Astronomy 162: Professor Barbara Ryden
Wednesday, February 5
``Though a good deal is too strange
to be believed, nothing is too
strange to have happened.''
- Thomas Hardy
- Most Type II supernovae leave behind
an extremely dense neutron star.
- A neutron star is a compact object supported by
- Rapidly rotating, strongly magnetic neutron stars
produce narrow beams of radiation.
(1) Most Type II supernovae leave behind an extremely dense neutron star.
Just a reminder: A type II supernova occurs when the iron core
of a supergiant star collapses to the density of
an atomic nucleus (a few hundred million tons per
cubic centimeter). At such tremendously high densities,
protons and electrons are fused together into neutrons.
The relevant reaction is this:
e- + p -> n + neutrino
About 1057 neutrinos are made in the
iron core, as the protons (p) are converted to neutrons (n).
The billion trillion trillion trillion trillion neutrinos
carry off most of the supernova's energy (photons are just
a minor byproduct of a supernova).
After its ``bounce'', the star's core settles down as a
sphere of tightly packed neutrons, known as a
A neutron star can be thought of as a single humongous atomic
nucleus (containing roughly 1057 neutrons)
with a mass between 1 and 3 solar masses, packed into a
sphere 5 to 20 kilometers in radius. To put things
into perspective, a neutron star is about as big as
the beltway around Columbus.
In addition to being amazingly
dense, neutron stars have other amazing properties:
The surface of a neutron star is not anyplace you would want to
visit. The gravitational acceleration is 100 billion g's (that is,
100 billion times the gravitational acceleration at the Earth's surface). The
escape speed at the surface of a neutron star is half the
speed of light (that is, 150,000 km/sec, versus a paltry 11 km/sec for
the Earth). On the surface of a neutron star, you'd be simultaneously
vaporized by the intense heat and squashed flat by the intense
- Rapidly rotating:
up to 1000 rotations/second, compared
to 1 rotation/month for the Sun.
- Strongly magnetized:
up to 1 trillion Gauss, compared to an average of
1 Gauss for the Sun (and 0.5 Gauss for the Earth).
- Very hot: initially 1,000,000 Kelvin at the surface,
compared to 5800 Kelvin for the Sun.
(2) A neutron star is a compact object supported by
Neutrons, like electrons, must follow the laws of quantum mechanics.
In particular, they must obey the Pauli exclusion principle, as
outlined in last Thursday's lecture.
The existence of neutron stars was actually first predicted
in 1933, only a year after the discovery of the neutron.
At a density of 1 ton/cm3, electrons are degenerate,
and provide degenerate-electron pressure.
At a density of 400 million tons/cm3, neutrons
are finally degenerate, and provide
The interior structure of a neutron star is fairly uncertain.
(We don't know a lot about how matter behaves at these amazingly
high densities.) One proposed model looks like this:
Just as there is an upper limit on the mass of a white
dwarf, there is an upper limit on the mass of a neutron star.
White dwarfs can't have M > 1.4 Msun; above
this mass, the degenerate-electron pressure is insufficient
to prevent collapse. Neutron stars can't have M > 3 Msun;
above this mass, the degenerate-neutron pressure is
insufficient to prevent collapse (the upper mass limit
for neutron stars is fairly uncertain). If a dense
object is too massive to be a white dwarf or a neutron
star, it's BLACK HOLE TIME (more about black holes next week..)
It's certainly true that the laws of quantum mechanics predict
the existence of neutron stars. However, how can we detect
them, to verify that they actually exist? Well, neutron
stars may be tiny, but they are also hot, and hence produce
a significant amount of blackbody radiation.
At a temperature of 1,000,000 Kelvin, the wavelength of maximum
emission is at 2.9 nanometers -- in the X-ray range. We can
hunt for hot neutron stars by looking for X-ray sources.
Although most of the light from neutron stars is emitted
at X-ray wavelengths, the
nearest neutron star can also be glimpsed at visible wavelengths.
- R = 15 km = 0.00002 Rsun
- T = 1,000,000 K = 170 Tsun
- Therefore, L = (0.00002)2 (170)4 Lsun
= 0.3 Lsun
(3) Rapidly rotating, strongly magnetic neutron stars
emit narrow beams of radiation.
Although neutron stars do emit blackbody radiation, they
are not simply boring spherical blackbodies, as stars are.
Neutron stars have additional ways of emitting electromagnetic
radiation. The strong magnetic field and rapid
rotation of a neutron star make it a very potent
electrical generator. (Here on Earth,
commercial electrical generators work
by rotating a series of magnets inside a coil
of wires. The essential point is that you need
to have a magnetic field in motion.) The
electric field generated by the rotating
magnetized neutron star is strong enough to
rip charged particles (such as electrons) away
from the surface of the neutron star.
The charged particles follow the magnetic
field lines to the north and south magnetic
poles of the neutron star. (Remember, when
I discussed the
magnetic field of the Sun, I pointed
out that charged particles move most readily
along the magnetic field lines, rather than
perpendicular to them.) The accelerated
particles produce intense but narrow
beams of radiation,
pointing away from the two magnetic poles.
We can see one of these beams of light
ONLY if it is pointing toward us, just
as we see the light from a flashlight only
when it is pointing toward us.
A complicating factor is that on a neutron
star, just as on Earth, the magnetic
poles don't coincide with the
rotational poles. Thus,
the beams of radiation pointing away from
the magnetic poles are at an angle to
the rotation axis of the neutron star;
as the neutron star rotates, the beams
swing around in a cone. If a beam happens
to sweep across our location in space, we
see a brief flash of light. (This is sometimes
known as the ``Lighthouse effect''. If you
are down by the shore at night, you see
lighthouses emit a blinking light. This
is not because the lamp in a lighthouse is
turned off and on, but because it inside
a searchlight which is rotated around and
around. As the beam of light from the
searchlight sweeps across your location,
you see a brief flash of light.)
Neutron stars whose beams of electromagnetic
radiation happen to sweep across us are
called pulsars; we'll
learn more about them in tomorrow's lecture.
Prof. Barbara Ryden
Updated: 2003 Feb 5
Copyright © 2003, Barbara Ryden