The Magnetic Universe
There is a tangled web of magnetism running across the Milky Way. Although you cannot see it, this fundamental force winds through the spiral arms and influences everything from the formation of stars to the galaxy’s structure. Where does it come from? How does it affect us? Astronomers do not know for sure, but in recent years they have come to realize how important cosmic magnetism is, and are working to unwind the answers.
The Earth’s interior essentially contains a giant magnet, and it exists because Earth’s rotation causes molten iron in its core to move, creating a ‘dynamo’ that generates the magnetic field. This field is important as it creates around the planet a shield from cosmic rays – charged particles from outer space that can damage electronics – and a steady stream of particles from the Sun called solar winds. The largest effect we see from these solar winds occur when the magnetosphere channels the charged particles toward the north and south magnetic poles, where they excite atoms and molecules in the upper atmosphere and cause the auroras.
Magnetic fields are present almost everywhere in the universe, and the biggest webs can span entire galaxies, as a significant fraction of a galaxy’s total energy can be tangled in its magnetic field. Bryan Gaensler, the director of the Dunlap Institute at the University of Toronto, says that the strength of the galaxy’s magnetic field is equivalent to the radiation pressure exerted by all the stars within it. As starlight is polarized, scientists use radio telescopes to measure polarization across the sky, mapping what magnetic structures look like in the galaxies. Since then, a startling picture of galactic magnetic structure has begun to emerge, and how they affect the formation of stars within them. The Whirlpool Galaxy (M51) has a smaller companion galaxy (NGC 5195) that seems bob on the end of one of M51’s spiral arms. The interaction between these two galaxies causes density waves to ripple through the Whirlpool; the magnetic fields not only follow the optical spiral structure but also compress the gas at the inner edges of the arms. This means that the strength of the galaxy’s magnetic field correlates with the density of interstellar gas forming stars.
While we are used to the classical picture where gravity is the master of the universe, it is apparent that magnetism has been a vital assistant in creating the cosmos we see today. The Milky Way, a typical spiral galaxy, has a spherical bulge surrounded by a flatter spiral arms spanning 150,000 light-years across, but only 1,000 light-years thick. Though gravity holds it all together, astronomers think that it is the magnetic pressure that provides a buoyant force that counters the gravity; otherwise the galaxy would deflate.
However, much still remains a mystery.
Credit: Astronomy (Abridged)