Astronomy 162: Professor Barbara Ryden

Thursday, February 6


``How far that little candle throws his beams!
So shines a good deed in a naughty world.''
- William Shakespeare, The Merchant of Venice, 5, 1

Key Concepts

(1) Pulsars are rotating neutron stars whose beams of radiation sweep across us.

Neutron stars have very strong magnetic fields, and are generally rotating quite rapidly. The electric field that is generated by the rotating, magnetic neutron star rips away charged particles from the surface of the neutron star, accelerates the particles along the magnetic field lines, and produces two beams of electromagnetic radiation, as shown in the diagram below.

Since the beams of radiation are not aligned with the rotation axis, they sweep around in a cone. If one of the beams sweeps across the Earth, we see a flash of radiation from the neutron star. (Most of the radiation is in the form of radio waves, but some is at shorter wavelengths.)

Neutron stars whose beams of radiation happen to sweep the Earth are known as pulsars. (PLEASE NOTE: the neutron star itself is not pulsating in and out, the way a Cepheid star does.) Pulsars are simply named for the ``pulses'' of radio waves that we see here on Earth. In fact, when pulsars were first detected, in 1967, nobody knew what was causing the pulses. Pulsars are easy to detect if you are doing a radio survey, since they are strong radio sources, and emit their bursts of radio emission at very regular intervals. (The first pulsar to be detected, for instance, had an interval between pulses of P=1.33730119 seconds. The pulsation was so regular that the discoverers gave the pulsar the joking name of ``LGM-1'', suggesting that it might be a radio beacon used by Little Green Men.)

The time between radio bursts, or ``pulses'', is typically a few seconds or less. For some pulsars, it's as short as a millisecond (a thousandth of a second). The time between bursts is equal to the rotation period of a neutron star; thus, some neutron stars are spinning on their axis a thousand times per second.

How did astronomers figure out that pulsars are rotating neutron stars? One important piece of circumstantial evidence is that some pulsars are inside supernova remnants. For instance, the Crab Nebula is a supernova remnant with a pulsar at its center. We see a burst of radiation from the Crab pulsar 30 times a second. (An animated cartoon of the rotating beams of the Crab pulsar is available at the Hubble web site.)

However, most pulsars are not actually inside supernova remnants.

(2) Pulsars rotate more slowly as they radiate away energy.

Most spinning objects tend to slow down with time. For instance, the pulsar inside the Crab Nebula currently has a rotation period of 0.033 seconds; however, the rotation period is increasing by 0.001 seconds per century.

What is slowing the rotation of the Crab pulsar (and other pulsars)? When investigating a political scandal, the best advice is ``follow the money''. When investigating an astronomical puzzle, a good piece of advice to follow is ``follow the energy''.

The large and luminous Crab Nebula is powered by the rotational energy of the tiny Crab pulsar.

(3) Pulsars in close binary systems accrete gas and emit X-rays.

The rotation of pulsars USUALLY slows down with time, as their rotational energy is converted to light and radiated away. However, if a pulsar is in a close binary system with an ordinary star, its rotation can speed up with time. If the ordinary star pours gas onto the pulsar, the force exerted by the falling gas can make the pulsar spin faster. (Similarly, in an overshot waterwheel, the force exerted by falling water makes the waterwheel spin faster. The pulsars with very short rotation periods (of a few milliseconds or less) have all been spun up by having gas dumped onto them.

Pulsars in close binary systems are also powerful energy sources. Consider dropping 1 gram of matter (about the mass of a paper clip) onto a neutron star from a great height. When the mass goes `splat' onto the neutron star, 30 trillion joules of energy are emitted. That's 40 times as much energy as you would get by fusing 1 gram of hydrogen into helium (and 200 million times as much energy as you would get by burning a gram of hydrogen). Thus, dropping matter onto a neutron star is an energy source even more efficient than nuclear fusion or fission. The matter that goes `splat' is heated and emits X-rays.

Pulsating X-ray sources, otherwise known as X-ray pulsars, are close binary systems in which an ordinary star pours gas onto a neutron star. The gas, following the magnetic field lines, is funneled to the magnetic poles of the neutron star, where it produces beams of X-rays. Thus, we see a burst of X-ray light if the beam of an X-ray pulsar sweeps across us (just as we see a burst of radio waves if the beam of an ``ordinary'' pulsar sweeps across us).

Prof. Barbara Ryden (

Updated: 2003 Feb 6

Copyright 2003, Barbara Ryden