The galaxy is the basic gravitationally-bound structure of stars and gas in the universe. They vary dramatically in character and in size, with the largest galaxies comprised of more than a trillion stars. It is not yet clear when in the history of the universe the galaxies began forming; galaxies are seen as far out as the current technology (the Hubble space telescope) permits. The only certainty is that all galaxies formed out of the same initial cloud of predominately hydrogen and helium. What distinguishes the galaxies that are close to our own, which is another way of saying galaxies that are the same age as our own, are their initial mass and angular momentum and their history of interactions with other galaxies.
The galaxies we see today fall into a very simple scheme that is characterized by a galaxies gas content. The galaxies can be subdivided into four types: elliptical, lenticular, spiral, and irregular.
The elliptical galaxy is as its name implies; the projection of this galaxy on the sky is an ellipse with a ratio of minor axis to major axis ranging from 0.3 to unity. The elliptical galaxy has little or no gas and little or no rotation. Is is composed of old stars.
The lenticular galaxy is a disk-shaped galaxy with little or no gas and few high-mass stars. The lenticular galaxy is a transition galaxy between the elliptical galaxy class and the spiral galaxy class.
The spiral galaxy is the most beautiful of galaxies. It is a disk with swirling arms of gas and bright stars surrounded by a spheroidal component of older stars. The arm structure in the disk can be complex and fragmented, or it can be simple, with a single pair of well-defined arms. Stars within the disk of a spiral galaxy are orbiting the galactic center in nearly circular orbits. Stars within the spheroidal component have a more random distribution of velocities.
Galaxies that cannot be placed in one of the previous-three classes get lumped into the irregular galaxy category. While some of these galaxies are actually pairs of galaxies in collision or are galaxies disrupted by other galaxies, the bulk of the irregular galaxies are galaxies rich in gas and dust, with numerous bright, young stars, and no symmetric structure. Irregular galaxies of this type are small, and therefore dim. The closest neighbors to the Milky Way galaxy, the Large and Small Magellanic clouds, are irregular galaxies. While few irregular galaxies similar to the Magellanic clouds are seen far from the Milky Way because of their low luminosity, they constitute more than 30% of the galaxies in the Milky Way's neighborhood.
These four classes of galaxy follow a progression that starts with the elliptical galaxies, which have little gas, dust, and new stars, to the lenticular galaxies, which also have little gas and dust, to the spiral galaxies, which have disks of gas, dust, and many new stars, to the irregular galaxies, which are rich in gas, dust, and new stars.
The type of galaxy one finds at a given location depends on the density of galaxies. This suggests that the interaction between galaxies over the evolution of the universe plays a central role in the evolution of individual galaxies.
In the low-density regions of our universe, where galaxies are not bound gravitationally to one another, the dominant type of galaxy is the spiral galaxy. Of the field galaxies, 80% are spiral galaxies, 10% are elliptical galaxies, and less than 10% are lenticular galaxies.
In the highest-density regions of our universe, where galaxies form gravitationally-bound clusters, the dominant type of galaxy is the lenticular galaxy, composing up to 50% of the cluster, followed by the elliptical galaxy, composing up to 40% of the cluster. Spiral galaxies compose only 10% of the core of a dense galaxy cluster.
The appearance of a single simple progression in galaxy type is surprising, given that there are two initial conditions for these galaxies and given that some of these galaxies are modified by their neighbors. An even more surprising aspect is the sameness of the structure of galaxies of a particular class. One point that is clear is that mass is not the determining factor in creating galaxies of a particular type, because all classes of galaxy have a broad range of masses.
The young stars found in spiral and irregular galaxies are a consequence of abundant gas; gas clouds form and gravitationally collapse, giving birth to young stars. As a consequence, one sees in irregular galaxies many young stars, particularly bright, blue, but short-lived O and B stars; these young stars make irregular galaxies very blue. At the other extreme, the absence of gas in the elliptical galaxies precludes the birth of new star, so that there are no O and B stars; this absence of young stars gives elliptical galaxies a red color.
The class of a galaxy is given by a letter and number notation. An elliptical galaxy is noted as En, where n is a digit ranging from 0 to 7 that gives the ellipticity of the galaxy: b/a = 1 - n/10, where a is the semimajor axis and b is the semiminor axis of the ellipse. A galaxy that appears purely isotropic is therefore an E0 elliptical galaxy, while a galaxy with b = 0.3 a is an E7 elliptical galaxy. The lenticular galaxy has a single notation: S0. Spiral galaxies have four symbols: Sa, Sb, Sc, and Sd, with the notation progressing from Sa galaxies, which have tightly-wound spiral arms and a spheroid of stars that is relatively bright compared to the disk stars, to the Sd galaxies, which have loosely-wound and clumpy spiral arms and a relatively dim spheroidal stellar component. The final type, the irregulars, are in general noted by Irr, although those that resemble the Magellanic clouds are noted by Sm or Im. The progression of galaxies from gas poor to gas rich is E0→E7→S0→Sa→Sd→Irr.