Elliptical galaxies are smooth and elliptical in appearance. There are four distinguishing characteristics of the ellipticals: (a) they have much more random star motion than orderly rotational motion (star orbits are aligned in a wide range of angles and have a wide range of eccentricities); (b) they have very little dust and gas left between the stars; (c) this means that they have no new star formation occurring now and no hot, bright, massive stars in them (those stars are too short-lived); and (d) they have no spiral structure. They are dead galaxies. If spiral galaxies are like rain forests, with cool life-giving interstellar clouds, elliptical galaxies are like deserts, with hot dry winds and little life. Elliptical galaxies are sub-classified according to how flat they are. The number next to the ``E'' in the tuning fork diagram = 10×(largest diameter - smallest diameter) / (largest diameter), so an E7 galaxy is flatter than an E0 galaxy. The flattened shape is not due to rotational flattening but to how the orbits are oriented and the distribution of the star velocities. Most ellipticals are small and faint. The dwarf ellipticals may be the most common type of galaxy in the universe (or maybe the dwarf irregulars are). Examples of elliptical galaxies are M32 (an E2 dwarf elliptical next to the Andromeda Galaxy) and M87 (a huge elliptical in the center of the Virgo cluster). 

Spiral galaxies have flattened disks with a spiral pattern in the disk. The spiral arms can go all of the way into the bulge or be attached to the ends of a long bar of gas and dust that bisects the bulge. The four distinguishing characteristics of the spirals are: (a) they have more orderly, rotational motion than random motion (the rotation refers to the disk as a whole and means that the star orbits are closely confined to a narrow range of angles and are fairly circular); (b) they have some or a lot of gas and dust between the stars; (c) this means they can have new star formation occurring in the disk, particularly in the spiral arms; and (d) they have a spiral structure. Spiral galaxies are sub-classified into ``a'', ``b'', ``c'', and ``d'' groups according to how loose their spiral arms are and how big the nucleus is. The ``a'' group spirals have large bulges and very tightly wound spiral arms and the ``d'' group spirals have almost no bulge and very loose arms. The Milky Way is between the ``b'' and ``c'' groups with a possible bar, so it is a Sbc or SBbc-type spiral galaxy. Most spirals are luminous. Some other examples of spiral galaxies are M31 (the Andromeda Galaxy) and M33 (a small spiral in the Local Group).

Some disk galaxies have no spiral arms and are called ``S0'' (``SB0'' if there is a bar) or lenticular galaxies. They are placed at the point in the tuning fork diagram where it branches off to the regular spiral or barred spiral pattern prong. Their gas and dust may been blown away by the galaxy moving quickly through the low-density intergalactic medium (hot, very thin gas between the galaxies) or used up in a rapid burst of star formation. 

Irregular galaxies have no definite structure. The stars are bunched up but the patches are randomly distributed throughout the galaxy. Some irregulars have a lot of dust and gas so star formation is possible. Some are undergoing a burst of star formation now, so many H II regions are seen in them. Others have very little star formation going on in them (even some of those with a lot of gas and dust still in them). Most irregulars are small and faint. The dwarf irregulars may be the most common type of galaxy in the universe (or maybe the dwarf ellipticals are). The estimates of the number of dwarf irregulars and dwarf ellipticals are based on the proportions of these types of galaxies in nearby groups. The dwarf galaxies far away are too faint to be seen and are, therefore, overlooked in surveys of the sky. Perhaps if the dwarf galaxies were brighter, Hubble would have arranged the galaxies in a different sequence instead of the two-pronged sequence. Examples of irregular galaxies are the Large and Small Magellanic Clouds (two small irregulars that orbit the Milky Way).

The clustering phenomenon does not stop with galaxies. Galaxy clusters attract each other to produce super clusters of tens to hundreds of clusters. Their mutual gravity binds them together into long filaments 300 to 900 million light years long, 150 to 300 million light years wide, and 15 to 30 million light years thick on average. Between the filamentary super clusters are HUGE voids with very few (if any) galaxies. The voids are typically 150 million light years across.