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. Maximum Mass of White Dwarfs = 1.4 M sun S. Chandrasekhar

What happens to a White Dwarf that gains more mass?

Question: What happens to a white dwarf when it accretes enough matter to reach the 1.4 M Sun 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 M Sun 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 +  No Pressure  Iron core collapses  Supernova

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 ~ 10 8 L galaxy ~ L sun, Core mass 1.4 M sun Burst lasted several seconds  Neutrinos diffused out Progenitor star (unexpected)  Blue not Red supergiant  Smaller, shock reached surface faster (2 hrs between &  )

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 (  v ~ h/m n  x) Because m n >> m e, must be squeezed much more to get large velocity & pressure

Neutron Star ~ Size of Lansing  eutron Star

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

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

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,  v ~ h/m n  x  Maximum possible velocity = speed of light  Maximum mass neutron star ~ 3 M sun

If supernove remnant mass > 3 M sun  Gravity overcomes Pressure  Remnant collapses  Gravity increases F gravity = G M 1 M 2 / D 2

Black Hole

Student Questions: 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 M sun )  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

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 M sun  Must be Black Hole

Do we see any Black Holes?

Black Holes are NOT holes in the Universe

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 = mc 2, energy and mass are same thing measured in different units

Mass warps Space - Time Warped Space - Time tells Mass how to Move Forget time, think just about warped space

Orbits in Warped Space - Time c = circular, e = elliptical, u = unbounded

Elevator & Rocket

Gravity = Acceleration Light Beam in an Elevator or Gravity

Gravity Attracts Light Light generates Gravity Reasonable since E = mc 2 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 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