“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.

“Home lies to the East!”

October 18, 2010 § Leave a comment

Solvay Public Conference, Sunday 17th of October 2010

Wolfgang Wiltschko: ‘The magnetic compass of birds’

Since the 1960s, Professor Wolfgang Wiltschko and colleagues of the Goethe-Universität Frankfurt am Main in Germany have been conducting experiments that have provided proof that birds use the Earth’s magnetic field to migrate.

Migratory birds are highly mobile organisms. In order to navigate successfully over long distances, they need to be able to do two things:

1) determine the course of the goal, e.g. 180° degrees South,and

2)  use a compass mechanism.

Arctic Terns (Sterna paradisaea), whose yearly migration takes them from their northern breeding grounds all the way to the Antarctic and back (a 44,300 mile round-trip), spend the long summers in the Arctic and migrate southwards at the beginning of autumn. Pied flycatchers (ficedula hypoleuca), who also make use of long northern summers to breed, migrate southeastward to Central Europe and then move again southwestward; thus bypassing the harsh natural barriers of the Alps and the Sahara desert to reach western Africa. This SE-SW (or NE-NW on the way back) pattern is one  often observed in migratory birds’ flightpaths.

According to Wiltschko, these birds all share an innate migratory compass. They use several such compasses: solar, stellar and the Earth’s magnetic field. It is the last one of these that Wiltschko homed in on in his conference yesterday at the Solvay Institute in Brussels.

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At about the time the word heimweh, a word of Swiss origin meaning homesickness or longing for home, was introduced to mainstream European vocabulary in the mid 1700s, it was realised that the Earth is a big magnet. This magnet has two magnetic poles, the North Magnetic Pole and the South Magnetic Pole, close to but not aligned exactly to the tilt of the Earth’s axis.

The Earth’s magnetic field extends thousands of kilometres into space. Wiltschko and colleagues have been studying birds’ behaviour relative to this magnetic field for the last forty-odd years and have determined that migratory birds display a reliable directional preference while migrating.

By catching the birds on their migratory paths and placing them in an enclosed space where the magnetic field has been artificially adjusted – say to 120° off Magnetic North – they have been able to deduce that these birds are using an innate compass to follow the Earth’s magnetic field in their seasonal migrations.

The European Robin (Erithacus rubicula), for example, which breeds in most parts of Europe, is a partial migrant (i.e. not all individuals migrate). The magnetic compass of these birds has been tested at an experimental station in Frankfurt-am-Main where robins are caught and tested en route from Scandinavia to northern Africa. By placing the robin in an enclosed funnel cage and sending an artificially adjusted magnetic current through the cage with a similar intensity to the Earth’s magnetic field at a given latitude, it has been found that the bird will move relative to the North-South vector (an imaginary line drawn along a magnetic field) of the adjusted field on being set free.

By analysing the magnetic compass of robins, it was discovered that it is primarily this vertical component of the mechanism (the North-South vector) that determines the preferred migratory paths of the bird; i.e. even if the direction of this vector is adjusted, the robin will still go ‘poleward’.

In another similar experiment carried out on domestic chickens (Gallus gallus domesticus) in Australia, a social stimulus was added to the directional one. By placing a newly hatched chick in a glass box containing a red ball and four screens – one at each of the four cardinal points (North, South, East and West) – after two days the chick will gravitate toward the ball as though it were its mother. The ‘mother’ (red ball) is then removed from the chick’s view and placed behind a screen in the northern corner of the box. The chick searches the box for the missing ball, and finds it behind the northern screen. When the ball is once again removed and this time the magnetic current is switched 90°, the chick will spontaneously move toward the adjusted north (in this case really the West or East) and look behind the screen there.

In both these experiments it can be deduced that the bird trusts the ‘vertical component’ of the magnetic field.  In migratory birds, however, one important anomaly results from this reliance on the vertical component; at the equator, the inclination of the Earth’s magnetic field is neutral (i.e. the field lines are approximately parallel to the Earth’s surface) and so migratory birds cannot use the magnetic field at the equator to navigate.  It is only when the birds move north or south of the equator, where the field lines are set at a steeper angle to the Earth’s surface, that they can perceive their latitude and direction again.

E come li stornei ne portan l’ali/ nel freddo tempo, a schiera larga e piena …”

“And as in cold weather, circling in serried groups, / starlings are lifted skyward on their wings …”

Dante, Inferno, V.

It has been shown that starlings introduced to Australia from the northern hemisphere have been able to adjust their compasses and fly southward (towards the pole) instead of northward (towards the equator) in their seasonal migrations.

It has also been discovered that birds can adapt to much weaker magnetic fields. As we have seen, the magnetic field at  equatorial latitudes is very weak; therefore, birds need to get tuned to a field before they can make use of it for navigation.

In other words, birds need to make a mental map of a region before they can navigate their way around it. In the same way that a child has an instinct to spontaneously learn language in order to be able to ‘navigate’ the immediate world around it, the birds integrate the ‘lay of the land’ across the target region. This innate compass (or ‘navigational language’) is progressively fine-tuned by adding to it features of the landscape, e.g. sounds, smells and geological features seen from above when the bird is in flight. The magnetic compass is thus the backbone of the birds’ navigational aptitude.

The effectiveness of the magnetic compass is also increased by addition of other compasses; stellar and solar. Similar mechanisms have been found and studied in bats, mole rats, butterflies and sea turtles.

Robin Hood

As often in experimental science, hunches give rise to unexpected discoveries. Wiltschko and his team discovered that the navigational abilities of robins were linked to the right eye (and thus to the left hemisphere of the brain). By placing a hood over the robin’s right eye – the left one was also tried, with little or no navigational impairment – it was discovered that the magnetic compass of these birds is lateralized in favour of the right eye.

From all of the above experimental results, which have also been carried out on numerous other species of birds, two main hypotheses present themselves concerning the magnetic compass of migrating birds:

1. Transitory magnetoreception*: a light-dependent process in photopigment in the birds’ eyes.

In this hypothesis, it is primarily light waves (or photons) that incline the bird to fly in a particular direction. The strongest argument for this hypothesis has come from experiments where the direction and intensity of light waves hitting the retina of the bird have been fiddled with. In these experiments, light at longer wavelengths (light at the red end of the spectrum) has been shown to disrupt the birds’ compass completely, whereas white and blue light have little or no adverse effect on the compass.

The exact point on the spectrum at which this disruption occurs has also been determined. It seems that in most cases, it is on the border between green and yellow light where the compass suddenly breaks down. It is therefore a tiny shift (a few nanometres) in the spectrum of the light entering the birds’ eyes that ends up disorienting the birds’ compass very significantly.

2. In the second, or permanent magnetoreception, hypothesis it is believed that the navigational aptitude in birds can be traced to the presence of magnetite (iron oxide) in their beaks. Wiltschko and his colleagues are convinced, however, that the magnetite found in birds’ beak serves only as a kind of rudder the bird uses to position itself correctly in flight; rather than functioning as an inbuilt magnetic needle.

Finally, as a zoologist, Wiltschko is keen to point out that this ‘phylogenetic’, or commonly inherited,  characteristic in modern birds can be traced back as far as the Late Cretaceous era; 70 to 90 million years ago. The inbuilt navigational devices of the early orders of  Galloanserea (fowl) and Neoaves – the two main orders from which modern birds have evolved –  can be seen in action today in our chickens, pigeons and Passerines (perching birds). All modern birds have magnetic compasses inherited from this time. It’s how they find their way home.

*  Magnetoreception is the ability to detect a magnetic field.

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