Ⅷ. External Galaxies

External Galaxies

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.

Types of Galaxies and their Categorization

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.

Our Local Galaxy Group and the Distribution of Galaxies in the Vicinity of Our Galaxy

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.

Galaxy Clusters

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.

Active Galaxies

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.

Hubble’s Law

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.

The Birth and Evolution of Galaxies

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.


  1. Explain how Hubble categorized the galaxies.
  2. What is the evidence that is used to support the claim that galaxy clusters contain a large amount of dark matter?
  3. While categorizing the galaxies, Hubble came to believe that spiral galaxies and elliptical galaxies were phenomena that occur in the process of evolution.Today, no one accepts the truth of this claim. What are the reasons that this claim was refuted?
  4. Discuss the reasons why elliptical galaxies contain such a low level interstellar matter.
  5. Up to the present, we had only a very inaccurate estimate of the Hubble constant. What is the reason that it is so challenging to determine the Hubble constant?
  6. It is thought that that galaxies experience far fewer collisions compared to stars. What is the reason for this difference?
  7. Use Hubble’s Law to estimate the distance to the farthest galaxy.
  8. Among the Cepheid variables, the brightest is said to have an absolute magnitude of approximately -15. A Hubble Space Telescope is able to observe dim astronomical objects only when the magnitude is at minimum approximately 25. What is the distance to the farthest galaxy in which we can observe Cepheid variables?
  9. Discuss why a black hole is able to emit a great quantity of energy in such a narrow area.
  10. Why is it necessary for a celestial body with a high light variation velocity to be small in size? Estimate the size of a celestial body with a light variable cycle of P.
  11. Cygnus A, which is a representative example of a radio galaxy, has a jet that extends around 50 seconds in the sky. If we assume that the distance of this galaxy is 225 Mpc, what would be the actual size of the jet?
  12. Quasars usually show redshifts near 2, and when their redshifts become larger or smaller, this number decreases. What conclusions can we draw based on this fact?
  13. What is signified by the fact that an enormous elliptical galaxy? with very large mass is located in the central region of a rich galaxy cluster?
  14. In contrast to Hubble’s law, the Andromeda galaxy and our own galaxy are moving toward one another. What is the reason for this movement?
  15. Explain the differences between the appearances of a spiral galaxy when photographed using a red filter compared to when photographed using a blue filter.
  16. Among elliptical galaxies, the one that appears the thinnest has a ratio of 3 between its long axis and its short axis. What does this fact indicate to us about the actual appearance of the elliptical galaxy?
  17. What is the method of measuring the mass of a black hole in the central region of a galaxy?
  18. What is the evidence that supports the claim that Quasars are phenomena that occur in far distant galaxies? ?
  19. Our own galaxy contains a black hole in its center. What would be the reason that this does not function as not an active galaxy core?
  20. The size of a globular cluster is generally around 20pc. In a certain galaxy, we have discovered numerous globular clusters, each with a size of 2 seconds. What is the distance to this galaxy?