Galaxies and the Milky Way

On a clear, dark night, far from city lights, a pale band of light arches across the sky. The ancient Greeks called it a river of milk. The Romans called it the Via Lactea, the Milky Way. What you are seeing is the combined glow of billions of stars, so distant and so crowded together that they blur into a single haze. You are looking at your own galaxy, from the inside.

The shape of home

The Milky Way is a barred spiral galaxy: a flat disc of stars, gas and dust about 100,000 light-years across, with a thick bulge of older stars at the centre and a straight bar of stars running through the middle. From the ends of the bar, spiral arms sweep outward. The disc is remarkably thin for its width, roughly a thousand light-years thick, like a dinner plate or a vinyl record.

The Sun sits in a minor arm called the Orion Spur, about 27,000 light-years from the centre, a little over halfway out. We orbit the galactic centre at around 230 kilometres per second, completing one lap every 230 million years or so. The last time the Sun was where it is now, dinosaurs had only just appeared.

Surrounding the disc is a vast, sparse halo of old stars and globular clusters, and beyond that a far larger halo of dark matter, invisible but holding the whole structure together with its gravity.

Counting the stars

Nobody knows exactly how many stars the Milky Way contains, because we cannot see through the dust to the far side, and because the faintest red dwarfs are hard to detect even nearby. Estimates range from 100 billion to 400 billion. Most of them are small, cool and dim. Bright stars like the ones that form the familiar constellations are the exception.

Between the stars drift clouds of gas and dust, the raw material for new stars, and the Milky Way is still making them at a rate of a few Suns' worth each year.

Kinds of galaxies

Galaxies come in three broad shapes. Spirals, like ours and Andromeda, are flat discs with arms, rich in gas and busy forming stars. Ellipticals are smooth balls or eggs of old reddish stars with almost no gas, having used it up or lost it long ago; the largest galaxies in the universe are giant ellipticals. Irregulars have no neat shape at all, often because they have been pulled about by a neighbour's gravity. The two bright smudges visible from the southern hemisphere, the Large and Small Magellanic Clouds, are irregular galaxies orbiting the Milky Way.

Size varies enormously. Dwarf galaxies may hold only a few million stars. The giant elliptical at the heart of the Virgo cluster, M87, probably holds several trillion.

The neighbourhood

Galaxies are social. The Milky Way belongs to the Local Group, a loose family of more than fifty galaxies spanning about ten million light-years. The two heavyweights are the Milky Way and Andromeda, each attended by dozens of small satellites. Andromeda is approaching us at 110 kilometres per second. In about 4.5 billion years the two will collide and, over a few hundred million years, merge into a single giant elliptical galaxy. Collisions between stars will be almost nonexistent, because the space between stars is so vast, but the beautiful spiral shapes will be lost.

The Local Group is a small part of the Virgo Supercluster, which is in turn an arm of an even larger structure named Laniakea, Hawaiian for "immense heaven". On the largest scales, galaxies are strung along filaments and sheets surrounding enormous near-empty voids, a pattern often compared to foam or a cosmic web.

How many galaxies?

When the Hubble Space Telescope stared at a patch of apparently empty sky the size of a grain of sand held at arm's length, it found thousands of galaxies. Scaling up, astronomers estimate the observable universe contains around two trillion galaxies. Each one is a city of stars with its own history, and in most of them, almost certainly, planets circle those stars.

What we still don't know

The spiral arms themselves remain a puzzle. They are not fixed collections of stars, since stars orbit at different speeds and would wind the arms up within a few rotations. Instead they are thought to be slow-moving waves of higher density, like a traffic jam that persists even as individual cars pass through it. Why some galaxies form arms, some bars, and some neither, and how the first galaxies assembled so quickly after the Big Bang, are questions the James Webb Space Telescope is now beginning to answer.