The Milky Way

On a clear, dark night far from street lights, you can see a pale, cloudy band stretching across the sky. People long ago called it the Milky Way.

Through a telescope the cloud turns out to be stars, far too many and too faint to pick out by eye. That band is our galaxy, seen from the inside.

The Milky Way is a huge, flat disc of stars, shaped a little like two fried eggs back to back. It holds a few hundred billion stars, and the Sun is one of them. We live inside the disc, about half way out from the middle. When we look along the disc we see the band. When we look up out of it we see only the few stars close by.

The whole disc turns. The Sun and its planets go round the centre at about 230 km every second, and one lap still takes about 230 million years.

Nobody has ever taken a picture of the Milky Way from outside. Nothing we have made has left it, or will for a very long time. Every picture of the whole galaxy is a drawing made from measurements.

Working out the shape of the Milky Way is like mapping a forest from one spot inside it. Nearby trees hide the distant ones, and you cannot step outside for a look. Dust between the stars blocks ordinary light across most of the disc, so much of the map comes from infrared light and radio waves, which pass through dust.

The parts.

Part	What it is	Size
Disc	Most of the stars, and nearly all the gas and dust. New stars form here	About 100,000 light years across and about 1,000 light years thick
Bulge and bar	A crowded mass of mostly old stars round the centre, drawn out into a bar	The bar is about 30,000 light years long, though estimates differ
Spiral arms	Bright lanes in the disc, marked by young blue stars and glowing gas	Wound right round the disc
Halo	A thin, round scatter of very old stars and more than 150 tight balls of stars called globular clusters	Reaches several hundred thousand light years out
Dark matter	Unseen matter known only by its pull. Most of the galaxy’s mass	Reaches further than the stars do

The disc is a hundred times wider than it is thick, the same proportions as a CD.

How many stars. Stars cannot be counted one by one, so the number comes from the galaxy’s mass and the mix of stars seen nearby. Estimates run from 100 billion to 400 billion. The whole galaxy, dark matter included, has a mass of roughly a trillion Suns. That figure is uncertain by a factor of two.

Where we are. The Sun is about 27,000 light years from the centre, close to the middle plane of the disc, on the inner edge of a minor arm. It moves at about 230 km/s round the centre and takes about 230 million years to go round once. Since the Sun formed it has been round about 20 times. The last time it was at this point in its orbit, the first dinosaurs were just appearing.

The arms are not fixed. Stars nearer the centre complete their orbits sooner than stars further out, so arms made of the same stars would wind up tight within a few turns. They are better thought of as traffic jams. Stars and gas move into an arm, slow down and bunch up, then move out the other side. The squeezing of gas in the arms sets off the birth of new stars, and the brightest young stars die before they can leave, which is why the arms shine.

The centre. In the direction of the constellation Sagittarius, hidden behind dust, is a black hole of 4.3 million Suns. It was weighed by following the orbits of stars round it for almost three decades, work that won a Nobel Prize in 2020. It is quiet at present, with very little gas falling in.

How the map is made. Between 2014 and 2025 the Gaia spacecraft measured the brightness of about 1.8 billion stars, and the distances and movements of about 1.5 billion of them, using parallax. That is the best map there has ever been, and it still covers only about one star in every hundred, mostly on our side of the galaxy. The far side of the disc is known far less well, and even the number of main spiral arms, two or four, is still argued over.

Neighbours and the future. About 60 small galaxies orbit the Milky Way. The two largest, the Magellanic Clouds, are easy to see by eye from the southern half of the Earth and lie about 170,000 and 210,000 light years away. The nearest large galaxy, Andromeda, is 2.5 million light years away and coming towards us at about 110 km/s. For years the two were expected to collide in 4 to 5 billion years. Newer measurements of the sideways movements of the galaxies make that less certain: a near miss is about as likely as a collision in the next 10 billion years. Either way, stars are so far apart that hardly any would ever hit each other.