Monsters in the Dark

I was about 10 years old when I saw the Milky Way for the first time. On holiday in the countryside, blissfully far away from the polluting airglow of city lights, the night sky was like nothing I’d ever seen before. There were more stars, of course, but there was also something wholly new: a vast silvery band of light, arcing across the sky.

For most of human history, the Milky Way was something of an enigma. Different cultures offered up a dizzying variety of folk tales and explanations. There’s the Roman story of a goddess spilling milk across the sky (from which we get our term “Milky Way”), and many tales of heavenly rivers in the firmament. My personal favorite is the Cherokee legend, in which a dog stole a basket of cornmeal, leaving a trail behind. The Cherokee call the Milky Way gi li’ ut sun stan un’ yi—”the way the dog ran away”.

It was the astronomer Thomas Wright who, in 1750, first described our galaxy as science now understands it: a vast disk of stars, held together by the same gravitational forces that define the Solar System, though operating on incomparably larger scales. Our Milky Way is a large-ish spiral galaxy, made up of around 200 billion stars and stretching 100,000 light years across. If these numbers seem huge to you, you’re not alone: up until the turn of the 20th century, astronomers assumed something so absurdly big must constitute the entire material Universe. So it was a considerable surprise in 1924 when Edwin Hubble demonstrated that our Milky Way is just one tiny corner of a Universe, containing hundreds of billions of galaxies of all shapes and sizes.

(continue reading)