Digital Universe Guide - Hayden Planetarium

92 3. THE MILKY WAY ATLAS
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 ◦ , 56 minutes declination.
Pulsars and Supernova Remnants Many pulsars are found in the snr group. 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 seconds (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