What is a galaxy, and why the Milky Way is special

If stars are the atoms of the sky, galaxies are the cities. A galaxy is a vast gravitationally-bound system of stars, gas, dust, and (we now know) dark matter, all held together and orbiting a common centre. Galaxies come in different shapes and sizes, and they’re the basic unit in which almost all of the universe’s visible matter is organised.

Our own galaxy is called the Milky Way. We live inside it, a little more than halfway out from the centre, on a fairly unremarkable arm of a fairly unremarkable spiral. That unremarkableness is itself kind of marvellous. We get to live inside the thing we’re trying to understand.

What galaxies look like

Galaxies come in a few broad flavours.

Spiral galaxies have a flat, disk-like shape with a bright central bulge and long, winding arms trailing outward. Blue, star-forming regions concentrate along the arms. The Milky Way, Andromeda, and most of the familiar galaxies in photographs are spirals.

Barred spiral galaxies are spirals whose central bulge is elongated into a straight “bar” of stars, with spiral arms trailing from the ends. The Milky Way is actually a barred spiral. We only confirmed our own galaxy has a central bar in the last few decades.

Elliptical galaxies are rounder, smoother, without clear arms or disks. Tend to be older, containing mostly cooler reddish stars, with less gas available for new star formation.

Irregular galaxies don’t fit any neat pattern. Often small, misshapen, sometimes the result of a collision or gravitational disturbance.

Dwarf galaxies are small systems, often orbiting larger galaxies. The Milky Way has dozens of dwarf galaxy companions.

The scale is hard to grasp. A typical spiral galaxy contains somewhere between 100 billion and 400 billion stars, and spans tens or hundreds of thousands of light years. The Milky Way is about 100,000 light years across. Light from one edge takes 100,000 years to reach the other.

Our galaxy, from the inside

The Milky Way contains between 100 and 400 billion stars. The number is hard to pin down precisely because we can’t see all of our own galaxy from our position inside it. Much of the view is blocked by dust.

Our solar system sits in a minor arm called the Orion Arm (sometimes called the Orion Spur), about 26,000 light years from the galactic centre. We orbit that centre once every 225 million years or so. Since Earth formed, we’ve completed roughly 20 galactic orbits.

At the centre of the Milky Way sits Sagittarius A* (pronounced “Sagittarius A-star”), a supermassive black hole about four million times the mass of the sun. It’s the anchor around which everything in the galaxy orbits. Every large galaxy we’ve studied has a supermassive black hole at its core.

The band across the sky

From Earth, the Milky Way appears as a faint glowing band stretching across the night sky. That band is our galaxy, viewed edge-on. The glow is the combined light of millions of stars too distant to resolve individually, plus glowing gas clouds and scattered light.

On a truly dark night, far from city lights, the Milky Way isn’t subtle. It’s a luminous river across the sky, with visible dark rifts (dust lanes blocking starlight behind them) and bright knots (star-forming regions). The most spectacular view is in the direction of the galactic centre, which runs through the constellation Sagittarius. In summer, from a dark site, the core of our galaxy rises low in the south (or high overhead from the southern hemisphere) and it’s breathtaking.

If you’ve never seen the Milky Way, it’s worth a dedicated trip to a dark site. City skies hide it completely. Under a truly dark sky, it commands the view. Often the single most memorable sight a new amateur astronomer experiences.

The cosmic neighbours

The Milky Way doesn’t orbit alone. It’s part of a small cluster of galaxies called the Local Group, containing around 80 known galaxies within a sphere roughly 10 million light years across. The two biggest members are the Milky Way and the Andromeda Galaxy (M31), each with tens of satellite dwarf galaxies.

Andromeda is the most distant object visible to the naked eye. From a dark sky, it looks like an elongated smudge near the constellation Andromeda. It’s 2.5 million light years away. The light from Andromeda reaching your eye tonight set out around the time Homo sapiens was still evolving. We’re seeing it as it was before humans existed.

Andromeda and the Milky Way are on a collision course. In about 4.5 billion years, the two will merge into a single, much larger elliptical galaxy. Our sun will probably be flung into a different part of the new galaxy. But because stars are so far apart from each other, very few will actually collide.

Beyond the neighbourhood

The Local Group is part of a much larger structure called the Virgo Supercluster, containing thousands of galaxies. The Virgo Supercluster is itself part of the even vaster Laniakea Supercluster. Laniakea is one of millions of similar structures across the observable universe.

Current estimates put the total number of galaxies in the observable universe at around two trillion. Most are small and faint, only detectable by deep-field observations from space telescopes. But they’re all real. They’re all places. Most of them contain hundreds of billions of stars.

It’s hard to look up at the night sky without thinking, briefly, about the sheer statistical weight of that number.

What to do with this

Two suggestions.

First: go somewhere dark once in your life. A genuine, low-light-pollution site, and see the Milky Way with your own eyes. Many people live their whole lives in light-polluted cities and never experience this. It’s visible from most deserts, mountain areas, and rural spots far from major cities.

Second: when conditions permit, find the Andromeda Galaxy with your naked eye. Northern-hemisphere autumn and early winter is the best season. You’ll be looking at another galaxy, with its own hundreds of billions of stars, without any equipment at all. That’s a remarkable thing for a human to do, and a decent answer to “does any of this matter?”

It matters. You’re an inhabitant of one galaxy, watching another, across 2.5 million years of travelling light. That’s your birthright for having eyes and a clear night.