This illustration shows the fundamental architecture of our island city of stars, the Milky Way galaxy: a spiral disk, central bulge, and diffuse halo of stars and globular star clusters. Not shown is the vast halo of dark matter surrounding our galaxy. A comprehensive survey that combines the observing prowess of both the Hubble Space Telescope and the European Space Agency's Gaia satellite measured the total mass of our galaxy at 1.5 trillion solar masses. Only a few percent of the estimated 200 billion stars in our galaxy contributes to this total. Most of the rest is locked up in invisible dark matter. The precision of Hubble and Gaia each allowed astronomers to measure the movement of the isolated spherical islands called globular star clusters. The more massive a galaxy, the faster its clusters move under the pull of gravity. And, this allows for the mass of our galaxy to be calculated.
