Since the 1920s, when the American astronomer Edwin Hubble developed the classification diagram of the galaxy, it has been the fundamental instrument used in creating order among the objects in space. This diagram contains three types of galaxies; elliptical, spiral, and irregular. The elliptical galaxies are the simplest: they have an oval form that is more or less flattened, they are brighter in the center than at the edges and they do not have any particular structures. These galaxies are classified in the catalogues with the letter "E" followed by a number between 0 and 7 that denotes the degree of flatness. For example, a galaxy classified as E0 appears to be completely round, while a galaxy classified as E7 is three times longer than its width. Spiral galaxies are more complex, and they can be divided into two categories: a normal spiral and a barred spiral. The normal spiral develops its arms from only one nucleus, while the barred spiral has arms that come out of the ends of a central bar. In the catalogues, spiral galaxies are classified respectively with the letters S and SB, followed by the lower case letters a, b, and c which describe the development of the arms. All galaxies that are not included in these categories are defined as irregular or peculiar and, for this reason, are not easily classified. The system currently used to identify every galaxy or nebula is the New General Catalogue in which individual objects are given a NGC code followed by an order number.
Since the 1920s, when the American astronomer Edwin Hubble developed the classification diagram of the galaxy, it has been the fundamental instrument used in creating order among the objects in space. This diagram contains three types of galaxies; elliptical, spiral, and irregular. The elliptical galaxies are the simplest: they have an oval form that is more or less flattened, they are brighter in the center than at the edges and they do not have any particular structures. These galaxies are classified in the catalogues with the letter "E" followed by a number between 0 and 7 that denotes the degree of flatness. For example, a galaxy classified as E0 appears to be completely round, while a galaxy classified as E7 is three times longer than its width. Spiral galaxies are more complex, and they can be divided into two categories: a normal spiral and a barred spiral. The normal spiral develops its arms from only one nucleus, while the barred spiral has arms that come out of the ends of a central bar. In the catalogues, spiral galaxies are classified respectively with the letters S and SB, followed by the lower case letters a, b, and c which describe the development of the arms. All galaxies that are not included in these categories are defined as irregular or peculiar and, for this reason, are not easily classified. The system currently used to identify every galaxy or nebula is the New General Catalogue in which individual objects are given a NGC code followed by an order number.
