Astronomers believe that the core collapse of a massive star gives birth to a neutron star. This event is marked by a supernova, where the energy released during the core collapse blows away the outer layers of the star. The radio pulsars found in supernova remnants is our evidence that this occurs. But only a small fraction of pulsars are found in supernova remnants. Shouldn't supernova remnants enshroud all pulsars if we believe them to be born in supernova explosions? Two factors cause most pulsars to live outside remnants: pulsars are given a kick at birth that causes them to move out of their remnants in a relatively short time, and pulsars live much longer than supernova remnants. For these reasons, the pulsars found in supernova remnants are young.
Supernova remnants look like bubbles in the interstellar gas. They appear when the stellar debris expelled in a supernova strikes the ambient gas, driving a shock wave into the ambient gas. What we see as a shell is the hot gas immediately behind the shock wave.
But supernova remnants are not the only nebula one finds around young pulsars. The most striking nebula containing a pulsar, the Crab nebula, is not a supernova remnant at all, but a wind of hot electrons and positrons expelled by the Crab pulsar. Many other young pulsars generate pulsar wind nebulae, many of which sit at the centers of supernova remnants. Without a pulsar to supply energy to a pulsar wind nebula, the nebula would rapidly fade away, with the visible light fading away in several hundred year, and the x-ray emission fading away in several years.
The page added this week to the web site describes the connection between pulsars and supernova remnants and character of the pulsar wind nebula.
Next Issue: The next issue of The Astrophysics Spectator is scheduled for February 21.
Pulsars, Pulsar Wind Nebulae, and Supernova Remnants. A small fraction of the observed radio pulsars live inside nebulae. Many of these nebulae are supernova remnants, nebulae generated by the shock wave of a supernova; the supernovae remnants that contain pulsars provide the evidence that pulsars are created in supernova explosions. Other nebulae found around pulsars are generated by the pulsars themselves. These pulsar wind nebulae are powered by the rotational energy lost by the pulsar in each nebula. Many young pulsars are surrounded both by a pulsar wind nebula and a supernova remnant. The best known example of a nebula surrounding a pulsar, the Crab nebula, is purely a pulsar wind nebula. Old pulsars are not found in nebulae because they have either outlived or outrun their supernovae remnants, and they are losing too little rotational energy to power a pulsar wind nebula. (continue)