Difference Between Galaxy and Constellation
Galaxy, a large star system that contains millions or billions of stars. A total of approximately 500 million galaxies is estimated to be within reach of the largest reflecting telescopes, which can probe to a distance of at least 5 billion light-years. The faintly glimmering band of the Milky Way across the sky at night marks the central plane of our own galaxy, the Milky Way system. All of the individual stars that are visible to the naked eye or are within reach of binoculars also belong to our galaxy, which contains in all about 100 billion stars. Our sun, a star of average mass and brightness, is situated in the outskirts of the galaxy, close to the central galactic plane. The greatest diameter of our galaxy is about 100,000 light-years.
In the Northern Hemisphere, one of the galaxies beyond our own star system can be seen with the naked eye—the galaxy in the constellation Andromeda. It is often referred to as Messier 31 because it is the 31st entry in the catalog of nebulae prepared by the comet-hunting French astronomer Charles Messier in the late 18th century. Messier 31 is similar to our galaxy but is about twice as large in diameter. A good pair of binoculars or a small telescope reveals two smaller galaxies that accompany Messier 31. Observers in the Southern Hemisphere can view the two companions of our own galaxy—the Large Magellanic Cloud and the Small Magellanic Cloud. All of these nearby galaxies appear nebulous to the naked eye or through telescopes, but on close photographic inspection they can be resolved into their individual stars. Galaxies at distances more than twice that of Messier 31 are too far away for individual stars to be seen.
Long before the invention of the telescope, people studied the stars. One of the things they observed was that stars rose in the east, moved across the sky, and then set in the west. They also noticed that as the stars moved, their positions in relation to one another never seemed to change. Instead, groups of stars seemed to form patterns in the night sky that did not change. These groups of stars came to be called constellations, a name that comes from a Latin word meaning “clusters of stars.”
People in ancient times believed that gods and spirits lived in the sky, so they named the constellations after mythological gods and heroes, revered animals, and familiar objects that the patterns of stars resembled. Many constellations we know today were named thousands of years ago, and the naming of constellations is one of the oldest traditions still recognized in modern times.
Today, astronomers know that bright groups of stars are not gods or spirits. They know that stars appear to move from east to west because of how the Earth rotates on its axis. They also know that the stars in constellations are not connected and that their positions change over long periods of time. Nonetheless, astronomers today still locate many objects in the sky by using the constellations as a guide.
One group of constellations was especially important to people. What made these constellations special was that the sun, moon, and planets appeared to move through them. These twelve constellations became known as the zodiac, meaning “circle of signs” or “circle of living things.” Many of them were named for animals.
Because of the mysterious movements of the sun, moon, and planets within these constellations, people began to use them to predict the future. This practice is called astrology, which is considered to be a type of fortune-telling. Astrology has no scientific basis but its practice has continued for more than 2,000 years.