The Milky Way
The Milky Way is the home of our Solar System along with at least 200 billion other stars and planets. It contains thousands of clusters and nebulas. It is the home of nearly all the objects of Messier’s catalog that aren’t their own galaxies.
The mass of our giant galaxy is somewhere between 750 billion and one trillion solar masses. The diameter is estimated to be about 100,000 light years. The galaxy has three main components: a disk, in which the solar system resides, a central bulge at the core and an all encompassing halo. The disk of our galaxy exhibits it’s spiral structure and is part of the prominent nuclear region which is part of a notable bulge/halo component.
The Milky Way is categorized as a Hubble type Sb or Sc spiral. It is unclear if the galaxy has a bar in its spiral so that is why its classification is unsure. The image above is that of the spiral galaxy M83, which is thought to be similar in size and shape to our Milky Way, causing some to classify our galaxy as a Abbc type spiral.
The bright ban of light seen at night, is usually what is referred to as the Milky Way. The bright plane is actually the disk of our galaxy. The disk is composed of mostly Population 1 stars, which are blue and fairly young, which range between a million and ten billion years old. The disk of our galaxy has four spiral arms, which contain interstellar matter, diffuse nebulae, and young stars and open star clusters emerging from that matter. The spiral arms are also where the active star formations take place. The arms are approximately 300 pc thick and 30 kpc in diameter.
The red stars, or the older stars, in our galaxy are located in the bulge component, which is the center of our galaxy. These Population II stars are thought to be 10 billion years old. The bulge component also contains the globular star clusters. It is estimated that our galaxy has about 200 globulars, but we know about 150. These globular clusters are consolidated toward the Galactic Center. Harlow Shapley concluded that the center of the Milky Way lies at a cpnsiderable distance in the direction of Sagittarius. He came to his conclusions because of the distribution of the clusters.
The Milky Way Galaxy belongs to the Local Group, a smaller group of 3 large and over 30 small galaxies, and is the second largest (after the Andromeda Galaxy M31) but perhaps the most massive member of this group. M31, at about 2.9 million light years, is the nearest large galaxy, but a number of faint galaxies are much closer: Many of the dwarf Local Group members are satellites or companions of the Milky Way. The closest of all is SagDEG at about 80,000 light years from us and some 50,000 light years from the Galactic Center, followed by the more conspicuous Large and Small Magellanic Cloud at 179,000 and 210,000 light years, respectively.
The Milky Way is a gravitationally bound collection of roughly a hundred billion stars. Our Sun is one of these stars and is located roughly 24,000 light years (or 8000 parsecs) from the center of our the Milky Way. Our solar system is thus situated within the outer regions of our galaxy, well within the disk and only about 20 light years above the equatorial symmetry plane but about 28,000 light years from the Galactic Center. Therefore, the Milky Way shows up as luminous band spanning all around the sky along this symmetry plane, which is also called the “Galactic Equator”.
Our galaxy is truly gigantic. Shapley came up with a process to determine just how big our galaxy is. First, he showed that most globular clusters reside at great distances, many thousands of parsecs, from the Sun. Second, by measuring the direction and distance of each cluster, he was able to determine their three-dimensional distribution in space. It was this way, that he was able to map out a truly gigantic, and roughly spherical, volume of space, about 30 kpc across. Thus, proving that our galaxy is enormous. It is truly an amazing thing to see lit up at night in our sky.