Pulsars and their discovery The discovery of pulsars is widely recognised as one of the great astronomical discoveries of the 20th century. Pulsars were an experimental discovery made in 1968 by Jocelyn Bell and Anthony Hewish. They were discovered using a radio telescope array just outside Cambridge. Bell detected lots of earth-based interference and also identified many scintillating sources. However after a while, Jocelyn Bell detected something that was neither interference nor a scintillating source. It disappeared for a while then came back, and she captured it on the chart as a series of pulses, which proved to be equally spaced corresponding to a frequency of about 3 seconds. Hewish believed them to be man-made, as with a gap of just 3 seconds the pulses seemed too close to be natural. Jocelyn Bell believed that it was possible that they originated from stars rather than man-made interference, so they investigated further. A different group of researchers then managed to pick-up the same signals, removing instrument error as a source. While continuing with her actual Ph.D. research, Bell identified a second signal close to Cassiopea A (itself a supernova remnant) and managed to capture the regular pulses about 1 second apart. This and additional discoveries convinced Bell and Hewish that the signals were not from distant life sources but emission from the neutron stars. Hewish and Bell published their results in Nature , which created immense interest. Anthony Hewish was awarded the Nobel Prize for Physics in 1974 for the discovery of pulsars, along with Martin Ryle (also of Cambridge) for his work with radiotelescopes and particularly aperture synthesis. Jocelyn Bell, did not share the prize, despite her instrumental role in the discovery of pulsars. Image of PSR 0540-69, a neutron star/SNR in constellation Dorado made by Chandra X-Ray Observatory PSR 0540-69 is a pulsar, that is rotating very rapidly, making a complete rotation every one-twentieth of a second. It is similar in many ways to the famous Crab Nebula pulsar. Both objects are spinning rapidly, are about 1,000 years old and are surrounded by a large cloud of gas and high energy particles. The surrounding cloud in both cases is powered by the conversion of rotational energy of the neutron star into high energy particles through the combined action of rapid rotation and a strong magnetic field. PSR 0540-69 is 180,000 light years away in the Large Magellanic Cloud, one the Milky Way's small satellite galaxies. - 2002 by Aryani Sumoondur - What are supernova remnants? Supernova remnants consist of expanding gas cloud, which was formerly the star's outer layers. The ejecta thrown off by the explosion plows through the ambient medium, producing a shock wave that sweeps and heats the circumstellar matter, creates an expanding shell of hot plasma with multi-million degree temperatures, and accelerates particles up to very high energies. The emission from this supernova remnant (SNR) will be seen, from radio to gamma-rays, for many thousand years. The core of a massive star that is 1.5 to 4 times as massive as the Sun ends up as a neutron star after the supernova. Neutron stars spin rapidly giving off radio waves.. If the radio waves appear to be emitted in pulses due to the star's spin, these neutron stars are called pulsars. The core of a massive star that has 8 or more times the mass of our Sun remains massive after the supernova. No nuclear fusion is taking place to support the core, so it is swallowed by its own gravity. It becomes a black hole which readily swallows any matter and energy that comes too near it. Some black holes have companion stars whose gases they pull off. As the gases are pulled down into the black hole, they heat up and give off energy in the form of X-rays. Black holes are detected by the X-rays which are given off as matter falls down into the hole.