Ken Tapping, 19thJanuary, 2016
In 1967 Jocelyn Bell, a radio astronomer at Cambridge, in England spotted something odd. The radio telescope she was using started picking up precisely regular pulses of radio noise. They were 1.33 seconds apart. Of course at Cambridge, in a country with a dense population, it was likely that these pulses were radio interference produced by human activity. For example the regular ticks of interference produced by the electric fences used by farmers to control the grazing of livestock. However, as the days passed, it became clear that the pulsing was turning up four minutes earlier each day; it was locked to the sky not the clock. More checking affirmed the pulses were coming from somewhere out in space, a long way away.
At the time no cosmic objects were known that could produce such precisely spaced pulses. Could they therefore be artificial, set up by a distant, alien civilization? Could they be navigation aids for interstellar travellers – “lighthouses”? When more of these “pulsars” were discovered, they were initially called, only partly whimsically, LGM-1, LGM-2 etc., where LGM was short for “Little Green Men”. We now know these objects are not artificial. We are detecting beams of radio emission from the rapidly rotating, collapsed cores of dead giant stars. When giant stars run out of fuel, they start to collapse, and a huge explosion occurs, not in the middle of the star, but part way out. Everything above the explosion is blown out into space. However the core of the star gets enormously compressed, so much that the atoms themselves collapse, forming a ball of neutrons; so we call these objects neutron stars. If the compression is even more extreme, the core could be crushed enough to form a black hole.
In the course of her show, almost every ice skater starts to spin with her arms out, and then she pulls them in. When she does this, her spin speeds up dramatically. This is due to a physical principle called the conservation of angular momentum. This principle applies to all rotating objects, such as stars. The Sun has a diameter of about 1.4 million kilometres. A typical neutron star is maybe 20 km in diameter. If we compressed the Sun down to a diameter of 20km, its rotation would speed up to about 2000 revolutions a second. The fastest pulsar discovered so far is spinning at 700 revolutions a second. There are almost certainly faster ones yet to be discovered.
When stars collapse, everything gets compressed, not only the material making the star, but also its magnetic field. It’s the Sun’s magnetic field that makes it look the way it does: a yellowish-white, sharply defined “ball in the sky”, with sunspots, loops and other structures. With no magnetic field the Sun would be a bright, fuzzy blob. When compressed down to a sphere around 20km across, the magnetic fields become very intense. These are spinning at the same rate as the neutron star and reach out into space, dragging at nearby material. This liberates huge amounts of energy and generates beams of intense radiation: radio waves, light and even X-rays. If one of these beams points in our direction, we see regular “flashes” just like what we see with lighthouses on the coast, but these cosmic lighthouses are definitely things to stay away from. The magnetic dragging at their surroundings and the energy released in radiation production comes from the rotation of the neutron star, slowing it down. However, a rotating mass that large will slow down very, very slowly, so in the short term, the timing precision of those pulses is possibly unique in astronomy. Having such precise and detectable clocks scattered around the universe is a powerful tool for studying the nature of space itself.
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