Ⅶ. Our Galaxy & Interstellar Matter

Our Galaxy & Interstellar Matter

A galaxy is the most basic of the celestial bodies that compose the universe, and our own galaxy is one among numerous galaxies that exist in the cosmos. In English, our own is written with a capital G as “Galaxy,” to distinguish it from a “galaxy” which refers to other ordinary external galaxies. The sun is located around 8.5 kpc away from the core of the Galaxy, and is rotating at a velocity of 220 km per second.

The Components of Our Galaxy

The major components of the Galaxy are stars, interstellar matter, and dark matter. Interstellar matter are categorized into gases an dust, and the dust occupies around 0.6% of the total mass of interstellar matter, exercising a very significant impact on our observations despite being very small in quantity. The identity of dark matter cannot be directly observed, but they are known to exist based on gravitational effects. It is estimated that around 90% of the total mass of our galaxy is composed of dark matter, indicating that the majority of the mass exists in the form of dark matter. In addition, there are magnetic fields in our galaxy, and though these are not matter, they do affect the dynamics of gases. Also, cosmic rays which move at an extremely high velocity are also an important factor that determines the balance of interstellar matter.

Interstellar Matter and Interstellar Clouds

Interstellar matter does not exist consistently dispersed through space and instead exist mostly in cloud formations that are referred to as interstellar clouds. There are many types of interstellar clouds, ranging from enormous molecule clouds with high density, mass, and size, to clouds with relatively small density and mass. The volume of interstellar clouds occupies only a few percentage of the total volume of our Galaxy, and the remaining space is filled with thin gases with low density but very high temperature. It is believed that stars are created within interstellar clouds with high density, and there are many clouds in which we can directly observe the actual activities of star creation.

Observation of Interstellar Clouds

The existence of gas is difficult to detect since gas absorbs almost no light in the range of visible rays. Hydrogen, which is the most common element in the universe, emits radio waves with a wave length of 21cm when in a neutral state. Therefore, with the development of radio telescopes, we came to know of the existence of interstellar gas. Interstellar clouds also contain many molecules such as hydrogen molecules, carbon monoxide, water vapor, etc.? The majority of these molecules also generate emission lines in the radio wave zone and therefore we observe them using radio telescopes. Based on the observation of emission lines, we are able to measure the motion, temperature, density, etc. of the interstellar clouds. However, because hydrogen, the most common molecule, does not give out any radio emission lines, it is difficult to observe directly. Recently, we have been able to use emission or absorption lines from hydrogen molecules that appear in the infrared zone.

Interstellar Dust

Interstellar dust comprises a very small portion of the total of interstellar matter, but they have the function of absorbing or dispersing stellar light. Interstellar dust is generally known to be composed of silicate compounds and carbon, and some heavy molecules that exist within the galaxy, such as calcium, aluminum, and silicon, are mostly captured within the dust and therefore almost none of these remain in the interstellar gas. The size of interstellar dust is approximately less than 0.1 microns, and since light with shorter wavelengths are more easily absorbed, light that has passed through clouds including interstellar dust changes in color to red. This phenomenon is called interstellar reddening. Due to interstellar reddening, the existence of dust has been known to us far earlier than the existence of gas. Dust is thought to be created by stellar wind coming from the surface of late type stars or by substances emitted at the time of a supernova explosion.

The Distribution of Stars and Interstellar Matter in the Galaxy

Our Galaxy is a spiral galaxy, and the majority of our stars and interstellar clouds are located in the very thin circular disk. Among the stars, early type stars that are very young in age have are very thin in their distribution along with interstellar matter. By contrast, very old stars form much thicker disks. Many of the stars are included in stellar clusters, which are groups of numerous stars. Stellar clusters that exist in our Galaxy can be broadly categorized into globular clusters, open clusters and stellar associations. Globular clusters are groups in the shape of spheres composed of hundreds of thousands of stars, aged 10 billion years or more. Globular clusters are not limited to the disk of the Galaxy, and are found distributed evenly across the entire spherical galaxy. In the early 20th century, the American astronomer Harlow Shapley observed that the distribution of globular clusters tended to be concentrated toward the Sagittarius in the celestial sphere, and thus first identified that the sun was far away from the core of the Galaxy. By contrast, open clusters are composed of a far fewer number of stars and their shapes are also relatively irregular, mostly being located on the disk of the Galaxy. The age of open clusters are far younger than the age of globular clusters, ranging from tens of millions of years to billions of years. Stellar associations are groups of stars comparatively small in scale ad composed of very young stars, and are also distributed on the Galaxy’s plane. Since the stars of stellar associations were created not long ago, they can usually be found in the regions where stars are being created. The core of the Galaxy has a nuclear bulge that is mostly composed of old stars.

The Core of Galaxies

Our Galaxy is very difficult to observe using visible light since there is much gas and dust in the direction of the nucleus. By measuring the distribution of gas using radio telescopes, we were able to learn the existence of molecular clouds surrounding the core of the Galaxy. Also, based on the movement of gas and stars very close to the core of the Galaxy, we came to know of the existence of a heavy black hole that is more than 2 million times the mass of the sun. This type of black hole in the center of a galaxy is commonly found not only in our own Galaxy but also in many external galaxies. The black hole with immense mass in the center of a galaxy is thought to be the cause of the powerful energy emitted from active galaxies or Quasars.

The Death and Birth of Stars

Stars have a limited life span. The stars that exist in our Galaxy are diverse in age. This indicates that the birth of stars has continued to occur. In actuality, among the interstellar clouds with large mass, there are many locations where stars continue to be created. By contrast, some stars are entering into the last phase of their evolution, having ended their life span. Stars with large mass experience a supernova explosion at the final stage of their evolution. In our Galaxy, there exist multiple supernova remnants, which are gases that have spread out after the explosion of a supernova. In this manner, a star is created by the contraction of an interstellar cloud but once it has completed its evolution, it returns the majority of its mass back into the interstellar cloud. Our Galaxy is thus a kind of ecosystem in which stars and gases circulate.


  1. Stars or gases that exist on the disk of our Galaxy generally are rotating around the center of the Galaxy. The rotation curve is a graph that shows the correlation between the distance from the center and the rotation velocity. What does the rotation curve for our own Galaxy look like, and what conclusions can be drawn based on this rotation curve?
  2. It is said that the degree of light absorption by interstellar dust is around 1 magnitude per 1kpc. Such interstellar absorption results in making the distance of a star seem farther than in actuality. If a certain star whose absolute magnitude we know based on spectral type is 1 kpc away in distance, how far away will the star appear to be when we observe it?
  3. What is the evidence that confirms the existence of a large quantity of dark matter in our Galaxy?
  4. How are we able to know that our sun is not located in the center of our Galaxy based on the distribution of globular clusters?
  5. How are we able to determine that the Sagittarius constellation is the center of the galaxy based on the observation of the Milky Way?
  6. How can we determine the age of our galaxy?
  7. Herschel concluded that the sun is in the center of our Galaxy based on observations of the distribution of stars in the sky. Explain the reasons why he reached this erroneous conclusion.
  8. What are some of the methods we can use to measure the mass of our Galaxy?
  9. Among the stars in our Galaxy, stars with small mass have a low content of heavy elements, while stars with large mass have a high content of heavy elements. Explain the reasons for this difference.
  10. What is the reason that a black hole is able to emit a great amount of energy?
  11. How can we confirm the existence of spiral arms in our Galaxy?
  12. The escape velocity for the earth is around 11km per second. How much time would it take to reach the center of the Galaxy riding a rocket that is traveling at the escape velocity?
  13. It is known that in our Galaxy, there is approximately one star per 1 pc3, and that the relative velocity between stars is around 30 km per second. Estimate how much time it would take for one star to collide with another star in our Galaxy.
  14. The radius of an interstellar cloud is known to be 10 pc, and its mass is approximately 10,000 times the mass of the sun. Estimate how long it would take for this interstellar cloud to contract. Assume that the cloud has almost no pressure.
  15. Discuss what kinds of observations must be made to find out whether interstellar gas and interstellar dust are mixed thoroughly together.
  16. List the differences between globular clusters and open clusters.
  17. The sun is approximately 4.6 billion years old. Estimate the number of times the sun has circled the core of the galaxy since the sun’s creation.
  18. What are some observations that can verify the existence of a black hole in the core of the galaxy?
  19. Draw an approximate diagram of our Galaxy.
  20. In general, younger stars tend to be located on the spiral arms, while older stars are not particularly concentrated in the spiral arms. What can we deduce from this fact?