Our galaxy is a spiral galaxy about 30,000 parsecs (about 560,000 million million miles or 900,000 million million kilometers) across, which consists of hundreds of billions of stars. Its disk appears as a faint white band that divides the night sky in two; this is the Milky Way. To each side of the band, the number of stars decreases rapidly.
The region in space occupied by the Milky Way is called the galactic plane. The center of nucleus of the galaxy, where the density of stars is at a maximum, lies in the direction of the constellation of Sagittarius. Minimum star density occurs in the opposite direction, toward Taurus.
The sun is off-center, about 10,000 parsecs from the nucleus, which is itself approximately 5,000 parsecs thick and 10,000 parsecs across. The galaxy has three spiral arms (each between 1,000 and 1,500 parsecs wide)—called the Orion, Perseus, and Sagittarius Arms—and the whole system is rotating in space. The group of stars containing the sun, in the Orion Arm, rotates once every 250 million years.
Distances within the galaxy are calculated with reference to certain objects, such as globular clusters. These clusters are symmetrical groups of stars that occur as a kind of halo around the region of the galactic center. From the earth, most can be seen on each side of the Milky Way in the part of the sky that lies toward the galactic center. The other principal type of stellar clusters—open clusters—are concentrated in the galactic plane and so, the most are seen from the earth within the Milky Way.
The galaxy may be classified in two regions according to the types of stars they contain. Population I areas have a great amount of interstellar material and “young,” bright, hot stars. Open clusters contain Population I stars. In Population II regions, on the other hand, most of the interstellar material has been used up in the formation of stars, which are therefore relatively “old” and are colder than Population I stars. Globular clusters contain Population II stars.
Evolution of the galaxy
Theories about how the galaxy evolved are based on two main observations: there are more heavy elements in Population I stars than in the Population II type; and Population II stars are moving faster through space than are Population I stars. It is thought that about 1010 (10 billion) years ago a great mass of light matter, mostly hydrogen, condensed out from the rest of intergalactic material. It then followed a similar evolutionary pattern to that of a star, which forms from interstellar material, and the young “protogalaxy” shrank.
About 6 X 109 years ago, Population II stars began to form, giving rise to the first globular clusters. At this stage of evolution, heavy elements were still not abundant. The galactic nucleus then formed in this contracting system. But the young galaxy still possessed much kinetic energy (energy of movement), which is why Population II stars tend to have higher velocities. Then star formation ceased as interstellar gas and dust was used up in the nucleus.
The rest of the disk-shaped part of the galaxy evolved from the further condensation of material outside the center of the protogalaxy. Population I stars began to form (and continued to form until relatively recently) from the abundant gas and dust. Previous generations of stars, at the ends of their life-cycles, created heavy elements in their dying stages, thus enriching the disk with the characteristic constituents of Population II stars.
The galaxy’s spiral arms are more difficult to account for. Magnetic fields may have dragged them out of the center and maintained their trailing arms. But speculation is hampered by the fact that, in any galaxy, the nucleus sometimes inexplicably releases vast amounts of energy. A complete understanding of this process could dramatically alter the various theories about the origin and evolution of our galaxy and others.
Our galaxy is a fairly typical, spiral-shaped galaxy, and is about 30,000 parsecs across and 5,000 parsecs thick (at its thickest point through the central nucleus). The top diagram shows an edge-on view of a galaxy similar to our own, showing the prominent nucleus and the flatter, more tenuous arms. The spiral shape of the arms is clearly visible in the middle diagram. (Although our galaxy has three spiral arms, their shape is very similar to that illustrated.) The bottom diagram is a schematic representation of our galaxy, showing the relative position of the sun (and, therefore, of the solar system). It lies about 10,000 parsecs away from the nucleus in one of the spiral arms (the Orion Arm). From the earth, most stars can be seen in the direction SB in the diagram, the least in the direction SA.
Observing the Milky Way
Ancient astronomers saw the Milky Way as a girdle across the heavens. Ptolemy, the famous ancient Greek astronomer, did not think it was a circle, but he did observe how, in places, it divided into two branches. Nobody explained its nature until the beginning of the seventeenth century when Galileo observed the Milky Way through a telescope and saw that it is made up of millions of individual stars. A century and a half later, Sir William Herschel measured the distribution of stars across the sky by means of star counts. He found that the numbers of stars decrease with increasing angular distance from the Milky Way.
The size and structure of the galaxy finally became apparent in the 1920’s, with the work of Edwin Hubble. Other spiral galaxies were observed as systems external to our own galaxy but recognized as of the same basic type. Then in the 1940’s, the new science of radioastronomy confirmed the spiral nature of the galaxy. The radiation from the cold clouds of hydrogen within the galactic plane revealed the nature of its spiral arms.
Despite these and more recent contributions to our knowledge of the galaxy, much of what we know about its macrostructure still comes from observations of other similar spiral galaxies. Because our own galaxy, although not especially large in cosmic terms, is too large for us conceivably to travel beyond and observe it from outside. Thus, although we know that the galaxy has three spiral arms, for example, we do not know exactly how these arms are positioned in space, nor their relationship to the galactic nucleus.