The giant spiral assemblage of several billion stars that is home to the Sun and its family of planets, including Earth,
is only one of the billions of star systems known to exist in the universe (see extragalactic systems). Because it is our star system, however, it is usually called the Galaxy. The Milky Way, another name for it, is the portion visible to the naked eye.In the 20th century astronomers determined that the Galaxy is a disk-shaped object, far larger than most of the
galaxies in its neighborhood (see Local Group of galaxies). Its visible disk is about 100,000 to 120,000 light-years wide but only about 2,000 light-years thick. A halo of other materials surrounds the disk and its spiral arms. Some astronomers suggest that the Galaxy is a barred spiral, with a long central bar from either end of which extends a large spiral arm.The total mass of the Galaxy can be measured by studying the
motions of individual stars and clouds of hydrogen gas in different parts of the Galaxy and by applying celestial mechanics to calculate a total mass that will account for the observed motions. The mass can also be determined from the motions of the Galaxy's small satellite galaxies, especially the nearby dwarf elliptical galaxies, and globular clusters. Computations by both methods agree that the Galaxy's mass is possibly 1,000 to 2,000 billion times the mass of the Sun. As the Sun's mass is about average for a star in the Galaxy, the total number of stars must also be of this order. Most of these stars are invisible from the Earth, however, because the solar system lies in the dense plane of the Galaxy, where interstellar dust obscures all but its nearer parts.The area around the Galaxy is populated by a small cluster called the Local Group, consisting of about 20 known galaxies. (More continue to be found.) Most of these neighbors, such as the _Magellanic clouds, are far smaller and less luminous than the Galaxy. The only other large galaxy is the Andromeda galaxy, which is more than 2 million light-years away. It is somewhat larger and more luminous than our own Galaxy and is visible to the naked eye. The Andromeda spiral differs slightly in shape from our Galaxy, having a larger, smooth, amorphous central bulge and spiral arms that are less patchy.The Sun lies from 28,000 to 30,000 light-years from the center of the Galaxy. From our vantage point, the Galaxy appears thicker toward its center, in the direction of the constellation Sagittarius, and somewhat thinner in other directions. Because of the obscuration by dust, however, which limits our view in all directions, it is difficult to realize from observation that we are not near the center of the system.