galaxy
/ GAL-uhk-see /
A galaxy is a vast city of stars — anywhere from a few million to many hundreds of billions of them — held together by gravity, along with great clouds of gas and dust and a hidden scaffolding of dark matter. The biggest galaxies, like our own, blaze with hundreds of billions of stars; but most galaxies are far smaller dwarfs. Look up on a clear, dark night and the faint, milky band arching overhead is our own galaxy, the Milky Way, seen edge-on from a seat inside it.
Galaxies come in a few basic shapes. Some are graceful spirals, with arms of bright young stars winding out from a glowing center, like cream just stirred into coffee. Others are smooth, featureless ellipticals — fat balls of older, redder stars. And many are irregulars, lumpy and shapeless, often because a near-miss with a neighbor pulled them out of joint. The universe holds something like two trillion of them.
A common mix-up is to picture a galaxy as a crowd of stars packed tightly together. In truth it is mostly empty space: stars sit so far apart that when two galaxies collide, almost no stars ever touch — the two simply glide through each other over hundreds of millions of years, reshuffled by gravity but rarely crashing.
The Milky Way holds 100–400 billion stars and spans about 100,000 light-years; our Sun takes roughly 230 million years to circle it once.
The word comes from the Greek galaxías, "milky," from a myth that the Milky Way was milk spilled from the goddess Hera. Until the 1920s many astronomers thought our galaxy was the whole universe — Edwin Hubble settled the matter by showing that the faint "spiral nebulae" were other galaxies, unimaginably far away.