Umn_supernova & Black Hole

8/10/2019 Umn_supernova & Black Hole

1/76

What is a white dwarf?


2/76

White Dwarfs

White dwarfs are

the remaining cores

of dead stars

Electron

degeneracy pressure

supports themagainst gravity


3/76

White dwarfs

cool off and

grow dimmerwith time


4/76

Size of a White Dwarf

White dwarfs with same mass as Sun areabout same size as Earth

Higher mass white dwarfs are smaller


5/76

The White Dwarf Limit

Quantum mechanics says that electrons must movefaster as they are squeezed into a very small space

As a white dwarfs mass approaches 1.4MSun, itselectrons must move at nearly the speed of light

Because nothing can move faster than light, a whitedwarf cannot be more massive than 1.4MSun, the whitedwarf limit (or Chandrasekhar limit)


6/76

What can happen to a white

dwarf in a close binary system?


7/76

Star that started with

less mass gains mass

from its companion

Eventually the mass-

losing star will becomea white dwarf

What happens next?


8/76

Accretion Disks

Mass falling toward

a white dwarf from

its close binary

companion hassome angular

momentum

The mattertherefore orbits the

white dwarf in an

accretion disk


9/76

Accretion Disks

Friction between

orbiting rings of

matter in the disk

transfers angularmomentum outward

and causes the disk

to heat up and glow


10/76

Thought Question

What would gas in a disk do if there were no friction?

A. It would orbit indefinitely.

B. It would eventually fall in.

C. It would blow away.


11/76

Thought Question

What would gas in a disk do if there were no friction?

A. It would orbit indefinitely.

B. It would eventually fall in.

C. It would blow away.


12/76

Nova

The temperature of

accreted matter

eventually becomes

hot enough forhydrogen fusion

Fusion begins

suddenly andexplosively, causing

a nova


13/76

Nova

The nova star

system temporarily

appears much

brighter

The explosion

drives accretedmatter out into

space


14/76

Thought Question

What happens to a white dwarf when it accretes enoughmatter to reach the 1.4MSunlimit?

A. It explodes

B. It collapses into a neutron star

C. It gradually begins fusing carbon in its core


15/76

Thought Question

What happens to a white dwarf when it accretes enoughmatter to reach the 1.4MSunlimit?

A. It explodes

B. It collapses into a neutron star

C. It gradually begins fusing carbon in its core


16/76

Two Types of Supernova

Massive star supernova:

Iron core of massive star reaches

white dwarf limit and collapses into aneutron star, causing explosion

White dwarf supernova:

Carbon fusion suddenly begins as whitedwarf in close binary system reaches

white dwarf limit, causing total explosion


17/76

One way to tell supernova types apart is with a light

curve showing how luminosity changes with time


18/76

Nova or Supernova?

Supernovae are MUCH MUCH more luminous!!!(about 10 million times)

Nova: H to He fusion of a layer of accreted matter,white dwarf left intact

Supernova: complete explosion of white dwarf,nothing left behind


19/76

Supernova Type:

Massive Star or White Dwarf?

Light curves differ

Spectra differ (exploding white dwarfs

dont have hydrogen absorption lines)


20/76

What is a neutron star?


21/76

A neutron star

is the ball of

neutrons leftbehind by a

massive-star

supernova

Degeneracy

pressure of

neutrons

supports a

neutron star

against gravity


22/76

Electron degeneracy

pressure goes awaybecause electrons

combine with protons,

making neutrons and

neutrinos

Neutrons collapse to the

center, forming a

neutron star


23/76

A neutron star is about the same size as a small city


24/76

How were neutron stars

discovered?


25/76

Discovery of Neutron Stars

Using a radio telescope in 1967, Jocelyn Bell

noticed very regular pulses of radio emissioncoming from a single part of the sky

The pulses were coming from a spinning neutronstarapulsar


26/76

Pulsar at center

of Crab Nebula

pulses 30 timesper second


27/76

X-rays Visible light
http://localhost/var/www/apps/conversion/tmp/scratch_3/crab_nebula_movie-CHANDRA.htm


28/76

Pulsars

A pulsar is a

neutron star that

beams radiation

along a magneticaxis that is not

aligned with the

rotation axis


29/76

Pulsars

The radiation beams

sweep through

space like

lighthouse beams asthe neutron star

rotates


30/76

Why Pulsars must be Neutron Stars

Circumference of NS = 2 (radius) ~ 60 km

Spin Rate of Fast Pulsars ~ 1000 cycles per second

Surface Rotation Velocity ~ 60,000 km/s

~ 20% speed of light

~ escape velocity from NS

Anything else would be torn to pieces!


31/76

Pulsars spin

fast becausecores spin

speeds up as

it collapses

into neutronstar

Conservation

of angular

momentum
http://localhost/var/www/apps/conversion/tmp/scratch_3/collapse_of_solar_nebula.htm


32/76

Thought Question

Could there be neutron stars that appear as pulsars toother civilizations but not to us?

A. YesB. No


33/76

Thought Question

Could there be neutron stars that appear as pulsars toother civilizations but not to us?

A. YesB. No


34/76

What can happen to a neutron

star in a close binary system?


35/76

Matter falling toward a neutron star forms an

accretion disk, just as in a white-dwarf binary


36/76

Accreting matter

adds angular

momentum to a

neutron star,increasing its

spin

Episodes of

fusion on the

surface lead toX-ray bursts


37/76

Thought Question

According to conservation of angular momentum,what would happen if a star orbiting in a

direction opposite the neutrons star rotation fell

onto a neutron star?

A. The neutron stars rotation would speed up.

B. The neutron stars rotation would slow down.

C. Nothing, the directions would cancel each otherout.


38/76

Thought Question

According to conservation of angular momentum,what would happen if a star orbiting in a

direction opposite the neutrons star rotation fell

onto a neutron star?

A. The neutron stars rotation would speed up.

B. The neutron stars rotation would slow down.

C. Nothing, the directions would cancel each other

out.


39/76

X-Ray Bursts

Matter accreting

onto a neutron star

can eventually

become hot enoughfor helium fusion

The sudden onset offusion produces a

burst of X-rays


40/76

What is a black hole?


41/76

A black holeis an object whose gravity is so

powerful that not even light can escape it.


42/76

Thought Question

What happens to the escape velocity from an object ifyou shrink it?

A. It increases

B. It decreases

C. It stays the same


43/76

Thought Question


A. It increases

B. It decreases


Hint:


44/76

Thought Question


A. It increases

B. It decreases


Hint:


45/76

Escape Velocity

Initial Kinetic

EnergyFinal Gravitational

Potential Energy=

=(escape velocity)2 G x (mass)

2 (radius)

Li ht


46/76

Light

would not

be able to

escapeEarths

surface if

you could

shrink it to

< 1 cm
http://localhost/var/www/apps/conversion/tmp/scratch_3/escape_velocity_and_r.htm


47/76


48/76

3MSun

Black

Hole

The event horizon of a 3MSunblack hole is also about

as big as a small city

Neutron star

E t
http://localhost/var/www/apps/conversion/tmp/scratch_3/schwarzschild_r.htm


49/76

Event

horizon is

larger for

black holesof larger

mass
http://localhost/var/www/apps/conversion/tmp/scratch_3/schwarzschild_r.htmhttp://localhost/var/www/apps/conversion/tmp/scratch_3/Spacetime_Mass_Rad_Orbit.htm


50/76

A black holes mass

strongly warps

space and time invicinity of event

horizon

Event horizon
http://localhost/var/www/apps/conversion/tmp/scratch_3/Spacetime_Mass_Rad_Orbit.htmhttp://localhost/var/www/apps/conversion/tmp/scratch_3/Spacetime_Mass_Rad_Orbit.htmhttp://localhost/var/www/apps/conversion/tmp/scratch_3/Spacetime_Mass_Rad_Orbit.htmhttp://localhost/var/www/apps/conversion/tmp/scratch_3/Spacetime_Mass_Rad_Orbit.htm


51/76

No Escape

Nothing can escape from within the event

horizon because nothing can go faster than light.

No escape means there is no more contact with

something that falls in. It increases the hole

mass, changes the spin or charge, but otherwise

loses its identity.


52/76

Neutron Star Limit

Quantum mechanics says that neutrons in the

same place cannot be in the same state

Neutron degeneracy pressure can no longer

support a neutron star against gravity if its mass

exceeds about 3Msun

Some massive star supernovae can make black

hole if enough mass falls onto core


53/76

Singularity

Beyond the neutron star limit, no known force can

resist the crush of gravity.

As far as we know, gravity crushes all the matter into

a single point known as a singularity.


54/76

Thought Question

How does the radius of the event horizon changewhen you add mass to a black hole?

A. Increases

B. Decreases

C. Stays the same


55/76

Thought Question

How does the radius of the event horizon changewhen you add mass to a black hole?

A. IncreasesB. Decreases

C. Stays the same


56/76

What would it be like to visit a

black hole?


57/76

If the Sun shrank

into a black hole, its

gravity would bedifferent only near

the event horizon

Black holes dont suck!


58/76

Light waves take extra time to climb out of a deep hole in

spacetime leading to a gravitational redshi f t
http://localhost/var/www/apps/conversion/tmp/scratch_3/time_near_black_hole.htm


59/76

Time passes more slowly near the event horizon
http://localhost/var/www/apps/conversion/tmp/scratch_3/time_near_black_hole.htm


60/76

Thought Question

Is it easy or hard to fall into a black hole?

A. Easy

B. Hard


61/76

Thought Question


A. Easy

B. Hard

Hint: A black hole with the same mass as the Sun

wouldnt be much bigger than a college campus


62/76

Thought Question


B. Hard

Hint: A black hole with the same mass as the Sun

wouldnt be much bigger than a college campus


63/76


64/76

Do black holes really exist?

Finding Black Holes


65/76

Finding Black Holes

How do we know black holes exist?

Can we search for the absence of light?

Finding Black Holes


66/76

Finding Black Holes How do we know black holes exist?

We can search for them inX-RAYBINARY STARS
http://localhost/var/www/apps/conversion/tmp/scratch_3/formation_xray_bin.htm


67/76

Black Hole Verification

Need to measure mass

Use orbital properties of companion

Measure velocity and distance oforbiting gas

Its a black hole if its not a star and itsmass exceeds the neutron star limit (~3MSun)


68/76

Some X-ray binaries contain compact objects of mass

exceeding 3MSunwhich are likely to be black holes


69/76

One famous X-ray binary with a likely black hole is in

the constellation Cygnus

Finding Black Holes


70/76

Finding Black Holes

We can see the effect that a black hole has on its stellar

companion in an X-ray binary: Cygnus X-1 was the first good candidate for a black hole

Keplers 3rd Law gives a mass > 3 M

for unseen companion

it can not be a neutron star

the only thing that massive, yet small enough to be invisible is a

black hole

Finding Black Holes


71/76

Finding Black Holes

The effect that a black hole has on its stellar companion in an X-

ray binary allows us to measure its mass (we measure the Dopplershifts).

The most accurate masses for binary stars come from eclipsingbinaries.

Although Cygnus X-1 is not an eclipsing binary system, there areother, better, black hole candidates which are.

A0620-00 is an x-ray emitting, eclipsing binary system with aninferred mass for the unseen companion of 8 solar masses.

Is this a black hole?


72/76


73/76

Gamma-Ray Bursts

Brief bursts of

gamma-rays

coming from space

were first detected

in the 1960s


74/76

Observations in the 1990s showed that many gamma-ray bursts were coming from very distant galaxies

They must be among the most powerful explosions inthe universecould be the formation of a black hole


75/76

What causes gamma-ray bursts?

S d G R


76/76

Supernovae and Gamma-Ray

Bursts

Observations show that at least some gamma-ray bursts

are produced by supernova explosions

Some others may come from collisions between

neutron stars

Umn_supernova & Black Hole

Documents

Transcript of Umn_supernova & Black Hole