When it comes to radio waves, most of us first think about the various devices that we use for communication, such as the radios in our cars and the phones in our pockets. However, there are many natural sources of radio emission, and as soon as we stop listening to ourselves we discover a universe full of signals from some of the most extraordinary places.
Our Sun is the brightest object in the radio sky. Its solar flares release enormous amounts of energy and produce crackling radio noise that can be detected easily with a small antenna.
Neutron stars are the collapsed remnants of once-bright stars. They are often visible only by their radio emissions, which sweep past the Earth like a lighthouse beam with each rotation of the star.
Supermassive black holes at the hearts of distant galaxies accelerate jets of material to almost the speed of light. This produces relativistic electrons that emit radio waves as they spiral around the embedded magnetic field.
Hydrogen, the most abundant element in the universe, can spontaneously emit a photon with a well-defined radio wavelength near 21 cm.
Finally, at the current limit of observable reality, the hot fireball that was our universe shortly after the Big Bang is visible today as a faint radio afterglow known as the cosmic microwave background.
To receive signals from the most...
