Some of the stars in the universe are part of multiple star systems known as star clusters. Most appear to be part of a binary system where two stars orbit a common center of gravity. A few are even part of a triple star system. But some stars are also part of a larger group. They can be found together in associations known as star clusters. Star clusters are groupings of stars held together by a common gravitational bond. They vary greatly in size and shape as well as the number of stars. They also vary in age from just thousands of years to billions of years old. Gravity is the force that binds these cosmic swarms together. Star clusters are among the most spectacular objects in the sky. Many of these clusters can be seen with the naked eye on a dark night. Astronomers have divided star clusters into two main types according to their shape and number of stars. The can all be classified as either open clusters or globular clusters.
Open clusters are also called galactic clusters. They usually contain somewhere between a dozen and a thousand stars. They are held together by mutual gravitational attraction and have a common center of mass. Open star clusters are composed of hot, relatively young stars and tend to be found within the spiral arms of our Milky Way galaxy. It is estimated that there are about 20,000 open star clusters in our galaxy. The reason open clusters are so young is because they don't last very long. Gravitational interactions between the stars and other objects will cause these clusters to eventually disperse over time. Open clusters are formed when several stars are formed at the same time from the same cloud of dust and gas. Our own Sun is part of an open cluster than includes other nearby stars such as Alpha Centauri and Barnard's star. All of these stars are believed to have formed from the same primordial nebula around 5 billion years ago. We know that the stars in open clusters are young because their spectrums indicate that an abundance of heavy elements. These elements are formed through many generations of star birth and death. Over billions of years, this stellar cycle produces star-forming clouds rich in heavy elements. Open clusters are formed from clouds like these.
Unlike open clusters, globular clusters are much older and usually contain between ten thousand to a million stars, which are gravitationally bound in a tight concentration. The stars are usually packed into a spherical arrangement with the highest density of stars occurs in the center of the cluster. Globular clusters are found just outside our galaxy. They orbit around the galaxy's central halo or bulge like a swarm of bees. Their concentration increases closet to the galaxy's center. They orbit the galaxy in highly elliptical orbits, which take them far outside the Milky Way. There are about 200 known globular clusters surrounding our galaxy. They have also been observed around many other galaxies as well. Spectroscopic studies show that the stars inside these clusters are much lower in heavy elements than stars such as the Sun. This means that they must be much older, because heavy elements are produced when stars end their lives in supernova explosions. Most globular clusters are believed to be between 14 and 16 million years old. They are believed to have formed from the same primordial matter that initially formed the galaxies.
Some star clusters, such as the Pleiades, have been known for some time. Others remained undiscovered until the invention of the telescope. Before that time, many of the globular clusters and smaller open clusters were visible only as fuzzy spots in the sky. Many were even mistaken for comets at one time or another. Charles Messier was one of the first astronomers to observe and catalog star clusters along with other deep sky objects. His catalog contains 51 of the brightest and largest star cluster in the night sky. Star cluster are among the easiest objects to observe in the night sky. Many open clusters, such as the Pleiades and the Hyades, are large enough to be seen with the naked eye. Binoculars will reveal a wealth of globular clusters when pointed toward the center of our galaxy near the constellation of Sagittarius. When seen through a telescope, globular clusters are arguably the most beautiful objects in the sky. Photographs do not do them justice. Atmospheric turbulence causes many of these photos to be lacking in sharpness. But when observed through a telescope from a dark location, they can take on the look of silvery glitter on black velvet. Observing star clusters can be one of the most rewarding experiences in the astronomy hobby.