How big is our galaxy?

Galaxies like our Milky Way are so big that light takes 100,000 years to cross it, travelling at 300,000 kilometres (186,000 miles) per second. The commonly used measure of distance – a light year – is equivalent to 9.5 trillion kilometres (5.9 trillion miles); with figures this large, it’s little wonder astronomers prefer not to use everyday units!

In a supergalaxy, large galaxies are separated by a couple of million light years at most – just a few times their own diameter. In contrast, light takes little more than a second to travel between the Moon and Earth, and about 500 seconds to arrive at Earth from the Sun.

To put it another way, if our world was a one- millimetre (0.04-inch) dot, the Moon would be just three centimetres (1.2 inches) away, but our galaxy would stretch as far as the Sun, and the Local Group supergalaxy would be about the size of our Solar System out to the orbit of Neptune.