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Pulsars are highly magnetized, rotating neutron stars that emit a beam of electromagnetic radiation. The observed periods of their pulses range from 1.4 milliseconds to 8.5 seconds. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name. Because neutron stars are very dense objects, the rotation period and thus the interval between observed pulses are very regular. For some pulsars, the regularity of pulsation is as precise as an atomic clock. Pulsars are known to have planets orbiting them, as in the case of PSR B1257+12. Werner Becker of the Max Planck Institute for Extraterrestrial Physics said in 2006, "The theory of how pulsars emit their radiation is still in its infancy, even after nearly forty years of work."
A quasi-stellar radio source (quasar) is a powerfully energetic and distant galaxy with an active galactic nucleus. Quasars were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than extended sources similar to galaxies.While there was initially some controversy over the nature of these objects — as recently as the early 1980s, there was no clear consensus as to their nature — there is now a scientific consensus that a quasar is a compact region 10-10,000 times the Schwarzschild radius of the central supermassive black hole of a galaxy, powered by its accretion disc.