Nuclear Density, Stellar Mass: Neutron Stars and Pulsars Continu© Wesley Colley
Feb 3, 1997
Ever since Jocelyn Bell first discovered a rhythmically varying radio source in the early 60's, pulsars have provided a wonderful observational laboratory and theoretical puzzle for astronomers and astrophysicists. Very quickly astrophysicists realized that pulsars were actually spinning neutron stars with magnetic fields and gravity two hundred billion stronger than Earth's. These objects are the remnants of stellar cores which have burned their nuclear fuel all the way from Hydrogen to Iron, leaving no energy to resist gravity. The star's core then imploded, and in less than a second, shrank to only 10 kilometers in diameter, but released 10 billion times more energy in neutrinos than the entire amount of energy ever to be deposited on the Earth by the Sun. These explosions (called supernovae, type II) outshine entire galaxies for months before cooling and exposing the remnant neutron star. The pulses come from the poles of the magnetic field which are not necessarily aligned with the rotation poles. Electrons accelerated to the speed of light radiate intense radio waves as they travel along bent magnetic field lines. Each time the pole sweeps by us, we see a pulse of radio emission.
These stars spin at rates up to an incredible 300 times per second. Moving an entire solar mass around at that speed is some feat. As the star collapsed, conservation of angular momentum left the star no choice but to spin at this incredible rate. The stars slow down as they radiate, but remain the most accurate clocks known to man.
Even more interesting things happen when the stars occur in pairs. Binary pulsars have been shown to slow down in their orbits around each other at exactly the rate predicted by Einstein's theory of General Relativity. These are the only proven sources of gravitational radiation, which is like ordinary light, but caused by gravity, instead of electricity. Because gravity is so weak, compared to electricity, we rarely observe the effects of its radiative nature.
Currently, massive search efforts for new supernovae and pulsars are underway, as astrophysicists push their knowledge of nuclear and strong force physics to understand the structure of these tiny little, superdense stellar remnants.
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