Pulsars
Pulsars
| Group Name | pul |
| Reference | ATNF Pulsar Catalogue
Manchester, Hobbs, Teoh, & Hobbs, Astronomical Journal, v129, pp. 1993-2006 (2005) |
| Prepared by | Brian Abbott (AMNH/Hayden) |
| Labels | Yes |
| Files | pulsar.speck, pulsar.label |
| Dependencies | none |
| Census | 1393 pulsars and labels |
Upon death, stars leave behind one of three possible remnants: a white dwarf star, a neutron star, or a black hole. Stars that are 1.4 to about 3 solar masses will become neutron stars in a violent explosion called a supernova. During a supernova, the core of the star collapses under such high pressure that the electrons, which normally remain outside the atomic nucleus, are forced to combine with the protons in the nucleus. Atomic nuclei break apart, producing what is called a degenerate state of matter. The collapse is halted when the material cannot be packed any tighter. At this point, the star has a radius of about 10 to 15 kilometers. The density of this material is so high that on Earth a teaspoonful would weigh about 100 million tons.
Just as ice skaters spin faster as they pull their arms in, dying stars rotate faster as they collapse. If the Sun were to suddenly collapse to a radius of 10 km, its rotation period would increase from its current 25 days to 1,000 times per second. Similarly, after a supernova, the neutron star is spinning fast from the rapid collapse, but it slows over time as it converts rotational energy into radiation.
Astronomers now know that pulsars are not pulsing but are spinning neutron stars whose beams of radiation point toward Earth as a lighthouse sweeps the horizon. Pulsars have strong magnetic fields that funnel beams of light from the magnetic poles. When these beams point to Earth, we see a strong radio signal.
Observing Pulsars
The first pulsar was discovered in November 1967 by Jocelyn Bell, who was then a graduate student at the University of Cambridge. Bell and Anthony Hewish investigated further and found the repeating signal had a period of 1.3373 seconds and originated from the same spot in the sky, night after night (Hewish won the 1974 Nobel Prize in physics for this discovery). The regularity of the signal led them to consider calling these objects LGMs—Little Green Men—implying these regular signals must come from intelligent beings. However, more were soon found in other parts of the sky flashing at different periods and the LGM name was dropped in favor of “pulsar.”
Pulsars are observed primarily in the radio spectrum, although some are seen in the visible, X-rays, and gamma rays. (The Crab Nebula Pulsar, the Vela Pulsar, a pulsar in the Large Magellanic Cloud, and the pulsar PSR 1939+2134 are all seen in the visible spectrum.) Pulsar signals are detected with radio telescopes including those at Green Bank, West Virginia (US); Arecibo, Puerto Rico; Jodrell Bank in the UK; and the Parkes Observatory and the Molonglo Observatory in Australia. The observing frequencies range from 400 MHz to 1520 MHz. The periods of most pulsars are between 0.03 and 0.3 seconds. This corresponds to a flashing between 3 and 30 times each second, a rate our eye cannot detect.
The basis for this catalog was compiled by Joe Taylor (Princeton), Richard Manchester (Australia Telescope National Facility), and Andrew Lyne (University of Manchester) and published in 1993. The Australia Telescope National Facility (ATNF) has taken this catalog and added many more pulsars which were mainly discovered by the ATNF. For this reason, you will notice there are many more pulsars in the southern sky. The labels take the form of right ascension in hours and arcminutes and declination in degrees and minutes. For example, PSR0334+2356 is a pulsar that lies at 3 hours 34 arcminutes right ascension and +23 degrees 56 minutes declination.
Pulsars and Supernova Remnants
Many pulsars are found in supernova remnants. Since supernova remnants have short lifetimes, we can assume that the pulsars seen in them are quite young. Once the supernova remnant disappears, the pulsar's rotation period continues to slow, and after about 1 million years the pulsar is no longer visible. Therefore, all the pulsars we see today must be the remnants of stars that have died over the previous 100,000 to 1 million years.
Pulsars in Globular Clusters
Pulsars result from the supernova explosions of stars that live only a few tens of millions of years after their formation. Why, then, do astronomers see so many pulsars in globular clusters that are more than 10 billion years old? The answer seems to be that these pulsars are drawing in material from a nearby companion star. This matter causes the star to spin faster, re-energizing the system. These are called millisecond pulsars for their periods, which can be as short as 0.002 second (2 milliseconds). More than 30 of these have been found and are easily seen to line up with the globular clusters in the Atlas. (Note that the distances can differ between the pulsar data and the globular cluster data, since they use different distance determination techniques.) Some examples include the globular clusters 47 Tuc, M5, and M13.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott