10,

Chaisson and McMillan,

6th ed. Astronomy Today,

© 2008 Pearson Prentice Hall

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Crab Pulsar

animation

The pulsar must be young because it is seen at visible and

X-ray wavelengths. Old pulsars emit mostly at lower

energy radio wavelengths.

Figure 22.4,

Chaisson and McMillan,

6th ed. Astronomy Today,

© 2008 Pearson Prentice Hall

and

Figure 13-19b,

Comins and Kaufmann,

8th ed. Discovering the Universe,

© 2008 W.H. Freeman & Co.

http://bcs.whfreeman.com/dtu7e/pages/bcs-main.asp?v=chapter&s=13000&n=00090&i=13090.05

What causes the radio pulses of a pulsar?

A. The star vibrates.

B. We observe pulses when one of the beams of

radio radiation emitted by the spinning star points

toward Earth.

C. The star undergoes periodic nuclear explosions

that generate radio emission.

D. The star’s dark orbiting companion periodically

eclipses the radio waves emitted by the main star.

E. A black hole near the star absorbs energy from it

and re-emits it as radio pulses.

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What causes the radio pulses of a pulsar?

A. The star vibrates.

B. We observe pulses when one of the beams of

radio radiation emitted by the spinning star

points toward Earth.

C. The star undergoes periodic nuclear explosions

that generate radio emission.

D. The star’s dark orbiting companion periodically

eclipses the radio waves emitted by the main star.

E. A black hole near the star absorbs energy from it

and re-emits it as radio pulses.

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Einstein’s Special Theory of Relativity

• You cannot determine if a frame of

reference is at rest or moving at constant

velocity

• All observers measure the same speed of

light in vacuum

• The distances and times between events

depend upon your frame of reference

• Length contraction and time dilation

• Time and space are linked together in a

single “fabric” called spacetime

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Einstein’s General Theory of Relativity

• You cannot determine if a frame of

reference is accelerating or immersed in a

uniform gravitational field

• Space and time are affected by large masses

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General Relativity • All matter warps spacetime.

– Like a weight on a rubber sheet.

– Warped spacetime affects the behavior of BOTH

objects and light in its vicinity.

Analogy: a rolling pool ball on an

uneven surface is deflected in

much the same way as a planet’s

curved orbit is determined by

warped spacetime near the Sun.

Figure 22.18b, Chaisson and McMillan, 6th ed.

Astronomy Today, © 2008 Pearson Prentice Hall

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Evidence for General Relativity • On May 29, 1919 Arthur Eddington carefully

measured star positions around the eclipsed Sun.

Precession of Mercury’s orbit, and that of a neutron star binary

system, offered further confirming evidence in support of General

Relativity

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Formation of Black Holes

• If core of star has M >3 M the neutron

pressure cannot hold up the core

– Nothing remains to stop collapse.

– Becomes a “singularity” -- object with infinite

density and infinitely small size.

– Rips a hole in the fabric of spacetime

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Why are Black Holes Black?

• Escape velocity = velocity needed to escape the gravitational pull of an object.

• As mass increases or size decreases, gravity on the surface of the star increases, and a larger velocity is needed to escape surface.

• When the escape velocity at the surface becomes greater than the speed of light, no light can escape.

escape

211 km/s for Earth

GMv

R

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Schwarzschild Radius

Normal star: Light

can escape surface

(vescape < c)

Radius > Schwarzschild Radius

Black Hole: Light

cannot escape surface

(vescape > c) Radius Schwarzschild Radius

Distance from the center of a supermassive object at

which the escape velocity would be equal to the speed

of light.

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Event Horizon

Black Hole

Light cannot escape

Light can escape

Event Horizon

• The event horizon of a black hole is one

Schwarzschild Radius away from its center.

– No events or communication inside the event

horizon can be observed.

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Evidence for Black Holes Isolated black holes are hard to observe, but we might

be able to detect gravitational lensing.

Figure 23.23,

Chaisson and McMillan,

6th ed. Astronomy Today,

© 2008 Pearson Prentice Hall

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Evidence for Black Holes

• We can observe how the black hole’s gravity affects

nearby objects.

– Unseen companion

– Accretion disk

– X-ray emission

Figure 14-15,

Comins and Kaufmann,

8th ed. Discovering the Universe,

© 2008 W.H. Freeman & Co.

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Cygnus X-1 • A flickering X-ray source that must be smaller than the Earth

• The X-ray source seems to force the nearby supergiant star to

wobble

• Conclusion: It’s a 30-solar-mass B0 supergiant and an 11-

solar-mass black hole that are orbiting each other

Figure 22.23, Chaisson and McMillan, 6th ed. Astronomy Today, ©

2008 Pearson Prentice Hall

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Black Holes in Galaxies

• BHs may have formed in center when galaxy formed.

• Mass of billions of stars in size of SS (Kepler’s 3rd Law).

• Black hole likely in center of the Milky Way.

Accretion disk

surrounding a

300-million-solar-

mass black hole

in the galaxy

NGC 7052.

The Schwarzschild radius of a body is

A. the distance from its center at which nuclear

fusion ceases.

B. the distance from its surface at which an orbiting

companion will be broken apart.

C. the maximum radius a white dwarf can have

before it collapses.

D. the maximum radius a neutron star can have

before it collapses.

E. the radius of a body at which its escape velocity

equals the speed of light.

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The Schwarzschild radius of a body is

A. the distance from its center at which nuclear

fusion ceases.

B. the distance from its surface at which an orbiting

companion will be broken apart.

C. the maximum radius a white dwarf can have

before it collapses.

D. the maximum radius a neutron star can have

before it collapses.

E. the radius of a body at which its escape

velocity equals the speed of light.

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Traveling into a Black Hole --

Tidal Forces

• Extremely large tidal forces near a BH.

• Difference in forces of gravity on near and

far side would pull object apart.

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Time Dilation

• From outside, observer sees clock on board tick more and more slowly than outside of craft.

• The closer to black hole, the slower time appears to run.

• At event horizon, time appears to stop!

– An observer far away never sees the craft fall into BH

– An observer inside the craft sees time proceed at its normal rate.

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Inside of a Black Hole

• Scientists to not know for sure what is

inside of a black hole.

• Theories of physics break down for such

high densities.

• Hard to make and test new model since

black holes cannot be directly observed.