But the cosmos is not only populated with stars. Since the Eighteenth century, astronomers have identified and classified other objects which, appearing in a telescope like diffuse and non-specific luminous objects, they called nebulas: together with the stars, the nebulas had to be part of a single and enormous system, what we call the Galaxy. But was this theory really correct? Was there only empty space beyond our cluster of stars? It was the successive generations of astronomers that answered these questions, one step at a time. First it was discovered that certain spherical clusters of stars, called globular clusters, did not seem to be a part of our system of stars; then that even some nebulas were not simply those inert masses of dust and gas that they seemed to be, but rather complex rotating systems of stars. In the beginning, it was thought that such systems were very small compared to our Galaxy, some kind of "satellite" systems. But finally they became convinced that, just like the solar system does not have a privileged position in the cosmos, even the Galaxy is nothing but one of the many islands of stars that inhabit the universe. And on the contrary, careful studies of the other galaxies have provided us with a much clearer idea of just what ours is made of. The closest isolated galaxy is Andromeda, an immense spiral structure with a diameter that is slightly larger than half of our own. It is 2 250 000 light years away and is part of a group of at least thirty galaxies, some larger and some smaller, called the "local group" because they are (relatively) close to us. Farther away, up to a distance of millions of light years, there are other groups, some of which contain hundreds and sometimes thousands of galaxies.
But the cosmos is not only populated with stars. Since the Eighteenth century, astronomers have identified and classified other objects which, appearing in a telescope like diffuse and non-specific luminous objects, they called nebulas: together with the stars, the nebulas had to be part of a single and enormous system, what we call the Galaxy. But was this theory really correct? Was there only empty space beyond our cluster of stars? It was the successive generations of astronomers that answered these questions, one step at a time. First it was discovered that certain spherical clusters of stars, called globular clusters, did not seem to be a part of our system of stars; then that even some nebulas were not simply those inert masses of dust and gas that they seemed to be, but rather complex rotating systems of stars. In the beginning, it was thought that such systems were very small compared to our Galaxy, some kind of "satellite" systems. But finally they became convinced that, just like the solar system does not have a privileged position in the cosmos, even the Galaxy is nothing but one of the many islands of stars that inhabit the universe. And on the contrary, careful studies of the other galaxies have provided us with a much clearer idea of just what ours is made of. The closest isolated galaxy is Andromeda, an immense spiral structure with a diameter that is slightly larger than half of our own. It is 2 250 000 light years away and is part of a group of at least thirty galaxies, some larger and some smaller, called the "local group" because they are (relatively) close to us. Farther away, up to a distance of millions of light years, there are other groups, some of which contain hundreds and sometimes thousands of galaxies.
