“Seul phare dans la nuit …”

November 9, 2010 § Leave a comment

Pulsars are a type of rapidly rotating neutron star; the remnants of supernova explosions. These “stellar corpses” represent, perhaps, the ultimate state of matter, as we know it, in the Universe. Akin to the regular pulses of lighthouses, pulsars emit beams of electromagnetic radiation that can be detected as radio waves here on Earth. The  rate at which these beams are emitted ranges from a few pulses to over 1,800 pulses per second. Listening to recordings of the waves emitted by a neutron star spinning 8 times per second, one is reminded of the steady click of a stylus on a scratched record. In contrast, the individual pulses of the fastest spinning pulsars can be heard only by slowing them down considerably, so that they sound like skipping CDs.

The existence of neutron stars was predicted in the early nineteen thirties by two American scientists; Baade and Zwicky. The third component of the atom, the neutron, – the electron and the proton were already known – had only recently been discovered. Baade and Zwicky believed that neutron stars, if they really existed, would consist entirely of neutrons packed very tightly; the remnants of exploded stars. Thirty four years later, in 1968, the Italian physicist Franco Pacini went so far as to say that if these stars existed, they were likely to rotate very rapidly and to emit electromagnetic waves.

Convincing evidence of the reality of these strange celestial bodies was found in 1969. Jocelyn Bell, a Belfast-born PhD student working at Cambridge in England, detected what she thought at first to be signs of interstellar communication: pulses of radio waves travelling across the Universe at fairly regular intervals. She and her colleague, Anthony Hewish, slowly gathered evidence for these “pulsed radio sources” on mile after mile of magnetic tape. To their dismay, they discovered that these pulses were not alien civilizations trying to contact the Earth (or each other), but hard evidence for the existence of swiftly rotating star remnants, from then on known as pulsars (a coinage derived from ‘Pulsating source of radio emissions’). Nevertheless, once other pulsars had been detected, it was perceived that some of them emitted radio waves in irregular blips, and not in the regular patterns that had been predicted. This meant that their physics was much more complicated – i.e. difficult to measure accurately – than had previously been assumed.

Crab Nebula

The Crab pulsar, at the heart of the Crab Nebula in the constellation of Taurus, is one of the best known pulsars. This is mainly due to the fact that it emits a very regular pulse – 30.2 times per second – and is easily detectable by radio-astronomical instruments on Earth. Like all known pulsars, it is thought to be the end product of a supernova explosion. Incidentally, the Crab nebula (and, by extension, the Crab pulsar within it) was the first astronomical object identified with a historical supernova explosion. It corresponds to a bright explosion visible in the sky in July of the year 1054, recorded by, among others, Chinese, Japanese and Persian astronomers. It has also been claimed that an obscure entry in a number of Irish monastic annals originally referred to the same event.

A supernova explosion occurs during the gravitational collapse of a star that has burnt out the nuclear fuel – mainly hydrogen and helium – in its core. Approximately 4 billion years in the future, our own Sun will die such a death. All the matter in our solar system, including ourselves – from gold and oxygen, to the molecules that make up our DNA – is the product of at least two previous stellar explosions. Depending on the mass of the star, the end product of its collapse can be either a black hole, a pulsar or a white dwarf. A star with roughly 6 to 8 times the mass of the Sun, or above, will become a black hole; ordinary matter being unable to resist the force of the gravitational collapse of such a massive body. A smaller or medium star, at least 1.4 times the mass of our Sun, will most likely become a pulsar: on collapsing it will blow off much of its excess material, and stabilise again to become an extremely dense body of roughly 20 – 30 kilometres across. One teaspoon of the matter of such a pulsar would have a mass equivalent to all the water in Galway Bay.

Pulsars are among the most extreme objects known in the Universe. Examples have been detected that have a surface gravitational force 1013 (ten with thirteen zeros after it) timesstronger than the Earth’s. As Einstein predicted in his theory of general relativity, space-time is extremely warped under such conditions. That is to say, light radiated from a star with a gravitational force of this magnitude bends such that parts of the normally invisible rear surface become visible. Time, on the other hand, as measured from a distant perspective (e.g. Earth’s orbit), will seem to pass more slowly in the star’s proximity. Pulsars have also been shown to have an atmosphere, and even a mantle and crust. The atmosphere of one of the roughly 2,000 known pulsars, for example, is thought to be composed of gaseous iron, about two inches thick, and extremely dense. The surface temperature of such a body is reckoned to be somewhere in the region of 1,000,000 Celsius, growing hotter as you move towards the core.

The most significant feature of pulsars, however, and the feature that distinguishes them from ordinary, slowly rotating neutron stars, is their massive magnetic fields. Millisecond pulsars – pulsars that rotate more than once every millisecond – have been shown to generate a magnetic field of over 1015 times greater than the Earth’s. As the pulsar spins, energy in the form of electrons and protons travels along the magnetic field at speeds sometimes approaching the speed of light. This energy is ejected from the magnetic poles producing an electromagnetic beam that we can detect on Earth as radio waves (provided the beams are pointing edge-on relative to our line of sight). The magnetic axis of the pulsar determines the direction of the electromagnetic beam, with the magnetic axis not necessarily being the same as its rotational axis. This misalignment causes the beam to be seen once for every rotation of the neutron star, which leads to the “pulsed” nature of its appearance: the so-called ‘lighthouse effect’.

At this point, two questions might spring to mind:

  • Why does a pulsar spin so fast? And,
  • What is a pulsar made of?

Basic physics teaches us that the larger the size of a body the slower it spins. When a figure skater extends her arms and legs, she turns more slowly than if she were to keep her limbs tightly tucked in towards her centre of gravity. In the same way, the velocity of a planet’s or a star’s spin depends on the distribution of its mass (how much space it occupies). Our Sun has a radius (the distance from its centre to its outermost surface) of approximately 700,000 kilometres, and a rotation period of 24.47 days (i.e. it takes around 24 and a half days for it to rotate once). Were you to retain the mass of the Sun but shrink it so that its diameter (2 × radius) was no more than 20 kilometres, it would spin much, much faster: about once every millisecond.

Three quarks for Muster Mark!
Sure he has not got much of a bark
And sure any he has it’s all beside the mark.

James Joyce, Finnegan’s Wake, p. 383.

The answer to the second question is: nobody knows for sure. There has been plenty of speculation, however. Some astronomers still believe that the cores of neutron stars are composed of quickly spinning neutrons (subatomic particles with a neutral electrical charge that are slightly heavier than protons). Others posit quarks (elementary particles that combine to form composite particles, the most stable of which are protons and neutrons, the building blocks of atomic nuclei).

Finally, millisecond pulsars might one day replace atomic clocks as the most accurate instruments we have to measure time. By comparing the rotational stabilities of these stars with one another and with terrestrial time scales, we might one day be able to narrow the margin of error in our time-keeping significantly. However, we would still need to adjust our timepieces by 1.3 seconds every million years: the current estimated rate at which pulsars slow down over time. We might take comfort in the fact that all civilizations have had to contend with the same problem; that nature does not produce regular astronomical patterns. Until we find a more accurate clock, we shall have to add 130,000 nanoseconds (a nanosecond is a billionth of a second) to our calendar once every century, or so, to keep the accounts balanced.

Rua Breathnach

Relative position of the Sun to the centre of the Galaxy and 14 pulsars with their periods denoted as included on the plaques sent aboard Pioneer 10 and Pioneer 11 spacecraft.

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