Pulsars - Neutron Stars Beaming Energy Our Way

2008-09-18

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Introduction

Pulsars are extremely compact objects (neutron stars) with a radius of about 10 km (6 miles). They emit strong beams of radiation along two opposite directions. If the axis happens to be in the line of sight with Earth, we observe these emissions. Due to rotation of the pulsar, this radiation is observed periodically. The time periods of rotation are generally of the order of few tens of milliseconds. Most pulsars emit in the radio region of the electromagnetic spectrum. Pulsars also have a very high magnetic field of about 1000 billion times that of the Earth. The discovery, observation and analysis of pulsars are one of the key research areas of modern day radio astronomy.

Timeline

JULY 1967: Jocelyn Bell Burnell and Antony Hewish discovered the first pulsar at the Cambridge radio astronomy observatory and named it LGM (Little Green Men) thinking that it was a torch from some Extra-Terrestrial civilization.

1968: Thomas Gold and Franco Pacini independently suggested that pulsars are rotating neutron stars.

1968: David H. Staelin and Edward C. Reifenstein discovered the Crab pulsar, the first pulsar that could be associated with a supernova.

1974: Antony Hewish became the first astronomer to be awarded the Nobel Prize in physics.

1974: For the first time a pulsar in a binary system was discovered by Joseph Hooton Taylor, Jr. and Russell Hulse.

1982: Shri Kulkarni and Don Backer discovered a pulsar with a rotation period of just 1.6 milliseconds.

1993: Nobel prize in physics was awarded to Taylor and Hulse for their discovery.

Classification

Although all known pulsars are assumed to be neutron stars, they have different means of producing energy for radiation. This forms the basis of their classification as follows:

Rotation powered pulsars: The pulsar’s rotational kinetic energy is converted to radiation resulting in slowing down of the pulsar.
Accretion powered pulsars: The pulsars that are apart of a binary system along with a normal star radiate due to the gravitational potential energy of the accreted matter.
Magnetars: Energy for radiation comes from the extremely strong magnetic field of these pulsars.
Thermal powered pulsars: When pulsars are formed, their temperatures are as high as 1000 million Kelvin. This heat energy is lost as radiation in these pulsars resulting in a cooling down process.

Formation

Pulsars are formed as a result of a tremendous explosion known as supernova. The supernova explosion marks the ’end of life’ for a star. It occurs in stars with about ten times the mass of the Sun due to a sudden, catastrophic gravitational collapse. Most of the outer mass is shed off in the explosion leaving behind a neutron star at the core. Since the neutron stars are extremely small by astronomical standards and the angular momentum of the initially huge star has to be conserved, the rotation velocity of neutron stars is very high. This gives rise to jets of particles shooting off from both the poles of its magnetic axis. If during the rotation period of the star, these jets point towards the Earth, they are recorded as pulses of electromagnetic radiation. This pulsating characteristic gives them the name pulsars (PULSating stARS).