The Milky Way

On a clear, dark night you can see a pale, milky band stretching across the sky. The ancient Greeks called it the Milky Way, and the name stuck. Galileo already saw through his telescope in 1610 that this band is made of countless separate stars. How huge that disc of stars is, and how far out in it we sit, only became clear a good 100 years ago. You are looking at our own galaxy, a gigantic disc of stars, gas and dust, seen from a planet right inside it. Every star you have ever seen with the naked eye belongs to it.

A view from the inside

Why a band? Imagine you are stuck inside a flat pancake of stars. If you look up or down out of the disc, you see only a few stars. But if you look along the disc, countless distant stars stack up behind one another into a bright shimmer. That shimmer is exactly the band in the night sky: the light of many millions of stars, too far away to make out individually.

Spiral arms and the disc of stars

Seen from the outside, the Milky Way would be a magnificent spiral galaxy: a disc around 100000 light years across with a thickened centre and sweeping spiral arms. A long bar of stars runs through that centre, and the arms start from its ends. Experts therefore call it a barred spiral galaxy, a special form of spiral galaxy. The gas is denser in the arms, so especially many new stars are born there, which is why they shine so brightly. In total the Milky Way contains, by current estimates, between 100 and 400 billion stars. Our Sun is just one of them, neither particularly large nor particularly central.

The Sun on its grand lap

The Milky Way does not stand still, it rotates. Our Sun sits about 26000 light years from the centre and races around it at roughly 230 km per second. Despite that speed, a single lap takes about 230 million years, a so called galactic year. When the Sun began its last lap at our current position, the first dinosaurs were setting off on Earth. Since its birth the Sun has completed roughly 20 such laps.