Stellar Remnants White Dwarfs – Supported by electron degeneracy pressure – Left over from low mass stars – What are they made of? Neutron Stars – Supported by neutron degeneracy pressure – Left over from high mass stars Black Holes – State of matter is not well understood – Left over from really high mass stars
White Dwarfs Exposed core of expired low mass star Outer layers of the star were expelled as a planetary nebula White dwarf
White Dwarfs Small (~Size of Earth) Massive (~1 Sun) Dense – 1 teaspoon weighs as much as a truck Made of carbon Hot – Some shine in UV and X-ray – Sirius is a binary system Sirius A: Brightest star in the sky Sirius B: White Dwarf Which is which? Photo from Chandra X-ray Observatory
White Dwarfs No fusion – Can’t fuse the carbon Electron degeneracy pressure: – Electrons can’t cram together any closer – Supports the star against gravity
Chandrasekhar Limit Compress a white dwarf, electrons go faster Eventually the electrons approach the speed of light – Nothing can exceed the speed of light – At 1.4 M Sun, electrons would reach the speed of light – No white dwarfs have ever been observed with masses > Chandrasekhar Limit
Accretion Disks White dwarf by itself: – Slowly fades away White dwarf with a friend: – More interesting! – Steals mass from its friend
Accretion Disks Mass starts off with small rotation As it falls in: – Conservation of angular momentum makes it rotate faster – Strong gravity of white dwarf makes disk spin fast – Faster rotation on inside, slower on outside LOTS of friction; the disk gets very hot
Nova White dwarf has strong gravity If it is accretes enough H, can fuse the H Nova: burst of fusion blows the layer off! 100,000x weaker than a supernova
White Dwarf Supernova Even with the recurring novas, white dwarfs gain mass Can suddenly fuse the carbon! – Just before Chandrasekhar Limit (1.4 M Sun ) “This day is the white dwarf’s last.” ** Sometimes called a “carbon bomb”
Type 1a Supernova and Distance White Dwarf supernovae are also called Type 1a Supernovae Because they are thought to occur when a white dwarf begins carbon fusion, we are always dealing with the same 1.4 M Sun ball of carbon and therefore the same explosion! By looking at the brightness of a Type 1a Supernova, we can figure out how far away it is. – They are bright; we use these to measure distances across the universe
Neutron Stars An iron core that collapsed into neutrons – Product of a supernova Around 10 miles in diameter Spinning FAST! Supported by neutron degeneracy pressure DENSE!
Neutron Star Gravity A teaspoon of neutron star weighs 10 million tons If it ran into Earth, Earth would be “squashed into a shell no thicker than your thumb on the surface of the neutron star”. A paper clip with the density of a neutron star would weigh as much as Mount Everest
Neutron Stars: Pulsars Jocelyn Bell built a radio telescope to study fluctuating radio sources She noticed a periodic signal in Cygnus – Precisely 1.337301 seconds apart! – Actually thought to be aliens
Neutron Stars: Pulsars Two of these objects were found in the Vela Nebula and the Crab Nebula (supernova remnants) So they’re neutron stars – But why don’t they all have this periodic signal?
Neutron Stars: Pulsars Neutron stars have strong, narrow cones of radiation Not always pointed along axis of rotation If the cones sweeps across us, it’s a pulsar
Neutron Stars: Pulsars Just like a lighthouse. All pulsars are neutron stars, but not all neutron stars are pulsars.
Neutron Stars: Pulsars Why does it spin so fast? – Sun spins once every 26 days – The neutron star (pulsar) spins up to 625 times per second! (fastest) Conservation of angular momentum: – It went from 109 Earths across to 5 miles across
Black Holes Gravity wins. Hands down. No thermal pressure. No electron degeneracy. No neutron degeneracy. This is a frontier of physics. It is NOT simple.
Black Holes Gravity is so strong that light cannot escape Light does not respond to gravity like, say, a ball. – It responds to space-time being curved. It follows what it thinks is a straight path.
Black Holes Event Horizon: – Anything inside the Event Horizon must move faster than light to escape. – Nothing moves faster than the speed of light. – (Think about the name! We can’t see events beyond the event horizon.)
Black Holes A solar-mass black hole is about 2 miles across A single, infinitely dense point at the center of the event horizon
Trip to a Black Hole Say we visit one. 1000 miles away: – Throw a blue clock out the window: – Slows down, turns red! – At 5 miles from event horizon, ticks half as fast Near event horizon: – Clock pretty much stops – Blue light has shifted to radio frequencies!
Trip to a Black Hole Your friend wants to go inside! Falls toward event horizon. – Feels time passing normally – Sees YOUR time going fast. Also sees you turning blue.
Trip to a Black Hole Friend’s point of view: Your friend passes through the event horizon Your point of view: Your friend approaches the event horizon more and more slowly – Their time slows from your perspective – Right at the event horizon, takes an infinite amount of time for anything to happen; they just hang there – They disappear because the light is infinitely red- shifted
Trip to a Black Hole But really, did your friend survive the trip into the event horizon?
Observing Black Holes The night sky is black. A black hole is black. …? Look for X-ray binaries with unseen companions Measure mass of unseen companion; exceeds allowed neutron star mass