Milky Way Galaxy The structure and dynamics of the Milky Way Galaxyastronomy

The first reliable measurement of the size of the Galaxy was made in 1917 by American astronomer Harlow Shapley. He arrived at his size determination by establishing the spatial distribution of globular clusters. Shapley found that, instead of a relatively small system with the Sun near its centre, as had previously been thought, the Galaxy is immense, with the Sun nearer the edge than the centre. Assuming that the globular clusters outlined the Galaxy, he determined that it has a diameter of about 100,000 light-years and that the Sun lies about 30,000 light-years from the centre. (A light-year is the distance traveled by light in one year and is roughly 9,460,000,000,000 km [5,880,000,000,000 miles].) His values have held up remarkably well over the years. Depending in part on the particular component being discussed, the stellar disk of the Milky Way system is just about as large as Shapley’s model predicted, with neutral hydrogen somewhat more widely dispersed and dark (i.e., unobservable) matter perhaps filling an even larger volume than expected. The most distant stars and gas clouds of the system that have had their distance reliably determined lie roughly 72,000 light-years from the galactic centre, while the distance of the Sun from the centre has been found to be approximately 25,000 light-years.