Compact
Stars
White dwarfs, neutron stars,
black holes: this is a list of objects
in which each is smaller, denser and
more extreme in its physical conditions
than the one before. The compression
is a result of the familiar force of
gravity, but the condensed stars which
result are outside our Earth-bound experience.
A matchbox-sized piece of white dwarf
material would weigh tens of tons, while
the mass of a battleship in neutron
star material occupies the space of
a pinhead.
A white dwarf, which is a star about
the size of the Earth but with a mass
similar to that of the Sun, is prevented
from shrinking further by `electron
degeneracy pressure' -- under the laws
of quantum mechanics, free electrons
can be packed only so closely together
and not more. In some stars, usually
more massive than white dwarfs, this
barrier is overcome when the electrons
combine with protons to form neutrons,
which can pack together 2000 times more
closely. A neutron star has one and
a half times the mass of the Sun, but
is only about 20 kilometres across.
Neutron stars are created when the core
of a dying, massive star collapses,
triggering the explosive ejection of
the outer parts of the star in a supernova.
Although neutron stars are so small
and cannot generate light by fusion,
some can be observed at great distances
by an entirely different kind of radiation,
a regularly pulsating radio signal.
These are the pulsars.
What
Are Pulsars?
Pulsars were discovered in 1967 by Jocelyn
Bell and Anthony Hewish at the radio
astronomy observatory (now the Nuffield
Radio Astronomy Observatory) at Cambridge.
Pulsar radio emission is very distinctive,
a uniform series of pulses, spaced with
great precision at periods of between
a few milliseconds and several seconds.
Over 700 radio pulsars are known. Some
pulsars have also been detected by optical,
X-ray and gamma-ray telescopes.
The regularity of the pulses is phenomenal:
observers can now predict the arrival
times of pulses a year ahead with an
accuracy better than a millisecond.
How can a star behave as such an accurate
clock? The only possibility for so rapid
and so precise a repetition is for the
star to be very small, rotating rapidly
and emitting a beam of radiation which
sweeps round the sky like a lighthouse,
pointing towards the observer once per
rotation. The only kind of star which
can rotate fast enough without bursting
from its own centrifugal force is a
neutron star.
Pulsars are rapidly rotating, very strongly
magnetised neutron stars, with fields
of strength reaching 1000 million Tesla
(10 million million Gauss. For comparison,
the Earth's magnetic field measures
less than 1 Gauss). These extreme properties
result from the compression of the original
star's core, which would have had a
weaker magnetic field and slower rotation.
They make neutron stars into powerful
electric generators, capable of creating
and accelerating charged particles to
energies of a thousand million million
Volts. These particles are the source
of the beams of radiation in radio,
light, X-rays and gamma-rays. They also
carry away much of the pulsar's energy
in the form of a fast wind. Their energy
comes from the rotation of the star,
which must therefore be slowing down.
This slowing down can be detected as
a lengthening of the pulse period. Typically
a pulsar rotation rate slows down by
one part in 10 million each year: the
Crab Pulsar, which is the youngest and
most energetic known, slows by one part
in two thousand each year.
How
Many Pulsars In Our Galaxy?
Pulsars are found mainly in the disc
of the Milky Way, within about 500 light-years
of the plane of the Galaxy. A complete
survey of the pulsars in the Galaxy
is impossible as faint pulsars can only
be detected if they are nearby. Radio
surveys have now covered almost the
whole sky, and over 1,000 pulsars have
been located. Their distances can be
measured from a delay in pulse arrival
times observed at low radio frequencies;
the delay depends on the electron density
in interstellar gas and on the distance
travelled. Extrapolating from the small
sample of detectable pulsars, it is
estimated that there are around 200,000
pulsars in the whole of our Galaxy,
taking into account those whose lighthouse
beams do not sweep across in our direction.
Each pulsar radiates for around 20 million
years; after this time it has lost so
much rotational energy that the particle
creation process begins to fail and
radio pulses are no longer produced.
If we know the total population (200,000)
and the lifetime (20,000,000 years),
we can deduce that a new pulsar must
be born in our galaxy roughly every
100 years (assuming that the population
remains steady).
If pulsars are born in supernova explosions,
then the rate of supernovae must be
at least as high as the pulsar birth
rate. In fact the supernova rate is
probably also around one every 100 years
or higher. Supernovae are spectacular
events. The last directly observed supernova
in our galaxy was Kepler's supernova
of AD 1604, but we do know that others
occur which are less spectacular or
which are hidden from us by interstellar
dust clouds.
The
Crab Pulsar
The Crab Nebula is the visible remnant
of a supernova explosion which was witnessed
in AD 1054 by Chinese and Japanese astronomers.
Near the centre of the Nebula is the
Crab Pulsar, which is the most energetic
pulsar known. It rotates 30 times per
second, and it is very strongly magnetised.
It therefore acts as a celestial power
station, its wind supplying enough energy
to keep all of the Nebula emitting radiation
over practically the whole of the electromagnetic
spectrum.
The Crab Pulsar radiates two pulses
per revolution: this double pulse profile
is similar at all radio frequencies
from 30 MHz upwards, and in the optical,
X-ray and gamma-ray parts of the spectrum,
covering at least 49 octaves in wavelength.
The visible light is powerful enough
for the pulsar to appear on photographs
of the Nebula, where it is seen as a
star of about magnitude 16. Normal photographs
smooth out the pulses, but stroboscopic
techniques can show the star separately
in its `off' and `on' conditions. Images
of the pulsar have also been made by
the Hubble Space Telescope.
Binary
Pulsars and General Relativity
Many stars are members of binary systems,
in which two stars orbit around each
other with periods of some days or years.
A number of binary systems are known
in which one of the stars is a neutron
star. Some of the most spectacular are
bright X-ray sources and are known as
X-ray binaries or X-ray pulsars. In
these systems, gas is being drawn from
the outer layers of a companion star
by the neutron star's gravitational
pull. As the gas falls towards the neutron
star, a large amount of energy is released,
mostly as X-rays. X-ray binary pulsars,
unlike radio pulsars, are powered by
the infalling matter rather than the
pulsar's rotation. They usually rotate
much more slowly than radio pulsars
and can be slowing down, speeding up,
or do both at different times.
Not all pulsars in binary systems are
accreting matter from their companion.
These are detected as radio pulsars
and most of them are `millisecond pulsars',
which rotate much more quickly (hundreds
of times per second) and have magnetic
fields thousands of times smaller than
normal pulsars. Millisecond pulsars
have even more precise rotation periods
than other pulsars and have extremely
long lifetimes because they are slowing
down only very slowly, around 1 part
in 10 thousand million every year. There
are probably at least as many of them
in the galaxy as there are normal pulsars.
Many millisecond pulsars are in binary
systems and it is believed that they
are probably the end product of X-ray
binary pulsars with relatively low mass
companion stars. Very recently, X-ray
observations have revealed millisecond
rotation periods in X-ray binaries,
making the final link with millisecond
radio pulsars. The transfer of matter
when the system is an X-ray binary speeds
up the neutron star to millisecond rotation
periods and leaves it eventually as
a rotation-powered millisecond radio
pulsar orbiting the remnants of its
companion, often now a white dwarf.
The most famous binary radio pulsar
is the Hulse-Taylor pulsar, PSR 1913+16,
which has another neutron star as its
companion. The pulsar is a normal radio
pulsar, not a millisecond pulsar, with
a rotation period of 59 milliseconds.
The two stars are so close that their
orbital period is less than eight hours,
but no matter streams between them;
they interact only by their mutual gravitational
attraction. The orbit of the pulsar
can be described in great detail because
the arrival times of its pulses at the
Earth are like the ticks of an accurate
clock moving very rapidly in a strong
gravitational field, which is the classical
situation required for a test of Einstein's
General Theory of Relativity.
According to non-relativistic, or Newtonian,
dynamical theory, the orbits of both
stars should be ellipses with a fixed
orientation, and the orbital period
should be constant. Measurements of
the arrival time of the pulses have
shown significant differences from the
simple Newtonian orbits. The most obvious
is that the orbit precesses by 4.2 degrees
per year. There is also a small, but
very important, effect on the orbital
period, which is now known to be reducing
by 67 nanoseconds (less than one ten-millionth
of a second) each orbit.
The reducing orbital period represents
a loss of energy, which can only be
accounted for by gravitational radiation.
Although gravitational radiation itself
has never been observed directly, the
observations of PSR 1913+16 have provided
good proof of its existence. It is appropriate
that this discovery, which is a further
confirmation of the predictions of the
General Theory of Relativity, was announced
in 1979, which was the centenary of
Einstein's birth.
PSR
B1828-11
Three Jodrell Bank scientists (Ingrid
Stairs, Andrew Lyne and Setnam Shemar)
have been studying 13 years' worth of
data from the pulsar PSR B1828-11. This
pulsar rotates 2.5 times per second,
but, unlike any other, wobbles regularly
with a period of about 1000 days. The
motion is very much like the wobble
of a top or gyroscope. This wobble,
or precession, has two manifestations:
it causes the observed pulse to change
its shape, and causes the time between
pulses to vary, becoming sometimes shorter,
sometimes longer.
In an article in the 3 August 2000 issue
of Nature, the Manchester astronomers
argue that these variations imply that
the neutron star, instead of being perfectly
spherical, is slightly oblate. Stairs
explains:
``The bulge in the neutron star causes
the angle between the pulsar's rotation
axis and its radio beam to change with
time, creating the wobbling effect that
we measure.''
Lyne emphasizes that the oblateness
is incredibly small:
``This star departs from being a perfect
sphere by only 0.1 mm in 20 km. On Earth
this would mean that no mountain could
be higher than 3 cm!''
The surprising aspect to the discovery
is not the small size of the wobble,
but that fact that it is seen at all.
Astronomers know from other long-term
observations, mostly done at Jodrell
Bank, that a pulsar is made up largely
of a neutron superfluid, with a solid
crust. Current theories predict that
the interaction between the superfluid
and the crust should cause any precession
to die out extremely quickly.
``But this pulsar is one hundred thousand
years old, and it's still wobbling!''
exclaims Lyne. ``We really don't understand
how this precession can be happening,
and theorists are going to have to do
some work to explain it,'' adds Stairs.
