 It is worth noting, however, that one does not need an extremely high density of matter to form a black hole if one has enough mass. If all the stars in the Milky Way gradually aggregate towards the galactic center while keeping their proportionate distances from each other, they will all fall within their joint Schwarzschild radius and so form a black hole long before they are forced to collide. Or if one has a couple hundred million solar masses of water at its standard density (1 gm/cm3)—so occupying in total a region of about 1027 cubic kilometers, the approximate size of the smallest sphere containing the orbit of Uranus—it will be contained within its Schwarzschild radius. (In this case, of course, the water would indeed eventually collapse on itself to arbitrarily high densities.) Some supermassive black holes at the centers of galaxies are thought to be even more massive than the example of the water, at several billion solar masses, though in these cases the initial density of the matter thought to have formed the black holes was extraordinarily high. 