The magnetic field at the Milky Way's core is at least 10 times stronger than that of the rest of the galaxy.
Credit: ASA/JPL-Caltech
As early as 1891, British physicist Arthur Schuster presented a paper to the Royal Institution of Great Britain, in which he suggested that the Sun and all other large rotating bodies in the universe are magnets.
Unknown to Schuster, a year earlier the great American inventor Thomas Edison had indeed proposed an experiment that could have shown the Sun to be a giant magnet.
However, it's unclear whether Edison ever built his device or made this measurement.
With the advent of the twentieth century, astronomers were able to confirm that planets and stars generate magnetism as they spin.
But Swedish physicist Hannes Alfvén (winner of the 1970 Nobel Prize for his research on magnetism) went much further, proposing in 1937 that the space between stars is also magnetic.
This prediction was confirmed in 1949, when two American astronomers, John Hall and William Hiltner, published back-to-back research papers showing that magnetism in space lines up interstellar dust grains like tiny compass needles.
These pioneering measurements have been greatly expanded over the last six decades, but even so we have only been able to crudely map out the magnetism of the entire Milky Way. The Milky Way's magnetism seems to have a rough overall spiral shape, broadly related to its glowing spiral pattern of stars and gas.
More broadly, we now appreciate that understanding the cosmos is impossible without understanding magnetism. Magnets play a vital role in key physical processes throughout the universe, from the formation of stars to the evolution of entire galaxies.
On the largest scales, much of the universe's mass consists of charged particles, whose movements are completely enslaved by whatever magnetism surrounds them.
Wherever gas flows, its movement is determined not just by gravity, but by magnetism too.
However, on the scale of entire clusters of galaxies, only rough measurements of magnetism have so far been made. And on the very largest scales, there have been tentative reports that the entire universe is magnetised.
Underpinning all this is a serious problem: we simply don't know what created this cosmic magnetism, or how it has maintained its strength over billions of years.
A big part of the problem is that magnetism is not directly observable, and must be studied through its effects on light and radio waves. One way to make these measurements relies on an obscure phenomenon known as 'Faraday rotation', in which light from a background object is subtly changed when it passes through a cloud of magnetised gas.
This effect allows us to determine the magnetism of gas or other matter in front of a bright background object, and can be measured by a radio telescope.
Faraday rotation thus allows astronomers to probe magnetic fields in regions that are otherwise invisible.
Until now we have only been able to measure the Faraday rotation effect in a limited number of directions.
But new wide-field surveys of the sky offer the opportunity to carry out a vast census of magnetism in tens of thousands of galaxies and galaxy clusters.
One of these is the 'POlarisation Sky Survey of the Universe's Magnetism', or 'POSSUM', which will be one of the main projects to be carried out with the Australian Square Kilometre Array Pathfinder (ASKAP), a new 36-dish radio telescope currently under construction in outback Western Australia.
Six of these 12-metre dishes are already installed, while the seventh will be erected later this month. The remainder will be constructed throughout 2011, with each new dish taking about two weeks to complete.
WHAT MAKES POSSUM possible is a major new technological development: at the focus of each ASKAP dish will be the 'phased-array feed', essentially a fish-eye lens camera that will provide a spectacular field-of-view covering an area of the sky 120 times larger than the full moon in a single exposure.
Commencing in 2013, this will allow ASKAP to survey the sky 100 times faster than any previous radio telescope.
Using sensitive new observations over the entire sky, POSSUM will allow us to study Faraday rotation in more than three million different directions.
With this spectacular data set, we can make the first full maps of the Milky Way's magnetism, can begin to determine the shape of the overall magnetic field of the universe, and can measure how magnetism in galaxies and gas clouds has changed and evolved over billions of years of cosmic history.
We don't how when or how the first magnetic fields were generated, or how they have stayed so strong and ordered over billions of years.
Overall, we as yet have only the barest understanding of what these celestial magnets look like, let alone what role they have played in the evolution of the cosmos.
With new projects like POSSUM, and with the even more ambitious surveys of magnetism being planned for the Square Kilometre Array radio telescope, we hope to finally hold the key to the magnetic universe.
Bryan Gaensler is an astronomer and Australian Laureate Fellow at the University of Sydney and project leader of POSSUM.
