The Milky Way Galaxy:

The white band  stretched across the night  sky  is, in  fact, a partial view of  the Milky Way Galaxy. Being inside  the Galaxy, we can see it only in parts. We cannot see the whole of it, the way we see the other galaxies. Visualising the whole Milky Way Galaxy and determining  its shape has not been easy. By watching a large number of galaxies distributed in all directions  in dozens of views as far as modem telescopes can see, scientists have been able to form a picture of what our galaxy must look like from outside. In ihis, they have also been helped by  the observations  about the stars in our galaxy, their distances and motion, etc. The picture of  the Galaxy constructed by the astronomers is shown in Fig. Doesn't it look somewhat like a disc or a gramophone record with a swollen centre? The Milky Way Galaxy contains about 100 billion stars. The stars are not uniformly distributed. You can test this yourself by a simple activity.  

concentration of  stars in certain parts of  the sky. There is a great concentration  of  stars towards the centre of  the Galaxy, which is located  in  the constellation of Sagittarius. The Sun is.situated  on its remote outskirts, about 30,000 light years away from the centre. Note  that when you see the portion of  the Milky Way in  the sky near Sagittarius, you would be looking toward the centre of  the galaxy. When you observe the portion near Orion, you would be seeing the "edge"  of the Galaxy, nearest  the Sun. The Galaxy, as you can sec in Fig. 9.8, is disc shaped. If  we could see our galaxy from the top, we would get the face-on view  (Fig. ). If  we could see it from the edge, we would get the edge-on view  (Fig.). In the edge-on view, the Galaxy consists of  two basic parts: the disc and the halo. The disc consists of  stars, as well as clouds of gas and dust called nebulae. It has a diameter of  100,000  light years, and a thickness of  about 5,000 light years. This collection of gas and stars rotates about the centre (also known as the nucleus) of the Galaxy, with each part moving at a different speed. The Solar System at a distance of  about 30,000 light years from  the centre, in  the outskirts of the Galaxy, also revolves. Moving at a speed of  250 km  per sec., it  takes roughly 200 million years to complete one revolution around the centre of  the Galaxy. There are individual  stars like the Sun as well as groups of  stars, called galactic clusters, that move together in the disc. Astronomers have  identified about  1000 galactic clusters  in the disc, each containing  10  to 1,000 stars.