Lecture 31 - Neutron Stars and Pulsars (4/3/96)
Seeds: Chapter 11
- Supernova Recap
- The end result of a star's life depends upon the mass of
- M < 0.01 Msun:
Planet (big Jupiter)
- 0.01 < M < 0.08 Msun:
Brown Dwarf (failed star)
- 0.08 < M < 0.25 Msun:
Helium White Dwarf
- 0.25 < M < 8 Msun:
C-N-O White Dwarf -> planetary nebula
- 8 < M < 10 Msun:
O-Mg-Ne White Dwarf -> planetary nebula
- 10 < M < 40 Msun:
Neutron Star -> supernova
- M > 40 Msun:
Black Hole -> supernova
- SN1987A began as a 20 Msun star, evolved to a red
giant of luminosity 60000 Lsun, burned increasingly
in its core, exploded in a supernova, and was observed here on
Earth on February 24, 1987.
- Neutron Stars
- Neutron stars have typical masses of 1.5 Msun within a radius
of 10-15 km.
- Neutron stars are very hot soon after their creation, but have
a tiny surface area and so are very faint.
- During collapse, conservation of angular momentum causes
neutron star to spin up in rotation frequency by the inverse
ratio of the radii squared.
- Also during collapse, the magnetic field is amplified by the
ratio of the densities (inverse radius cubed).
- There is tremendous spin and magnetic energy in the neutron star,
not to mention the gravitational energy in the immense surface
- Thus, we might hope to detect emission that taps one of these
- In 1967, Hewish and Bell discover "pulsed" radio emission from
unknown astronomical objects. These are later identified as
from rapidly rotating neutron stars. Hewish later received the
Nobel Prize for this discovery.
- These objects are called pulsars after the pulses
received once every rotation.
- The pulses are due to a beam of radiation emitted from the
magnetic poles of the pulsar (which like the Earth's are
not aligned with the rotational axis) which sweep by the
Earth like a searchlight.
- This radio emission comes from synchrotron radiation
from high energy particles caught in the strong magnetic
fields coming out of the pole of the pulsar.
- There is a pulsar in the center of the Crab Nebula. This
is what provides the power for the nebula emission, through its
magnetic and rotational energy.
- The Crab pulsar is rotating with a period of 0.033 seconds (!),
but is slowing down due to the loss of energy to the nebula.
Eventually it will be spinning too slowly to emit much radiation
and will disappear from the skies.
- Currently over 500 pulsars are known, with periods ranging from just
over 0.001 seconds to tens of seconds. Pulsars have been seen
pulsing at optical, X-ray, and Gamma ray wavelengths. Some are
even in binary systems (how did they survive the supernova?).
- The Structure of Neutron Stars
- A neutron star is made, basically, of neutrons. It is like a giant
solar-mass atomic nucleus!
- The conditions inside a neutron star are far removed from what
we can study in our laboratories here on Earth. There is much
interest, theorising, and speculation concerning the internal
state of neutron star matter.
- However, there is some internal structure to the neutron star.
- The outer kilometer or so is a crust made of heavy nuclei
(like iron) and electrons.
- Below this, there is a zone 3-5 kilometers thick of superfluid
neutrons. Superfluidity is a strange state of matter where quantum
effects allow bizarre behavior to occur.
- In most of the central zone of the neutron star, superfluid neutrons
are joined by a small fraction of superconducting protons and
- Some astrophysicists speculate that at the heart of a neutron star
is a core made of an even stranger state of matter.
- Accretion Disk Radiation
- The pulsar emission taps into the magnetic and rotational
energy of the neutron star. There is also a great deal of
energy in the gravitational field also.
- The surface gravity of the neutron star is so great that a
few grams of matter released from 1 AU away will impact the
surface with the energy of a several megaton nuclear explosion!
- Gaseous material gravitationally swept up near a neutron star will
be accelerated to high velocities.
- In addition tidal forces near the neutron star will be strong enough
to rip apart solid or gaseous bodies, and disperse the matter into
a rotating disk surrounding it.
- This accretion disk, which is similar to the protostellar
disks we studied earlier, will be heated by the gravitational
acceleration and tidal forces to high temperatures (millions of
- This high temperature gas will emit X-rays, which can be seen with
X-ray telescopes on Earth-orbiting satellites. We have located many
neutron star systems this way.
- We have found a number of neutron-star / normal star binary systems
where gas from the normal star has made its way to an accretion
- Neutron stars are pretty extreme objects, but we believe there
are even more extreme objects out there!
Next Lecture - Black Holes
The Structure of Neutron Stars
Accretion Disk Radiation
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Steven T. Myers (email@example.com)
Last revised 08Apr96