Announcements
• Reading for next class: Chapter 19
• Star Assignment 9,
due Monday April 12
Angel Quiz
• Cosmos Assignment 1,
Due Monday April 12
Angel Quiz
Death of Stars
1) White Dwarf
2) Neutron Star
3) Black Hole
4) Nothing
WHITE DWARFS corpse of small mass stars
• Core contracts until electrons are squeezed
so much, their velocity increases according
to the Uncertainty Principle
• Produces extra Pressure, stops contraction,
at Size about Earth
• White Dwarf slowly cools & becomes
redder
A white dwarf is about the same size as Earth
More Massive White Dwarfs are
Smaller
• More Mass
 More gravity
Need larger Pressure
Squeeze electrons
more to increase
their speed and
pressure
Smaller White
Dwarf
Maximum Mass for White Dwarfs
• Pressure of “degenerate”
electrons can only support
so much mass before
electron speed would =
speed of light. Electrons
get squeezed onto protons.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
• Maximum Mass of
White Dwarfs
= 1.4 Msun
S. Chandrasekhar
What
happens
to a
White
Dwarf
that
gains
more
mass?
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Question:
What happens to a white dwarf when it accretes
enough matter to reach the 1.4 MSun limit?
A. It explodes
B. It collapses into a neutron star
C. It gradually begins fusing carbon in its core
Question:
What happens to a white dwarf when it accretes
enough matter to reach the 1.4 MSun limit?
A. It explodes (White Dwarf SUPERNOVA)
B. It collapses into a neutron star
C. It gradually begins fusing carbon in its
Fate of Large Mass Stars
Core contracts & gets
hotter
Onion like layered
structure
Fuse heavier nuclei up
to Iron
Iron core shrinks, but
can’t fuse to heavier
nuclei & release
energy
Fate of Large Mass Stars
Iron core shrinks
e- + p -> n + n
No Pressure
Iron core collapses
Supernova
QuickTime™ and a
Sorenson Video 3 decompressor
are needed to see this picture.
What is the source of Energy for a
Supernova Explosion?
a) Chemical Energy?
b) Nuclear Energy?
c) Gravitational Potential Energy?
d) Dark Energy?
e) Thermal Kinetic Energy?
What is the source of Energy for a
Supernova Explosion?
a) Chemical Energy?
b) Nuclear Energy?
c) Gravitational Potential Energy
d) Dark Energy?
e) Thermal Kinetic Energy?
Test Supernova Theory
• Supernova 1987A close by in Magellanic Cloud
• Burst of neutrinos observed
Core collapsed and became very hot
Energy ~ 108 Lgalaxy ~ 1019 Lsun, Core mass 1.4 Msun
• Burst lasted several seconds
Neutrinos diffused out
• Progenitor star (unexpected)
Blue not Red supergiant
Smaller, shock reached surface faster (2 hrs between n & g)
Supernova are the source of all heavy
elements
• Explosion returns to
space the elements
produced nuclear fusion
during a stars life: C, N,
O, Ne, Mg, Si, S, Ca, Fe
• Elements heavier than
iron are only made
during supernova
explosions
What is left after a Supernova
Explosion?
1. Neutron Star
2. Black Hole
What is a Neutron Star?
• Ball of neutrons
• Remnant core of a massive star
supernova
• Supported by Pressure of degenerate
neutrons (Dv ~ h/mn Dx)
Because mn >> me, must be squeezed much
more to get large velocity & pressure
Neutron Star ~ Size of Lansing
Neutron
Star
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Discovery
• Theorized by J. Robert
Oppenheimer and Volkoff
in 1930s
• Discovered by Jocelyn
Bell Burnell
• Part of her PhD thesis
• Found regular pulses of
radio waves
Quic kTime™ and a
TIFF (Unc ompres sed) dec ompres sor
are needed to see this pic ture.
Crab
Pulsar,
f=30 /s.
P = 1/30 s
How do we see Neutron Stars?
• Gravity near NS very
strong (mass of Sun in
Size of Lansing)
• Gas falling into NS
(from companion binary
star) speeds up to almost
speed of light, becomes
very hot
• Emits x-rays in beam
along rotation axis,
~ lighthouse beacon
QuickTime™ and a
Animation decompressor
are needed to see this picture.
X-rays
Visible light
Test of Neutron Star Model
• Observe Crab Pulsar is slowing down
• Is slowing down because losing rotational
KE. Calculate rate of energy loss from rate
slowing down based on assumption is NS
• Compare rate of energy loss to observed
rate of energy emission from entire Crab
nebula
• They agree!!!
Must be NS
Maximum Mass of Neutron Stars
• Neutron stars are supported against gravity
by the pressure of “degenerate” neutrons
• More Mass
More Pressure
neutrons move Faster
neutrons more Squeezed together,
Dv ~ h/mn Dx
Maximum possible velocity = speed of light
Maximum mass neutron star ~ 3 Msun
If supernove remnant mass > 3 Msun
Gravity overcomes Pressure
Remnant collapses
Gravity increases
Fgravity = G M1 M2 / D2
Black Hole
Student Questions:
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What is a black hole
Do they exist
How do they form
Explain curved space-time
Is a BH a hole in the universe
How can we know anything about them
How can we find them
Can one live forever inside them
What is on the other side
Why does time run slower
How can more heat make gravity stronger
Where does stuff go that falls into them
What is a Black Hole?
• An object whose GRAVITY is so strong,
not even Light can escape it (that is you
would have to go faster than the speed of
light to escape)
Question:
What happens to the escape velocity from an
object if you shrink it?
A. Increases
B. Decreases
C. Stays the same
Question:
What happens to the escape velocity from an
object if you shrink it?
A. Increases
B. Decreases
C. Stays the same
Formation of Black Holes
If the collapsing core of a massive star which
produces the supernova explosion has more
mass than the pressure of degenerate neutrons
can support (> 3 Msun)
Nothing can stop its collapse
The escape velocity reaches the speed of light
Nothing can go faster than the speed of light
Black Hole
Surface of a Black Hole
• Surface where
escape velocity = speed of light
is surface of a Black Hole,
called Event Horizon
• Outside Event Horizon can escape,
inside can not
Question:
•
What happens to the SIZE of a BH if it
gains more mass?
a) Increases
b) Decreases
c) Stays the Same
Question:
•
What happens to the SIZE of a BH if it
gains more mass?
a) Increases (Gravity stronger, so escape
velocity = speed of light farther away)
b) Decreases
c) Stays the Same
If nothing can escape from a BH,
How do we know its there?
If gas falls into a BH
BH gravity makes it speed up
Conservation of Angular Momentum makes
it form an Accretion Disk, orbiting at nearly
the speed of light
Friction makes it very hot
Emits X-Rays
Black Hole Accretion Disk
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
How do we know it’s a Black Hole?
• Only Neutron Stars and Black Holes have
strong enough gravity to make infalling gas
hot enough to emit x-rays.
• If can determine mass of suspect (in a
binary system) & Mass > 3 Msun
Must be Black Hole
Do we see any Black Holes?
Black Holes are NOT
holes in the Universe
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
What
would
you see
as you
approach
a Black
Hole
What happens as you fall into a BH?
• Tides: gravity is
stronger on your feet
than your head,
because they are closer
• Gravity is towards
center of BH, squeezes
you from sides
What do your classmates see?
To answer this need to know a little of
Einstein’s theory of Motion and Gravity:
• Gravity is Motion in Warped Space - Time
• You can’t tell the difference between
acceleration by gravity and any other
constant acceleration
• E = mc2, energy and mass are same thing
measured in different units
Mass warps Space - Time
Warped Space - Time tells
Mass how to Move
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
Forget time, think just about warped space
Orbits in Warped Space - Time
c = circular, e = elliptical, u = unbounded
Elevator & Rocket
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
Gravity = Acceleration
Light Beam in an Elevator or Gravity
QuickTime™ and a
GIF decompressor
are needed to see this picture.
Gravity Attracts Light
Light generates Gravity
Reasonable since E = mc2
• Black Holes Gravity attracts light
• Light loses energy escaping from environs
of a Black Hole
• Escaping Light is redshifted to longer
wavelengths and periods
Your classmates would see you
slow down as you approached
the BH event horizon
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Can use period of light as a clock
Redshifted light oscillates with a longer period
Time appears to run slower near event horizon
You would appear to stop and hover (& fade
out) as you approached the Event Horizozn
What would you notice as you passed
the Event Horizon
Nothing special
• For you time does not slow down in a BH.
• You quickly crash into the previous matter
inside the BH
(But you couldn’t tell us about it)
What can we know about Black Holes?
•
•
Nothing can escape from inside an Event
Horizon
Long range forces can exert influence
outside Event Horizon
1. Gravity
2. Electric Force
•
Can determine:
1. Mass
2. Charge
3. Spin
Mini Black Holes can Evaporate
Mini BH produce strong tides (stellar BH
don’t have strong enough tides)
Lose energy by work of tidal gravity on
material outside the event horizon
Since energy = mass, they lose mass and
get smaller
Evaporate