The Milky Way Galaxy is the home galaxy of our solar system. It is a large spiral galaxy containing an aggregation of gas, dust, an estimated 400 billion stars, and thousands of globular clusters and nebulae. The Galaxy's mass is probably between 750 billion and one trillion solar masses, and its diameter is about 100,000 light-years light year is equal to 5,865,696 million miles (9,460,800 million kilometers)). All the objects composing our Galaxy orbit about their collective center of mass, called the galactic center. The Galaxy is bound together by the gravitational attraction between its parts, and its rotational motion prevents it from collapsing on itself.
The Milky Way is a term used to described how the stars of our galaxy appear to the naked eye from a position on the earth, away from the light pollution caused by modern cities. For thousands of years, people looked at the sky and saw a smear of light stretching across the sky that looked as if someone had spilled milk on the dome of the sky. In 1610 Galileo Galilei discovered (with a telescope) that the Milky Way was actually the integrated light of countless stars, individually too faint to see with the unaided eye. Nearly 200 years later, William Herschel used a more powerful telescope to count faint stars in different directions of the Galaxy. From this work Herschel believed that the stars forming our Galaxy were grouped in a flattened distribution, with the Sun near the center and the concentration of stars marking the directions of the Galaxy's greatest extent. One hundred years later, in 1917, Harlow Shapley discovered that globular star clusters are distributed over a roughly spherical volume that was surrounding the center of the Galaxy. It was also discovered that the Sun was not near the center of the galaxy, but on the fringes of one of the great galactic arms.
The Milky Way Galaxy has three major components: a nucleus, a thin disk containing spiral arms, and an extended halo. The first major component of the Galaxy is the relatively massive "nucleus." The center of the galactic nucleus is obscured by interstellar dust particles that absorb both the visible and the ultraviolet light radiated by its components. However, scientists have been able to record and study emissions from the region at radio, infrared, X-ray, and gamma-ray wavelengths. The strong emissions of infrared radiation and X-rays, in particular, seem to indicate the presence of rapidly moving clouds of ionized clouds. These gas clouds are thought to be circling a supermassive object, possibly a black hole. The nucleus is a flattened spheroid of dimension 3,500 by 20,000 light-years. It is composed of closely packed old stars (about 10 billion years old) and hundreds of globular star clusters.
The thin and flat "disk" of the Milky Way Galaxy, its second component, contains five spiral arms (Cynus, Centarus, Sagittarius, Orion, and Perseus) that consist of diffuse nebulae, interstellar matter, and young and intermediate age stars (estimated at ages between a million and ten billion years old). These spiral arms are regions of active star formation. The disk is about 100,000 light-years in diameter and on the average 10,000 light-years thick (increasing up to 30,000 light-years near the nucleus).
Our solar system is situated within the outer regions of the Galaxy, well within the disk and only about 20 light-years above the equatorial symmetry plane. The solar system is about three-fifths of the way from the center of the Galaxy (about 26,000 to 36,000 light year from the center), located near the inner edge of the Orion arm. In its nearly circular orbit, the solar system takes about 220 million years to revolve once around the Galaxy.
The third component of the Galaxy is a diffuse, spherical, dark "halo" of star clusters that encompasses the Milky Way. It extends out in all directions from the edge of the visible portion of the Galaxy about 130,000 light-years. The halo contains a low density of old stars mainly in globular clusters. The stars are mostly dark matter (non-luminous), with their existence inferred from the gravitational pull on the visible matter within the Galaxy. The particular constituents, shape, and extent of its materials are still not known.
The Milky Way Galaxy belongs to a group of galaxies known as the Local Group. The group contains three large and over 30 small galaxies. The Milky Way is the second largest galaxy after the Andromeda Galaxy (also called M31), followed by the Triangulum Galaxy. M31 is about 2.9 million light-years away, the closest large galaxy to the Milky Way Galaxy. A number of faint galaxies are much closer. In fact, many of the dwarf local group members are satellites or companions of the Milky Way. SagDEG is a small galaxy that is currently in a close encounter with the Milky Way, and thus our closest known intergalactic neighbor at a distance away of 80,000 light-years. In turn, the Local Group belongs to a much larger group of galaxies known as the Virgo Supercluster.
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