Magnetism and Radio Astronomy

A powerful magnetic field surrounding a black hole was the first radio source ever discovered in space, but we didn’t realize that at the time. In 1933, Karl Jansky detected radio waves coming from the center of our Galaxy. Grote Reber observed the same source in other radio frequencies, only to discover that the radio waves did not behave as if they were coming from a radiating object.

A decade later, physicists figured out that magnetic fields make radio waves. Radio astronomers then put various pieces together to determine that what Jansky found and Reber examined in the heart of our Galaxy was the enormous magnetic field surrounding a supermassive black hole.

Magnetism Makes Particles Emit Radio Waves

One of the great things about giant objects in space is that many of them have a magnetic field. In other words, inside them is a means of making them behave like a big magnet that can affect particles thousands to billions of miles away from them.

The means of making an enormous magnetic field are still not clear. It appears to require a constant movement of materials against each other. Why should movement have anything to do with making a magnet?

EM radiation Consider the little charged electrons that swarm around all atoms: If a force can get them moving, then they will create a current of electricity. Thanks to Einstein’s maths, we know that a strong enough current also creates a bubble of magnetism around it, and vice versa. This electricity-magnetism give-and-take creates electromagnetic radiation, also known as light

synchtrotron Whenever electrons are accelerated, they yelp electromagnetic radiation. The faster the electrons are accelerating, the higher the frequency of the light they yelp. Electrons ripped from their atoms and hurtled in a magnetic field are accelerated to nearly the speed of light. They radiate waves of light like patrons screaming on a roller coaster ride. Radio telescopes pick up the longer wavelengths of light, and X-ray telescopes pick up the shorter ones. (Here's a more advanced and complete look at the types of radio waves.)

Magnetic Spheres

The churning insides of a star or planet often create flows of charged particles, currents of electricity, that spray magnetic forces out from them. In space, these magnetic field bubbles expand out and are called magnetospheres. In the case of our Sun, its radiation swells its magnetic bubble well out beyond the planets– in other words, we live inside it!

We also live inside the magnetosphere of the Earth. Worry not, because without the Earth’s magnetic field, we’d be blasted by billions of harmful charged particles barrelling out from the Sun. The Earth’s magnetic field acts like a shield against this, and may be the reason why life has been so successful here and not on the worlds that do not have magnetospheres of their own.

All of the giant planets in our Solar System have a magnetic field around them. And, strangely, so does the smallest planet, Mercury. Jupiter’s magnetosphere is the largest by far, and causes all kinds of problems for the little unprotected moons that go around it.

Radio telescopes help us learn about the hidden insides of stars and planets by measuring the strength and changes of their magnetospheres.

Super Magnets

Other stars besides our Sun also make their own magnetic fields. The faster-spinning stars pump out the most powerful fields. For example, imagine a big star, say three or more times the mass of our own Sun.

Eventually, a star of this size will explode off its outer gases and leave behind a collapsed core called a neutron star. The smaller core spins a lot faster than the star did, because it got whipped up during the explosion. Its magnetic field strength is compressed and also increases during the collapse. Radio telescopes discovered these bizarre star corpses by the sweeping of their magnetic poles -- we see the radio waves pouring out every time the fountain sweeps past. They look like lighthouses, which is why we named them "pulsars."

Radio telescopes also discovered neutron stars that have truly enormous magnetospheres. We call them "magnetars." If you let go of a paper clip near the surface of a magnetar, it would yank the paperclip with a force greater than 150 million times what you’re used to gravity doing here. And if a magnetar were located at about half the distance to the Moon, it would easily erase your credit cards and pull keys right out of your pocket.

Most magnetars spin back down again over time and lose their strong fields. However, every so often some energetic neutron stars can spin up to become magnetars. When they do, any burps of gas on their surfaces get shot out along their magnetic field lines and burst out as Gamma-rays, a form of light more intense than X-rays!

Black holes have the most powerful magnetic fields, because they are yanking so much material near and around them. That constant movement of materials past each other is moving electrons into strong currents. So, huge fields of electricity run through the collection pits around black holes, in turn building giant bubbles of magnetism around them that hurl particles at relativistic speeds, enough to broadcast electromagnetic radiation across the galaxy.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.