Birth of the Milky Way

The story

The stripe of milk in the sky

If you move away from city lights on a night with no Moon, you can see a wide pale stripe across the sky. The ancient Greeks called it the “milky circle”; in the myth it was milk spurting from the breast of the goddess Hera. The Greek word is also where the name of all galaxies comes from. In 1610 Galileo Galilei pointed his telescope at that stripe and realised that it was made of countless stars, too small and too close together to be told apart with the naked eye.

What we see is the Milky Way seen from the inside. We are in a disc of stars: when we look along the disc we see a huge number of them, one behind another, and together they make the stripe. Astronomers estimate that our galaxy contains between 100 and 400 billion stars and is about 100,000 light-years across, which means light takes about 100,000 years to cross it. The Sun is in a spiral arm, about 26,000 light-years from the centre, and goes once around the galaxy in about 230 million years.

A galaxy built from pieces

The Milky Way was not born with the shape it has today. For a long time astronomers knew little about how it began, but in recent years there has been a breakthrough, thanks to the European Space Agency's Gaia satellite, launched in 2013. Gaia has measured the position and movement of almost two billion stars, building the most detailed three-dimensional map ever made.

That data showed that the thick disc of the galaxy began to form about 13 billion years ago: the core of the galaxy was already forming less than a billion years after the Big Bang. After that the galaxy grew in a messy way, like a village that spreads by swallowing up the villages around it. Groups of stars and gas, that is, smaller galaxies, came close and were caught by the gravity of the Milky Way.

One of the most important meetings happened about 10 billion years ago, according to estimates: a dwarf galaxy called Gaia-Enceladus (or the Gaia-Sausage, from the shape its stars make on graphs) merged with the Milky Way. Some of its stars were scattered into the halo, the outer, almost spherical region of the galaxy, and their “backwards” motion is what allowed them to be recognised. Other dwarf galaxies are still falling in today: the Sagittarius Dwarf is slowly being torn apart.

The disc and the arms

With time the gas settled into a thin, spinning disc, and new stars kept being born in it. The spiral arms are waves of density that pass through the disc, like waves of traffic on a road: the gas is squeezed and new stars are born. At the centre there is a bar of stars and, at the very middle, an enormous black hole called Sagittarius A*, with a mass about 4 million times that of the Sun. In 2020 the Nobel Prize in Physics went to Reinhard Genzel and Andrea Ghez for following the orbits of stars around that black hole for years.

A 20th-century discovery

Until 1920 many astronomers thought the Milky Way was the whole Universe. In 1924 Edwin Hubble, using the big telescope at Mount Wilson in California, measured the distance to a bright smudge called Andromeda and realised that it was too far away to belong to our galaxy: it was another galaxy. In one go the Universe became enormous: the Milky Way was one galaxy among many. Today it is estimated that the observable Universe holds hundreds of billions of them, or more.

The Milky Way is not standing still. It is approaching Andromeda, the nearest big galaxy, at a speed of about 110 kilometres per second. In a few billion years the two galaxies might merge, although recent studies say this is not certain. There is no danger for the stars: they are so far apart from one another that almost none would collide with another.