Astronomy 162: Professor Barbara Ryden

Wednesday, February 5

NEUTRON STARS

``Though a good deal is too strange to be believed, nothing is too strange to have happened.'' - Thomas Hardy

Key Concepts

(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 neutron star. 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 gravitational force.

(2) A neutron star is a compact object supported by degenerate-neutron pressure.

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 degenerate-neutron pressure.

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.

(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 (ryden@astronomy.ohio-state.edu)

Updated: 2003 Feb 5

Copyright © 2003, Barbara Ryden