Neutron Stars:

1054 - "guest star" in Taurus (Chinese Observation)

• visible in day (like Venus) - 2 weeks
• took year to fade

Lord Rosse saw gas remnants - 19th century - (He was really Irish!)

• Crab Nebula
• it expands! We can measure the expansion then extrapolate back -- gas has been expanding ~1000 years. Can go back to supernova of 1054.

Fricke & Bothe - 1933

• supernova event is often the collapse of normal star to neutron star

Neutron star

• made out of neutrons
• ~10 to 14 miles in diameter
• density is that of the nucleus of atom
• collapse of high mass stars
• Chandrasekhar mass limit -- need to have at least this mass to form
• neutron star [1.4 M_sun] (Chandrasekhar was 19 years old!!)
• Don't know upper limit of neutron star mass -- 2 to 3 M_sun maybe, probably more

Didn't seek out neutron stars because astronomers didn't expect to find them. The discovery came as a surprise.

1967 Jocelyn Bell (graduate student)

• radio wave telescope
• regular pulses from a spot in the sky
• LGM -- thought at first these were signals from sentient life
• called "pulsars" from pulsating radar sources

Rotating neutron star emits stream of charged particles -- in part due to strong magnetic field. We see radio waves from these streams of charged particles.

Interesting points:

1. Can't see pulsars that don't radiate in our direction. So ... spotting a supernova without a pulsar is not surprising.

2. Beacon lasts longer than supernova remnants, which disperse in relatively short time. So ... not surprising to find pulsars without supernova remnants.

3. To verify their ideas astronomers needed to find a pulsar in the middle of a supernova remnant. They found one in the Crab Nebula! Saw pulsing image on TV screen. Pulsed in optical as well as radio frequency. Fastest known pulsar at that time; pulses 30 times per second. Now have seen some that pulse ~1000 times/sec.

Black Holes:

If the neutron star is > 2 to 3 M_sun then collapse continues and the star becomes a black hole.

• light can't escape from black hole
• don't have to have seen supernova -- can have gravitational collapse directly to black hole -- there is no rebound, it stalls. Computer models indicate this is "easier" or "more likely" source of black holes. Few computer models show successful explosion to black hole.
• event horizon -- 10 to 12 miles from center.
• NOT VACUUMS OF SPACE!!
• not positive they exist. 99% certainty -- need more evidence such as witnessing formation

Normal star can be seen orbiting a "dark spot" in space.

• matter from star can be attracted to black hole.
• waiting matter -- accretion disk
• falling matter gets hot -- emits x-rays.

Cygnus X1 (first x-ray source in Cygnus) -- binary star -- one probably black hole because velocity of rotating one we can see is so high.

Inside Black Holes:

• curved space-time
• most black holes rotate because formed from rotating stars (non Schwartzchild -- these were the first proposed black holes - 1916); they are Kerr black holes.
• core - singularity -- pull of gravity is so strong that anything pulled in will be destroyed. Can not be observed -- don't occur as naked objects in nature.

Draw Picture Here: (Label ergo sphere, event or Schwartzchild horizon, core)

• light below the event horizon never escapes
• from event horizon to about twice as far out can escape if moving fast enough!
• ~1000 miles away -- enormous tidal forces (elongated and squished thin)

Q: Do black holes emit gravitational radiation?

A: Astronomers are looking for evidence.