Pulsars

• Pulsars are rapidly rotating neutron stars that emit radio waves in beams from their magnetic poles. The magnetic poles are not aligned with the rotation axis, as illustrated below. Thus, the radio beam sweeps around as the neutron star rotates, a thousand times every second. As the figure shows, we see a flash of radio emission every time the beam sweeps past us. Think of them as unearthly lighthouses!

  A supernova explosion forms a pulsar
  [MPIfR]

• Why do pulsars spin so fast? They spin quickly for the same reason that a figure skater spins faster when she pulls her arms in tightly to her torso. When a rotating object shrinks in size, it spins faster. The physical principle is called the conservation of angular momentum. But a neutron star has collapsed a long way from a normal star – from a radius of one million kilometers to 10 kilometers. A simple calculation shows that the Sun, which rotates once every 27 days, would rotate more rapidly than 1,000 times per second if it collapsed to the size of a neutron star.

• SS 433, one of the strangest objects in the sky, was discovered in 1978 by astronomer Bruce Margon (University of Washington). He recognized that its spectrum had strong emission lines at unusual wavelengths, and after taking repeated spectra he saw that the wavelengths of the emission lines shifted to the red and blue with a period of 164 days. These emission lines were the Balmer lines of hydrogen, and the wavelength shifts were caused by a changing Doppler shift. These shifts could be understood if the source was squirting out twin jets of hydrogen at a speed of a quarter of the speed of light! Moreover, the jets were precessing, as if the jet nozzle was moving in a conical pattern every 164 days. After carefully analyzing his observations, Margon found that the source of the jets was orbiting a massive star with a period of 13 days. Afterwards, radio astronomers and X-ray astronomers were able to obtain images of the SS 433 jets. Despite decades of intensive observation and analysis, SS 433 remains a mystery. We suspect that the source of the jets is a neutron star, but we can't rule out the possibility that it's a black hole. We don't know why it squirts out the twin jets, and we don't understand why the jets precess. Despite extensive searches, astronomers have failed to find anything else quite like it.

  The strange object SS 433, a massive star (left) and a neutron star (right) with jets. [NASA/GSFC]