That bright strip across the sky with its extraordinary abundance of stars and clusters was a clue to the nature of our galaxy. 

 

It was obvious to astronomers for quite a long time that most of the stars were gathered together into a flats, layer or disk and that we were within that disk. But we still don't know whereabouts in the galaxy we are. And then in the early 20th century an American astronomer, Harlow Shapley, hits on a way of trying to find out where the centre of the galaxy might be. He used objects called globular clusters, which are actually found all over the sky. 

 

Bright sources containing thousands of stars, globular clusters are spread out in the sphere around the Milky Way's central disk. Shapley realized they were in effect signposts to where the centre of the galaxy could be found. 

 

He plotted where the clusters were and he found although they were spread all over the sky, they were concentrated in a particular direction. And that told us that we weren't at the centre of the galaxy, but the centre of the galaxy was in this direction here. So at last, astronomers knew exactly where the centre of the galaxy was and they also knew pretty much how far away it was.