The universe contains numerous galaxies. Even up to the early 20th century, we had almost no knowledge of the existence of external galaxies, but once we were able to measure distances through Cepheid, etc. it became known that many of what we had considered nebulae were actually external galaxies. External galaxies are very diverse in all aspects, including their shapes, sizes, and masses, etc. Many galaxies are gathered together in our universe.

According to their external appearance, galaxies are broadly categorized into spiral galaxies, elliptical galaxies, and irregular galaxies. Spiral galaxies are in the structure of a very thin disk with spiral arms. Elliptical galaxies appear in the shape of an ellipse and are far more gently sloping compared to spiral galaxies. ?Irregular galaxies refer to those that are irregular in shape unlike spiral galaxies or elliptical galaxies. Spiral galaxies are more specifically categorized according to the degree of looseness or tightness of the spiral arms, while elliptical galaxies are subdivided according to the degree of contortion. ?In general, elliptical galaxies are composed of stars of old age, the movements of the stars are relatively irregular, and it does not contain a lot of cold interstellar matter that are capable of forming new stars. By contrast, old and young stars coexist in spiral galaxies, indicating that the activity of star birth continues to occur today in these galaxies. Spiral galaxies contain a lot of gases capable of creating stars, and the stars move on a circular orbit on the disk. While elliptical galaxies have a very wide range of mass, spiral galaxies have a relatively narrow range of mass. The spiral arms of the spiral galaxies are formed by spiral density waves and rotate at a constant velocity unlike the stars.

Near our own galaxy, there exist numerous galaxies including very large galaxies such as the Andromeda galaxy and small ones such as the Magellan galaxy. We refer to these several tens of galaxies that exist near our own galaxy as our local group. Our galaxy and the Andromeda galaxy the largest galaxies in the local group, comprising the majority of the mass. In general, a group composed of several tens of galaxies is referred to as a galaxy group. The diameter of the local galaxy group is around 1 Mpc.

Groups consisting of hundreds to ten thousands of galaxies are referred to as galaxy clusters. In particular, clusters with a high number of clusters are called rich galaxy clusters. The Virgo galaxy cluster is the closest to our own galaxy, being around 178 Mpc away. Also, the Coma Berenices cluster is located around? 90 Mpc away, and this galaxy cluster is estimated to be composed of ten thousands of galaxies. The diameters of galaxy clusters are within the range of several Mpc. Galaxy clusters are organically interrelated and thereby form a far larger structure that we refer to as a super cluster of galaxies. The length of a super cluster of galaxies occupies around 10% of the size of the observable universe. In the central region of the majority of rich galaxy clusters, there is an enormous elliptical galaxy with great mass.

While the universe contains galaxy groups or super clusters composed of many galaxies, there are also voids where there are almost no galaxies. While galaxies are distributed like complex filaments, nearly all voids form a spherical shape. The existence of these galaxy groups or voids is understood to be the result of group formation, a process in which? galaxies are created with the concomitant gravitational effects.

Among galaxies, there are those that emit an enormous energy in a narrow region in their center. Examples of such active galaxies include Quasars or the Seyfert Galaxies. Quasars are radio galaxies that appear as small as a point source, and are located at a very far distance. The Seyfert Galaxies are those that appear to have the shape of a spiral galaxy but which emit an especially great amount of energy from its central region. An important characteristic of active galaxies are that the region from which they emit their energy is limited to the central region of the galaxies, and that they have many celestial bodies with light variations in very fast cycles. Such characteristics are best explained by the energy generated by black holes using gravitational fields. The mass of the black holes that exist in the center of active galaxies must be very heavy, from 10 million to 1 billion times the mass of the sun. Recently, it has become known that black holes exist not only in the center of active galaxies but in the center of the majority of other galaxies as well.

Galaxies are moving away from one another at a velocity proportional to their distance, and this is referred to as Hubble’s Law. Hubble’s Law is evidence that the universe is expanding. Using Hubble’s Law, we are able to estimate the distance to a very far galaxy. Hubble’s Law and Cosmic Background Radiation are important observed data that support the Big Bang theory of cosmology. The velocity of galaxies’ receding movements is observed by redshifts in the spectrum. A redshift indicates the degree to which a wavelength has lengthened, and in cases where the value is much smaller than 1, the receding velocity is the quantity obtained by multiplying the velocity of light to the redshift. However, in cases where galaxies recede at a velocity close to the velocity of light, the redshift can have a value much greater than 1. According to the Big Bang theory, a celestial body with a redshift of z exhibits a light that started off in a past when the size of the universe was 1/(1+z) times its current size. The distance to a far galaxy is expressed simply using the redshift. Among the celestial bodies that have been hitherto discovered, the largest redshift is around z~6, and these show us the state of the universe when its age was around a billion years old.

When the universe was first created, the distribution of mass was very consistent, but there was a very small degree of density fluctuation. It is thought that as time passed, regions with high density contracted to create galaxies or galaxy groups, and the regions with low density became voids. A large amount of dark matter is required to form a galaxy. Depending on the properties of the dark matter, the formation of the galaxy and its evolution varies greatly. According to the cold dark matter theory that is favored by many scholars today, after galaxies relatively small in size were created in the universe, these collide, combine and grow into a larger galaxy. However, no observed data has been able to verify the existence of this process. However, it appears indisputable that the collision and merger between galaxies is an important factor that determines evolution.