Milky Way

The Milky Way is the galaxy that contains our Solar System, along with hundreds of billions of other stars and vast clouds of gas and dust. It is a large spiral galaxy, roughly a hundred thousand light-years across, and its name comes from the faint, milky band of light that stretches across the night sky. That band is the combined glow of countless distant stars seen from our position inside the galaxy's disk.

Because we live inside the Milky Way, understanding its shape has been far harder than studying other galaxies seen from outside. Only through careful measurement, and by comparing it with similar galaxies, have astronomers pieced together a picture of its structure and history.

The shape of the galaxy

The Milky Way is a barred spiral galaxy. It has a central bar-shaped region of stars, from which spiral arms curve outward through a flattened disk. The Solar System sits in the disk, roughly two-thirds of the way out from the centre, in one of the galaxy's arms. The disk contains most of the galaxy's stars, gas and dust, and appears as a broad band of light because we view it edge-on from within. Around the disk is a fainter halo of old stars and globular clusters, and beyond that is a massive, invisible halo of dark matter that shapes the galaxy with its gravity.

Stars, gas and dust

The Milky Way contains an enormous number of stars, ranging from dim red dwarfs to brilliant blue giants. Stars are born in cold, dense clouds of gas and dust called nebulae, and they die in different ways depending on their mass, some quietly and others in spectacular supernova explosions. Between the stars are vast quantities of gas and dust, the interstellar medium, which blocks our view of the galactic centre in visible light but glows brightly in infrared and radio wavelengths. This dust is why the dark patches visible along the Milky Way band appear to divide it.

The centre of the galaxy

At the heart of the Milky Way lies a dense concentration of stars and, at its very centre, a supermassive black hole. By observing the motions of stars whipping around this region, astronomers have inferred that the black hole is millions of times more massive than the Sun. The galactic centre is hidden from optical view by intervening dust, so it is studied using infrared, radio and other wavelengths that pass through the dust.

Our place and motion

The Solar System orbits the centre of the Milky Way at a great distance, and one full circuit, sometimes called a galactic year, takes hundreds of millions of years. The Sun and its planets move at high speed along this path, carrying us through the galaxy's disk. The neighbourhood of the Sun is relatively calm compared with the crowded, energetic centre, which is one reason Earth has enjoyed a long, stable environment suitable for life.

How we study it

Astronomers map the Milky Way by measuring the distances, motions and colours of stars, by observing radio emissions from gas clouds, and by tracking the positions of objects over time. Missions that measure stellar positions with great precision have refined our knowledge of the galaxy's structure and the Sun's path through it. Comparing the Milky Way with other spiral galaxies helps scientists fill in the parts we cannot see directly.

Dark matter

One of the great surprises in modern astronomy is that the visible stars and gas do not account for the galaxy's gravity. Stars far from the centre orbit faster than expected from the visible mass alone, and the galaxy behaves as if it contains far more matter than we can see. This unseen material is called dark matter. Its nature remains one of the biggest open questions in physics, but its gravitational effects on the Milky Way and other galaxies are well established.

Common misconceptions

Some people think the Milky Way is a nearby object like a star cluster or nebula; it is our entire galaxy, and we are inside it. Others confuse it with the Solar System. Images often show a neat, bright spiral, but a realistic view from Earth is the soft band of light across the sky. Another common error is believing the Milky Way is unique; it is one of countless galaxies in the universe, most of which are far larger or smaller.

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