The Milky Way galaxy

Milky Way Galaxy, named for the hazy band of light that stretches across the dark skies of the night, is a very large structure made up of more than 200 billion stars (including our own Sun and planets) and a huge amount of gas and dust all bound together by the force known as gravity. Although this system (usually referred to as a spiral galaxy, much like the Andromeda galaxy) has created much wonder and awe since the beginning of human civilization, it was not until 1610 that its true nature was discovered by Galileo when he turned his small refracting telescope toward the night sky to reveal that the Milky Way was composed of countless individual stars, nebula and other astronomical bodies, many of which remained mysterious and unknown well into the 20th century.

With the advent of large reflecting telescopes, such as the Mount Palomar reflector in the 1950's, it was determined that the distribution of stars in our galaxy was "shaped like a flattened disc similar to a double convex lens" and if viewed from above appeared "nearly circular with a concentration of stars in the center forming a spherical nucleus and other stars forming spiral arms." Thus, the entire structure has often been compared to a giant pinwheel (Pasachoff, 1990, 56). The diameter of this structure has been estimated to be approximately 100,000 light years across with its maximum thickness estimated to be between 5,000 and 15,000 light years. In other words, a spaceship traveling from one end of the Milky Way galaxy to the other would take about 100,000 years to complete, presuming that the spaceship was traveling at the speed of light (186,000 miles per second). Our Sun with its system of planets is located about 30,000 light years from the galactic center and fairly close to the galactic equatorial plane which runs through its center.

Looking from the Earth at the Milky Way, one obtains two distinct views -- first, in the direction of the poles, the density of the stars decreases dramatically, meaning that at the poles, the stars are far apart and are set against a very dark background. Second, looking along the equatorial plane, the closest stars are situated against a faint but luminous band, being "the blended light of billions of stars in the galaxy's disc. In 1785, astronomer William Herschel, by using a 48-inch reflector telescope of his own construction, counted the stars in more than 600 regions of the sky and concluded that "the majority are concentrated along a plane and that the galaxy is disc-shaped with the Sun at its center" (Parker, 1988, 156).

This was the generally-accepted view of the Milky Way until the 1920's when American astronomer Harlow Shapley at the Mount Wilson Observatory determined the true size and shape of our galaxy by studying Cepheid variables in globular clusters, "swarms of stars crowded together in spherical shapes which are scattered throughout the galaxy." Thus, Shapley determined that our Sun lies some 25,000 light years away from the galactic center (Parker, 1988, 157).

Overall, the shape of the Milky Way galaxy suggests that like all other spiral galaxies, it is rotating, due to the existence of the central bulge which would not exist otherwise. The axis of rotation is perpendicular to the equatorial plane and in this respect, it is similar to the Earth's rotation around its axis. However, the Milky Way galaxy does not rotate as a solid body, for the stars rotate around the center in much the same way as the planets move around the Sun in a counterclockwise direction. Stars that are closer to the center move at greater speeds, while those closer to the edge move much more slowly. In essence, the entire Milky Way galaxy is also moving and has been estimated to take some 3 billion years to make one complete revolution within the universe itself. In comparison, our Sun takes about 250 million years to complete one revolution around the center of the galaxy which is often referred to as a galactic year (Verschur, 1990, 178).

Although numerous studies have demonstrated that the Milky Way galaxy is spiral in shape, it is relatively impossible to view its entire structure with optical devices or visible light experiments, due to the Earth being located inside the plane of the galaxy and immersed in one of its spiraling arms. Also, the presence of interstellar dust greatly limits our ability to view the galactic disc in its entirety. Thus, our present knowledge on the true structure of the Milky Way galaxy has been greatly increased through the study of radio waves emitted by interstellar hydrogen atoms. These radio emissions penetrate the interstellar dust and can be detected by using ultra-sensitive radio telescopes which have shown that the Milky Way galaxy is indeed spiral in shape.

As to the mass of the Milky Way galaxy, the galactic disc itself holds between 100 and 200 billions stars, vast expanses of gas and dust, enormous interstellar clouds, high-energy cosmic rays, magnetic fields and radiation in all wavelengths from gamma rays to radio waves. Recent studies have confirmed that the bulk of the galactic mass lies within what is known as the galactic halo, "a huge shell of invisible or dark matter that surrounds the galaxy" (Pasachoff, 1990, 189). At present, it is estimated that the Milky Way galaxy contains material equivalent to more than 200 billion solar masses, meaning that it contains enough mass to create 200 billion suns (Pasachoff, 1990, 191).

The nucleus of the Milky Way galaxy which lies in the direction of the constellation of Sagittarius is composed of billions of stars set against dark, obscuring patches of dust. In some areas, this dust is so dense that it prevents any direct observations with optical instruments of the nucleus. However, by observing other nearby spiral galaxies, such as the Andromeda galaxy which lies some 20 million light years from the Earth, it is rather easy to determine what the Milky Way galaxy looks like from the outside. Observations made with other astronomical devices like radio, infrared, X-rays and gamma ray detectors have allowed astronomers to "pierce the dark veil and see into the heart of the Milky Way galaxy" (Parker, 1988, 198). Recent observations have revealed that one of the strongest radio sources known as Sagittarius a is very active. Also, studies of the galactic nucleus have shown that more than 1 million solar masses of hot gases are constantly being ejected from the galactic center which may indicate that the center of the Milky Way galaxy is very similar to a black hole (Verschur, 1990, 213).

Besides the existence of stars in the Milky Way galaxy, there are numerous other astronomical structures, such as star clusters and various nebula. There are actually two types of star clusters, being open or galactic clusters and globular clusters. These two types are very different in appearance and each represents different stages in the evolution of the stars that surround them.

Open clusters are generally loosely assembled and are composed of young stars that are only found along the galactic plane. Some are very inconspicuous and contain less than twenty stars that hardly stand out against the darkness of space; others are very rich and dense clusters that contain upwards of 1000 stars. Two very prominent open clusters are the Pleiades in the constellation of Taurus (which can be seen with the naked eye) and the Wild Duck cluster. Both of these structures range from about 15 to 40 light years across (Pasachoff, 1990, 241). In contrast, globular clusters are extremely old structures that are found primarily in the galactic halo that surrounds the galactic nucleus. As many as a million stars are condensed into these types of clusters, in fact so dense that individual stars cannot be seen with even the most powerful Earth-based telescopes. The diameters of these clusters range from about 75 light years to as much as 400 light years and can be as far as 300,000 light years from the Earth.