The Milky Way

Our galaxy from the inside

The Milky Way is the galaxy that contains the Sun, Earth and every star you can see with the naked eye. A galaxy is a huge system of stars, gas, dust and dark matter held together by gravity.

Because we live inside it, we cannot step outside and take a picture. Astronomers have pieced together its shape by measuring the distances and motions of stars and gas clouds, a bit like mapping a forest while standing among the trees.

Its structure

The Milky Way is a barred spiral. At the center is a bulge of old stars crossed by a bar, an elongated band of stars several thousand light-years long.

Around it spreads a thin, flat disk about 100,000 light-years wide, where most young stars, gas and dust are found. Spiral arms wind through the disk; they are regions where stars and gas bunch up, lit by bright young stars.

A faint, roughly round halo surrounds everything. It holds old stars, ancient globular star clusters, and most of the galaxy's dark matter, an invisible material detected only by its gravity.

Where the Sun is, and how it moves

The Sun lies in the disk, about 26,000 light-years from the center, in a minor arm called the Orion Arm or Orion Spur. It is roughly halfway between the center and the visible edge.

Like everything in the disk, the Sun orbits the center, moving at about 500,000 mph (around 230 km per second). Even at that speed, one lap takes about 230 million years. The last time the Sun was in this part of its orbit, the first dinosaurs were appearing.

The black hole at the center

In the direction of the constellation Sagittarius lies Sagittarius A*, a supermassive black hole about 4 million times the mass of the Sun. Thick dust hides it from ordinary telescopes, so astronomers watch it in infrared and radio light.

By tracking stars racing around it for decades, Reinhard Genzel's and Andrea Ghez's teams proved something very massive and compact sits there; they shared the 2020 Nobel Prize in Physics. In 2022 the Event Horizon Telescope released the first image of its shadow.

It is fairly quiet compared with the black holes that power bright galaxies elsewhere. It poses no danger to Earth.

How and when to see the Milky Way

The band of the Milky Way is the combined glow of billions of distant stars in the disk, seen edge-on. Dark lanes across it are dust clouds blocking the light behind them.

Darkness is everything. City light hides the band completely, so travel to a rural area, ideally far from towns. A light-pollution map helps you find dark spots. Choose a clear night near new Moon, or when the Moon has set, because moonlight washes the band out.

From the Northern Hemisphere, the brightest part, toward the galactic center in Sagittarius and Scorpius, is best from roughly May to August. Look south in the hours after full darkness; in late summer it is well placed in the evening. In winter you see a fainter part of the band running through Cassiopeia, Perseus and Orion.

Give your eyes 20 to 30 minutes to adapt and use only a red light. Binoculars sweeping along the band reveal dense star clouds and clusters. For photos, a camera on a tripod with a wide lens and exposures of about 15 to 25 seconds will capture it.

Gaia: mapping our galaxy

ESA's Gaia spacecraft, which operated from 2013 to 2025, measured the positions, distances and motions of nearly 2 billion stars with extraordinary precision. It is the most detailed map of the Milky Way ever made.

Gaia showed that our galaxy is still settling after swallowing smaller galaxies in the past, and that the disk is slightly warped and rippling. Astronomers will be working through its data for years.