The Milky Way

A galaxy seen from the inside

The Milky Way is the galaxy we inhabit, and the only one we can study from within. To the unaided eye it appears as a soft, misty band crossing the sky: the combined light of countless stars too faint to resolve individually. Dark rifts within the band are clouds of interstellar dust blocking the starlight behind them.

Because we sit inside the disc, mapping its full structure took centuries. Radio and infrared surveys can see through the dust, revealing a barred spiral: a central bar, two major spiral arms, and several smaller arms, all set within a much larger, fainter halo of old stars and dark matter.

Our place in it

The Sun lies in the Orion Arm, a minor spiral arm about 26,000 light-years from the galactic centre. It takes the Sun roughly 230 million years to complete one orbit of the Galaxy, a period astronomers call a galactic year. The Solar System is partway through its current lap.

Most of the stars we can see with the naked eye are our closest galactic neighbours, all within a few thousand light-years. The nearest system to our own is Proxima Centauri, just 4.24 light-years away. The Galaxy as a whole is far larger: the visible disc spans about 100,000 light-years, and its dark-matter halo extends much farther still.

The galactic centre

At the heart of the Milky Way, in the constellation Sagittarius, lies a supermassive black hole called Sagittarius A* (pronounced "A-star"). Stars seen orbiting it at high speed give a mass of about four million Suns packed into a region smaller than the orbit of Mercury.

Surrounding the black hole is a dense hub of old stars, gas, and dust, largely hidden from visible light. The Event Horizon Telescope produced the first image of Sagittarius A* in 2022, confirming the shadow of a black hole at the centre of our own Galaxy.

Satellite galaxies and neighbours

The Milky Way is the second-largest member of the Local Group, after the Andromeda Galaxy. Around it orbit dozens of smaller satellite galaxies, the most prominent being the Large and Small Magellanic Clouds, visible from the southern hemisphere.

In roughly four to five billion years, the Milky Way and Andromeda are expected to make their first close pass, then merge over several billion more years. The stars themselves will almost never touch.

A future collision with Andromeda

The Andromeda Galaxy is approaching the Milky Way at roughly 110 kilometres per second. In about four to five billion years, the two giant spirals are expected to pass close enough to become gravitationally bound, then merge over several billion more years.

The dark-matter halo

Everything we can see, stars, gas, and dust, makes up only a small fraction of the Milky Way's mass. The galaxy is embedded in a vast halo of dark matter that extends far beyond the visible disc and shapes how the Galaxy rotates and how its satellites orbit.

How we study it

Because we cannot photograph the Milky Way from outside, astronomers reconstruct its shape with star counts, radio maps of gas, and distances measured by the Gaia mission. All-sky infrared surveys such as 2MASS and WISE peer through the dust, while radio interferometry traces the spiral arms and the motion of gas around the centre.

The medallion on this page is an illustration of a barred spiral galaxy, based on the nearest observed analogues, not a photograph of the Milky Way itself.