The Milky Way Galaxy.
The Milky Way galaxy is rotating along with the Sun at a speed of 230 km per second in a near-circular orbit around the galactic center. It takes about 220 million years to complete one orbit! Stars close to the galactic center have shorter orbital periods than those placed far away but they do not behave like the planets do traveling around the Sun. (The planets when placed farther from the Sun have lower orbital speeds). Весаuse the mass of the galaxy is spread over the vast distances and the speed of a star is governed not only by its distance but also by the mass that lies inside it as also by the mass that lies outside. So greater the distance of the star, greater is the mass acting upon it. Star speeds close to the galactic center are high and decrease as they move to the outer fringes but as they farther out speeds increase to about 230 km per second at Sun s distance and to some 300 km/sec at a distance of 60,000 light-years from the center. This, therefore, also implies that a large fraction of the mass in the Galaxy lies at the fringes for if this were not so the speed of the stars beyond the Sun would begin to decrease.
The Galactic center is marked by a powerful radio source known Sagittarius A. And as much of these emissions are from a source as small as our Solar System the center of the Galaxy must contain a black hole of several million solar masses, The Milky Way Galaxy is also known by its Indian name as ‘Akash Ganga’.