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Black Holes Black Holes and Neutron and Neutron

StarsStars

Dead Stars

Copyright – A. Hobart

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GoalsGoals

• What are black holes?• How do we see black holes?• What happens when black holes are in

binaries?• Supermassive Black Holes

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Density• Density = mass per volume• From Red Giant cores to White Dwarfs to

Neutron Stars, density has been increasing.• As density increases, the force of gravity on

the surface increases.• The greater the force, the higher the

escape velocity:– How fast you need to go in order to escape the

surface.

• How dense can something get?• How strong can the force of gravity be?• What if the escape velocity is faster than

light?

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Singularity

• When a high-mass star’s core is greater than ~3 x Msun, then, when it collapses, neutron degeneracy pressure can’t balance gravity.

• The star collapses to form a singularity.

• No size at all.• Density infinite.• Escape velocity > c

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Black Hole Diagram

.

SingularityEvent Horizon

Schwarzschild Radius

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Schwarzschild Radius• Distance from object where vesc > c

Object Mass Radius

Earth 6 x 1024 kg 1 cm

Jupiter 300 x Earth 3 m

Sun 300,000 x Earth

3 km

SunS M

Mkm3R

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Concept Test

• A black hole is best defined as:a. a star which sucks all matter into itself.b. a window to another Universe.c. any object which is smaller than its event

horizon.d. the final result of all stellar evolution. e. none of the above

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Seeing Holes

• Can’t see black hole itself, but can see matter falling into a hole.

• Gravitational forces stretch and rip matter: heats up.

• Very hot objects emit in X-rays (interior of Sun)

• Cygnus X-1.http://www.owlnet.rice.edu/~spac250/steve/ident.html

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Binaries• Gravitational tides pull matter off big low

density objects towards small high density objects.

Cygnus X-1

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Holes Don’t Suck

• Newton’s Laws of gravity only depend on mass and separation.

• Kepler’s Laws of orbits only depend on mass and separation.

• At 1 AU, force of gravity from a 1 Msol B.H. is same as from a 1 Msol star.

• At surface of each, force of gavity is very different!

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m

Tides

M m

2d

MmGFgravity

m

•While each m is attracted to each other m, the difference in force from M is greater.•The closer you are to the object M, the more extreme this is!

M m m m

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Tides

Frictional Heating

Accretion disk

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Concept Test

• We can see X-rays from black holes because?a. X-rays are more energetic than visible light

and so can escape from the event horizon.b. X-rays can pass through ordinary matter

showing us things we can’t normally see.c. Light given off by objects as they enter the

event horizon are gravitationally redshifted to X-rays.

d. Material flowing into a black hole is heated so much that the thermal radiation peaks in X-rays.

e. None of the above

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Cygnus X-1• 1970s• Intense source X-

rays.• “Near” star

HDE226868.

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HDE226868

• Doppler shifts of HDE226868

• Like before, we get mass of star and unseen companion.

Mass Total

)(Separation)(Period

32 AU

yrs

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The Companion• Result:

Period = 5.6 daysTotal Mass ~ 28 x Msun

• From spectral type of HDE226868 we estimate its mass ~18 Msun.

• Companion M = 10 Msun!• Massive!• But where is its light?• Dark!• Can’t be a normal star, or even

neutron star.

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X-ray Source?

• Star brightness fluctuates every 5.6 days.

• X-rays drop off every 5.6 days!• Companion must be source of X-rays!

• REH = 30 km!

SunEH M

Mkm3R

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Supermassive Black Holes

• Photograph the center of a galaxy.

• Make spectrum of light from center.

Velocity

Dis

tan

ce

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Heart of Darkness

• From Doppler shift get a velocity.

• From picture get a separation.

• From Kepler’s Laws get a Total Mass.

Mass Total

)(Separation)(Period

32 AU

yrs

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The Dark Truth• Observe:

V = 400 km/s within 26 LY of center.• So:

– Period = 121,600 yrs– Separation = 26 LY = 1,600,000 AU

• Total Mass in central pixel:300,000,000 x Mass of Sun!

• But where’s all the light?• Small, massive, dark black hole?

• REH = 6.5 AU!

Mass Total

)(Separation)(Period

32 AU

yrs

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Homework #13Homework #13• For 10/22: Read B18.4: Voyage to a Black Hole• I am the captain of a starship that gets stuck just

outside the event horizon of a black hole. When I am rescued, what am I most likely to find:a. I have aged a day, but 1000 years have passed for

everyone else.b. A day has passed for everyone else, but I am now an old

man.c. While I have stayed the same age, I think everyone else

has grown old. Strangely, everyone else thinks they have stayed the same age, but I have grown old.

d. While I have grown old, I think everyone else has stayed the same age. Meanwhile, everyone else thinks the reverse.

e. None of the above.