| Themes > Science > Astronomy > Modern Astronomy > Cosmology > Gravitation and the General Theory of Relativity > Pulsars | |||||||||||||||||||
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The first pulsar discovered was found by Anthony Hewish
and Jocelyn Bell at the Cambridge radio astronomy observatory in 1967. The most
famous pulsar was discovered shortly after that. It lies in the Crab Nebula
(M1), which is about 7000 light years away in the constellation Taurus. The Crab
Pulsar rotates about 30 times a second, emitting a double pulse in each rotation
in the RF through gamma-ray spectrum. The adjacent image catches the Crab Pulsar
(in the yellow region in the center) pulsing in X-rays. In visible light, the
Crab Pulsar appears to be a magnitude 16 star near the center of the nebula (see
below), but stroboscopic techniques show it to be pulsing. Powering the Crab NebulaThe tiny Crab Pulsar, which is not much more than 10 kilometers in diameter, powers the enormous energy output of the Crab Nebula, which is 10 light years in diameter. To set things in perspective in terms of relative sizes, this is as if a 1 kilometer wide volume of space were radiating strongly at various wavelengths and most of the power were being supplied by a single hydrogen atom at the center of that volume!Recently, Hubble Space Telescope observations illustrated in the figure below and the animation adjacent right have shown that the Crab Nebula undergoes substantial changes in time as energy is being fed into it by the pulsar . For example, energy ripples outward at speeds near half that of light through the nebula in the vicinity of the pulsar. Here is a movie (550 kB MPEG) of the ripples moving outward in the Crab Nebula from central pulsar partially illustrated in the above animation. The movie imagines that you are close to the Crab Pulsar and slowly pull away, allowing you to see more and more of what is going on to power the inner part of the nebula and to account for the detailed activity seen in the preceding animation (more info). The Sound of PulsarsSince pulsars pulse in the RF part of the spectrum, it is possible to "listen" to them by connecting the amplified signal from the radio telescope to a radio speaker . Here are sound files for three pulsars of very different period.
The names for these pulsars are derived from their position on the celestial sphere. The first part of the number gives the approximate right ascension in hours and minutes. The second part of the number gives the declination (with a plus or negative sign). For example, the pulsar labeled PSR 0329+54 is located on the celestial sphere at right ascension 3 hours and 29 minutes and declination +54 degrees. Here is a map from the Princeton Pulsar Group of pulsar positions given in right ascension and declination (the links from this map to some databases with further information do not always work at present). Change in Spin Rate As a pulsar radiates away its energy, its spin rate
decreases slowly. This change is very slow, but can be measured very precisely.
The rate of change in the rotational period for a radio pulsar is very important
for several reasons. One is that it can be used to estimate the magnetic field
associated with the neutron star. The resulting measurements indicate that
pulsars have enormous magnetic fields, typically in the range of 10^8 - 10^12
gauss (there is some evidence that the fields in neutron stars may be as large
as 10^14 gauss). In addition, the change in the spin rate for pulsars has been
used to detect indirectly the radiation of gravitational waves (a key prediction
of the General Theory of Relativity), and to infer the possible existence of
planets orbiting a neutron star (see "New Views of Neutron Stars", L. Boldsten
and T. Strohmayer, Physics Today, Feb. 1999, p. 40). Guest Star in
Sung dynasty chronicles (1054 AD).
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