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Magnitude (astronomy), term used in astronomy to designate the brightness of a star . Magnitude, also called apparent magnitude, describes the brightness of a star as viewed from the earth . The term absolute magnitude refers to the brightness of a star as viewed from a standard distance of 10 parsecs, or about 32 light-years. The ancient Alexandrian astronomer Ptolemy originally divided all visible stars into six magnitudes: the brightest were called first magnitude, those barely visible to the naked eye were called sixth magnitude, and the other visible stars were assigned intermediate positions. After the introduction of the telescope in the 17th century, this system of magnitudes was used and extended to the fainter stars in different ways by different astronomers. In the 19th century a standard system was finally adopted under which a star of any given magnitude is 2.512 times as bright as a star of the next higher magnitude; thus, for example, a star of the second magnitude is 2...

Binary Star, two stars that are bound to each other by gravity and orbit about a common center of mass . Binary star systems are quite common and the pairing of stars appears to be random in most cases. Astronomers estimate that approximately one-fourth of the visible stars belong to a binary system. The time it takes for one star to orbit the other can range from hours to centuries depending on the distance between the two stars and their masses. Some binary pairs, called interacting binary systems, are so close that they exchange material. Binary stars are very useful to astronomers because they are the only stars of which astronomers can directly determine mass.

Orbit (astronomy and physics), path or trajectory of a body through space. A force of attraction or repulsion from a second body usually causes the path to be curved. A familiar type of orbit occurs when one body revolves around a second, strongly attracting body. In the solar system the force of gravity causes the moon to orbit about the earth and the planets to orbit about the sun, whereas in an atom electrical forces cause electrons to orbit about the nucleus. In astronomy, the orbits resulting from gravitational forces, which are discussed in this article, are the subject of the scientific field of celestial mechanics. An orbit has the shape of a conic section—a circle, ellipse, parabola, or hyperbola—with the central body at one focus of the curve. When a satellite traces out an orbit about the center of the earth, its most distant point is called the apogee and its closest point the perigee. The perigee or apogee height of the satellite above the earth's surface is often give...

Orbit ->> LAWS OF MOTION Early in the 17th century, the German astronomer and natural philosopher Johannes Kepler deduced three laws that first described the motions of the planets about the sun: (1) The orbit of a planet around the sun is an ellipse. (2) A straight line from the planet to the center of the sun sweeps out equal areas in equal time intervals as it goes around the orbit; the planet moves faster when closer to the sun and slower when distant. (3) The square of the period (in years) for one revolution about the sun equals the cube of the mean distance from the sun's center, measured in astronomical units . The physical causes of Kepler's three laws were later explained by the English mathematician and physicist Isaac Newton as consequences of Newton's laws of motion and of the inverse square law of gravity. Kepler's second law, in fact, expresses the conservation of angular momentum. Moreover, Kepler's third law, in generalized form, can be stat...

Astronomical Unit (AU), unit of distance used in the measurement of orbits and trajectories within the solar system . One AU is the average distance between the earth and the sun . Its value has been established as, roughly, 149,600,000 km (92,956,000 mi) by means of radar -ranging studies of nearby celestial objects such as Venus or passing asteroids; these studies have enabled astronomers to determine the scale of the solar system with great accuracy.

Nebula, in astronomy, a localized conglomerate of the gaseous and finely divided dust particles that are spread throughout interstellar space. Before the invention of the telescope, the term nebula (Latin, “cloud”) was applied to all celestial objects of a diffuse appearance. As a result, many objects now known to be star clusters or galaxies were called nebulas. Nebulas exist within other galaxies as well as in our own Milky Way galaxy. They are classified as planetary nebulas, supernova remnants, and diffuse nebulas, including reflecting, emission, and dark nebulas. Small, very bright nebulas known as Herbig-Haro objects are found in dense interstellar clouds, and are probably the products of gas jets expelled by new stars in the process of formation. Diffuse nebulas are extremely large structures, often many light-years wide, that have no definite outline and a tenuous, cloudlike appearance. They are either luminous or dark. The former shine as a result of the light of neighboring ...

Globular Cluster, compact, spherical group of stars , containing many thousands or even millions of stars. Astronomers have found more than 200 globular clusters in the Milky Way Galaxy (the earth’s galaxy). Most galaxies contain globular clusters and some galaxies contain thousands of such clusters. Most of the known globular clusters in the Milky Way move around the center of the galaxy in orbits that take them far outside the Milky Way. By finding the center of their orbits, the American astronomer Harlow Shapley of Harvard University located the Milky Way’s center in 1918. Globular clusters are the oldest structures associated with our galaxy. They contain only Population II stars—the oldest stars in the universe. All globular clusters in the Milky Way and the neighboring Andromeda Galaxy seem to be about the same age, suggesting they were created by conditions within galaxies while the galaxies were young. The diameters of globular clusters average about 50 light-years. The stars...