- 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
difficult to see directly (although their hot surfaces
can emit X-rays), but with some
- 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.
- 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
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...
page by luca bombelli <bombelli at olemiss.edu>,
modified 16 apr 2013