A guide to PULSARS.


What are Pulsars?

Pulsars are sources of raid, sharp, intense, and very regular pulses of radio radiation. The pulse periods of different pulsars range from a little longer then 1/1000s to about 10s. As pulsars evolve, their pulse rate increases as they age. Calculations have shown that the typical lifetime of a pulsar is 10 million years.

Below is an X-Ray image of the pulsar EXO 2030 + 375. This shows the pulsar when it is in the process of producing one of its pulses of radio radiation.

Pulsars are formed from the charged particles (protons & electrons) given off by neutron stars, after a supernova. The electrons move close to the speed of light and emit electromagnetic energy. The radiation released is very directional, and if the magnetic poles do not coincide with the axis of rotation, the rotation carries each magnetic pole into our view, one after the other, similar in fashion to a lighthouse. As such, a pulsar is essentialy a rotating neutron star.


General information on Pulsars.

Astronomers currently belive that new pulsars are born somewhere in our Galaxy every 30 to 100 years. Pulsars typically have velocities of about 100 km/s, and are probably accelerated to these high velocities at the time of the supernova explosion. Because the lifetime of a pulsar is around 100 times longer than the length of time needed for the expanding gas from the supernova to properly disperse into the surrounding space.

Most pulsars lie fairly close to the plane of the Galaxy. Due to no pulsars having been seen beyond a few hundred parsecs above or below the plane of the Galaxy, astronomers have concluded that they must stop their emmission of pulses before travelling long distances.

The energy radiated by a pulsar, from matter ejected from the star interacting with the stellar magnetic field, robs the stars of rotational energy, and so, this is why their pulses slow down with age. Pulsars that are many thousands of years old have lost too much energy to properly emit pulses in visible and x-ray wavelengths, and can only be observed using radio waves.


Some unusual pulsars.
The Binary Pulsar PSR 1913 + 16.

When this pulsar was discovered in 1974 by R.A. Hulse and J.H. Taylor in Massachusetts, it showed cyclic variations ove a time period of 7 hours 45 minutes. These period changes are due to the pulsar's revolution about another object.

Analysis has shown that this pulsar is in mutual revolution, in that 7 hours 45 minutes time period, around an invisisble (at least to us) companion, that, becuase of its mass, is probably a white dwarf or another neutron star.

The image below, is a radio image of the Binary Pulsar 1913 + 16. The image shows the increased pulse given out by this binary pulsar.

As the pulsar and its companion spiral together, their period of revolution shortens. This shortening is only about one-ten-millionth of a second per orbit, but the effect cumulates over time. During the first seven years that this system was observed, the time during which the two objects were closest together shifted by more than a full second, relative to to when it originally would have occurred if the period had remained constant. Because of pulsars' ability of good timekeeping, any shifts like this one, are easily observed.


The Pulsar 1257 + 12.

This pulsar was found to be actually accompanied by at least two planets. The planets were deduced to be there by observing if the pulsar revolved in a small orbit around the center of mass of the system defined by the positions and masses of the pulsar and planets. As the pulsar moved in its orbit, the intervals between successive pulses varied, being slightly longer when the pulsar moved away from us, and shorter when the pulsar was moving towards us.

Below is a radio image of the pulsar 1257 + 12. On this image, it looks unremarkable, but it is not any particular propert of the pulsar itself that is the main attraction, but the planets orbiting it.

It was through analysis of these intervals between pulses, that astronomers concluded that the pulsar is being orbited by at least two objects with masses of around 2.8 and 3.4 times that of the Earth. Their orbital periods were worked out to be 98 and 67 days respectively, and the sizes of the orbits were found to be similar in size to Mercury's orbit. It is also possible that this system may also contain a third planet with an orbital period of about one year.

The planets would not be hospitable for life as we know it, as the pulsar emits most of its energy in the form of an intense stellar wind, which would blast the planet with the high energy particles moving close to the speed of light. The planets must have been formed since the supernova explosion (which they wouldn't have survived), either from material blasted away from a nearby companion star, which by now, has been completely vaporized by the pulsar.

This discovery is important in that it suggests that the formation of planets may be relatively easy, and that potentially habitable planets may be very common. Another important question to be asked is whether most pulsars that rotate hundreds of times per second have planets or whether this particular pulsar is unique.

Written by Jon Talpur at Keele University, 1997.


Links

Here are some further sites about pulsars:

AP Dictionary of Science & Technology definition of a Pulsar

A Pulsar with planets

Princeton Pulsar group


Bibliography

"Exploration Of The Universe" by George O. Abell, David Morrison & Sidney C. Wolff

Credits

X-Ray pulsar image of EXO 2030 + 375 taken from the ASCA Guest Observer Facility.