In 1933, an Oklahoman physicist, Karl Jansky, made headlines when he announced that the we could listen to the galaxy. An employee of Bell Labs, Jansky had been investigating the background noise that interfered with transatlantic radio telephone calls. Much of the noise, he found, was from thunderstorms, but some was from another, unidentified source. Jansky eventually he determined that the noise was being produced not by a terrestrial source but by the Milky Way; he was the first real radio astronomer, the first to successfully learn about celestially bodies by analyzing radiation (such as radio waves or X-rays) that they emitted outside the visible spectrum. Jansky wanted to investigate further, but Bell Labs, having received the answers they needed, put Jansky on another project; he never did radio astronomy again. Thirty years later, two Bell Labs scientists made a discovery using radio astronomy that would win them the Nobel Prize. Arno Penzias and Robert Wilson were using a sensitive, horn-shaped antenna to study the Milky Way when they, too, were puzzled by background noise. The two investigated further and eventually published their findings; others had observed the effects of the cosmic microwave background before, but they had observed it with better equipment, eliminated all known possible causes of the noise (starting with the pigeon dung that occasionally fouled their antenna), and had the fortune to be working at the same time as cosmologists Bob Dicke and Jim Peebles, who had recently theorized that a certain amount of background heat — less than three degrees Celsius above absolute zero — could be expected if the Big Bang model of the universe’s creation were true. Penzias and Wilson scooped Peebles and Dicke on the experimental proof, but the final nails in the coffin of the Steady State Theory of the universe, and made the most important astronomical discovery of the century.
The other great discovery made with radio telescopes was the pulsar. Cambridge graduate student Jocelyn Bell Burnell, who later credited the investigation to her "truly remarkable depth of ignorance", discovered a regular pattern of equally-spaced pulses. She and her advisor (Anthony Hewish, who would go on to share in the 1974 Nobel Prize; Burnell, as a mere grad student, was not honored for the discovery) originally speculated that the regular patterns emitted by pulsars were signs of alien intelligence, but, as they discovered more, decided that they had to be a natural phenomenon. If you’re using a Windows PC, you can try a simulated pulsar hunt in the comfort of your own home. The pulsars turned out, in the end, to be rapidly rotating neutron stars — totally collapsed stars, made from the densest material in the universe.
The only thing that might be denser is the material at the core of a black hole. Black holes, it seems, make a sort of music. It’s not really music, of course, but the similarities between the patterns of radiation emitted by different types of black holes strongly suggest to astronomers that there are physical similarities as well. Thousands of years after people told first themselves stories about patterns in the stars, we can hardly see the constellations any more. But thousands of years after Pythagorus and his followers started thinking about numbers, tones, and the cosmos, we can finally listen in on the stars and hum along to the music of the spheres.