Our Galaxy has a shape that is similar to that of our distant neighbors, but we can only see a small part: that large, faint luminous band that completely encircles the night sky. This band makes it seem that the Galaxy is simply a thin disk of stars mixed with gas and dust, but this is because the Earth is inside, in an outlying position. In reality, the Galaxy is very similar to a gigantic top, with a large bulge in the central part. This bulge, which is shaped like a slightly flattened sphere with a diameter of 30 000 light years, has many parts that lie inside each other. The outer edge has enormous dark clouds of gas which partially block our view and surround a kind of "halo" of very old stars which are clustered around each other. Farther in, there are at least four disks of molecular gas, especially hydrogen, millions of compact stars and clouds formed by ionized gas which orbit at high speeds inside the galactic center. Here there may be superdense and massive objects whose mass might be millions of times greater than the Sun. Instead, towards the outer portion, large arms extend outwards in a spiral. Most of the stars are small and red, and not very bright, but some are young and brilliant or "middle aged", like our own; these are the easiest to observe and have helped us, with their luminous signals, to collect an amazing amount of information: from the chemical composition of the star to its age, from its distance to the processes which take place within its interior.
Our Galaxy has a shape that is similar to that of our distant neighbors, but we can only see a small part: that large, faint luminous band that completely encircles the night sky. This band makes it seem that the Galaxy is simply a thin disk of stars mixed with gas and dust, but this is because the Earth is inside, in an outlying position. In reality, the Galaxy is very similar to a gigantic top, with a large bulge in the central part. This bulge, which is shaped like a slightly flattened sphere with a diameter of 30 000 light years, has many parts that lie inside each other. The outer edge has enormous dark clouds of gas which partially block our view and surround a kind of "halo" of very old stars which are clustered around each other. Farther in, there are at least four disks of molecular gas, especially hydrogen, millions of compact stars and clouds formed by ionized gas which orbit at high speeds inside the galactic center. Here there may be superdense and massive objects whose mass might be millions of times greater than the Sun. Instead, towards the outer portion, large arms extend outwards in a spiral. Most of the stars are small and red, and not very bright, but some are young and brilliant or "middle aged", like our own; these are the easiest to observe and have helped us, with their luminous signals, to collect an amazing amount of information: from the chemical composition of the star to its age, from its distance to the processes which take place within its interior.
