Neutron Stars

• What are they? Extremely compact and dense star remnants, made mostly of neutrons, held up by neutron degeneracy pressure which balances gravity.
• How do they form? From the collapsed cores of massive stars after supernova explosions; It appears that during the formation process they receive a kick that gives them a speed of up to 1000 km/s, for reasons that are not understood.
• Mass and size: 1.4 to almost 3 solar masses, 20-30 km diameter; The more massive, the smaller they are! Their density is billions of tons per cc, their gravity tens of millions of times stronger than Earth's.
• Important facts: Fast rotation (spin-up from collapse, and possible later accretion); Magnetic fields up to a trillion times the Earth's (or at least that's what our models tell us).
• Different kinds: They depend on the radiation they emit, mostly very high energy; [Two types are Soft Gamma-ray Repeaters (SGR), or Anomalous X-ray Pulsars (AXP), which are actually magnetars].
• How do we find them? They are so small that are difficult to see directly (although their hot surfaces can emit X-rays), but with some luck...

Pulsars

• What are they? Objects that emit regular pulses of radio waves, first detected in 1967.
• Examples: The center of the Crab nebula M1 or the Vela nebula (both are SN remnants); about 1800 are known as of 2009, some in globular clusters.
• Periods: Most range from 0.03 to 0.3 sec (although a few are much longer), and are very stable: They are the most precise clocks known!
• Explanation: They are spinning neutron stars with hot spots; In addition to their thermal radiation, they have rotating radio beams, like lighthouses.

• How can they be in binaries? A neutron star can replace another star; Some may acquire "planets" from remains of nearby stars.

  Neutron Star Binaries

• What do they do that is different? Matter accumulates in accretion disks, which produce flashes [and can also cause the NS to spin up – millisecond pulsars], and they emit bursts of X-rays or gamma rays.
• Matter jets: The magnetic field can also produce jets of particles, as in the case of SS443.
• What if one is a pulsar? We can tell it's in a binary indirectly, from Doppler changes in their pulse periods; A double pulsar was discovered in 2003.
• And so? We can tell very precisely if the orbit is changing in any way.

Beyond Neutron Stars

• What if the star's core has more than 3 solar masses? The collapse continues.
• Down to what? Quark stars? Maybe [some evidence for them was reported recently]; But when matter collapses beyond a certain point, the only possibility is the ultimate collapse to a black hole...