ASTR 1040 October 24 th Homework 4 due
ASTR 1040 – October 24 th) Homework 4 due Thursday (26. Lecture in Planetarium next Tuesday (Halloween) Second mid-term exam Thursday November 2 Website http: //casa. colorado. edu/~wcash/APS 1040. html
Neutrino Astronomy Ground Level Fill mineshaft with water Put photo detectors around the inside
Neutrino Going Through Water n n n e+ e (scattering) n light produced by secondary interactions
Super Kamiokande
Detection of SN 1987 A • Kamiokande II • IMB Japan Ohio 12 events 15 s 8 events 5. 6 s Mont Blanc neutrino detector saw a marginal signal 4. 7 hours earlier. Real? ? ? In 1987 A we detected the formation of a new neutron star!
Pulsars • • • Neutrons Stars considered unobservable Forgot the effect of magnetic fields When a magnetic spins it creates electric fields Electric fields create accelerated electrons Accelerated electrons create strong radio signals Pulsar is a spinning, magnetized neutron star
Intense Magnetic Field not necessarily aligned with spin axis Particles get thrown out along the polar axes (cannot cross field lines) Beam radio signal along magnetic axis too.
From Above Every time beam sweeps by we see a pulse
Lighthouse Analogy Some of the Cosmic Rays that reach Earth could have been created in Pulsars
Pulse Trains Listen to the Pulsars: PSR B 0329+54 1. 4 Hz Vela Pulsar 11 Hz
Center of the Crab Nebula Chandra (X-rays) and HST (Visible)
Pulsar Power • • Energy Source is the Rotation As Pulsar emits, rotation slows As pulsar slows, it becomes less luminous Rapidly fade out Period Glitches or Starquakes Time
The Binary Pulsar Pulses get closer on approaching side of orbit (Doppler) Can map out orbit Two neutron stars spiraling toward each other. Will merge into black hole in about 200, 000 years Friction that causes the inward spiral caused by Gravity Waves Period First confirmation of Einstein’s prediction Time
An X-ray Pulsar Hercules X-1 Discovered in 1971 Pulses every 1. 24 seconds in the x-ray Eclipses every 1. 7 days due to binary orbit Turns off every 35 days due to accretion disk wobble The Rosetta Stone of X-ray Astronomy Proved the bright x-ray sources were mass transfer binaries with a neutron star. There about 100 of these “Classical X-ray Binaries” in the Milky Way
X-ray Bursters Intensity Time Every few hours the x-ray source will “burst” for 30 seconds Explanation: Helium accumulates on surface of the neutron star in binary Periodically it explosively burns Nuclear release insufficient to blow it out into space Collapses back and cools A Neutron Star Nova
Black Holes • Are there any Black Holes? YES. Just 20 years ago, the answer was “probably”. The term didn’t exist until 1969
• What Happens when a neutron star exceeds its Chandrasekhar Limit? • Must Collapse • No known force powerful enough to halt the collapse • Result: Black Hole
Things that don’t happen • Collapse to Singularity • Winks out of universe • Explodes by some new force • Einstein’s prediction wasn’t a “sure thing”
Formation Neutron Star Collapses Now it’s inside event horizon and can never be seen again.
A Newtonian Black Hole Energy Falling To Surface Kinetic Energy OR if v=c
Schwarzschild Radius Two errors cancel. This is the radius of the “Event Horizon” The event horizon is a true singularity in space-time. It is a place where time and space cease to exist.
Geometry of Black Hole Can you boost the signal? No, that doesn’t help. Space Curves in on itself There’s no path out!
Curved Space
Rubber Sheet Analogy
Properties - Size The radius of a black hole is 3 km per solar mass There is no limit on size or mass. Note: Volume rises as the cube of the mass. Implies the larger the black hole gets, the lower is its density. A sphere of water the size of Saturn’s orbit would be a black hole!
Properties – Escape Velocity No way. Not even light can escape. No signal can escape No particle Nothing
Properties – Orbital Period Material orbiting a black hole will have milli-second periods
Properties - Basic 1. Mass (Schwarzschild Black Hole) 2. Electric Charge (doesn’t happen) 3. Angular Momentum (Kerr Black Hole) No “Surface” Features No Magnetic Fields No Pulsing “No Hair”
Jump on In You won’t make it, but it would be quite a ride!
Properties – Energy Emitted • Energy Released from accretion • about 0. 1 mc 2
Time Dilation Now it gets weird! Time does not run at the same rate everywhere in the universe. Twins are not always the same age. Clocks run a little bit faster in Colorado than in Washington DC. Folks in DC are actually a little slower than us. About one part in a trillion. That’s a millisecond over a lifetime.
Time Dilation Some event takes dt∞ out in free space. Same event takes place at radius r from center of a black hole. Now view it from free space. Takes dt instead. Longer. It looks like things are moving slower. If you are near the black hole, the rest of the universe appears to be moving faster.
Time Dilation As r approaches Rs dt gets longer and longer. When r reaches the event horizon, time stops. We know how to make a time machine with a forward switch only! Just fly to a black hole and orbit above the surface. But you can fall in really fast as viewed from outside.
A “frozen star” If time stops above the surface, it can’t go in. How does the hole form? Is the material still stuck in temporal limbo just above the surface? What’s inside? If nothing can get out, then how can gravity?
Hawking Radiation The vacuum makes pairs of electrons and positrons that pop into existence and then annihilate without any net effect. Above a black hole, one can get sucked in. The other annihilates above the surface to cause radiation. Since its close to the surface, the light gets redshifted escaping, but it carries energy with it!
Temperature Hawking derived that the temperature of a black hole is thermal. Energy for the radiation comes from the mass of the hole. The black hole shrinks with time. That’s a really long time unless the black hole is tiny. A 1 kg black hole would last only 10 -16 s.
Artist’s impression of Cyg X-1 (NASA) 6 M 0 BH orbiting an O 9 star. 2000 pc away in Cygnus
- Slides: 38
