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30.3 Star Groups Constellations • Constellations: patterns of stars and the region of space around them. • Standard set of 88 constellations set by group of astronomers in 1930. – Acts as a road map to locate particular stars. • Orion • Big Dipper • Pleiades- “seven sisters” 7 bright stars Naming Constellations • Modern names come from Latin. • Names come from… – Real animals e.g. Ursa Major, “The Great Bear” (The Big Dipper) – Imaginary animals e.g.Draco, “The Dragon” – Ancient Gods – Legendary Heroes Multiple-Star Systems • Binary Stars : pairs of stars that revolve around each other and are held together by gravity. • Barycenter: center of mass. i.e. the point at which the stars revolve around. • Estimated that more than half of all observed stars are multiple-star systems. Star Clusters • Clusters: groups of hundreds or thousands of stars • Globular Clusters: Spherical shape and can contain up to 100,000 stars. • Open Clusters: loosely shaped and rarely contains more than a few hundred stars. Galaxies • Galaxy: a large-scale group of stars, gas, and dust that is bound together by gravity. • Major building blocks of universe • Typical galaxy is 100,000 l.y. in diameter and contains 200 billion stars • Estimated that universe contains hundreds of billion of galaxies Types of Galaxies • Spiral Galaxies: has a nucleus of bright stars and flattened arms that spiral around the nucleus • Elliptical Galaxies: vary in shape from nearly spherical to very elongated. Extremely bright in the center and have no spiral arms. • Irregular Galaxies: has no particular shapes. Have low total masses and are fairly rich in dust and gas. More rare than other galaxies. • Elliptical Galaxies-Virgo Clusters • Irregular Galaxies The Milky Way • Spiral galaxy in which the sun is one of hundreds of billions of stars. • All stars orbit around the center of the Milky Way Galaxy. • Takes sun 225 million years to complete one orbit. • Closest galaxies are 170,000 l.y. away from Earth. • Milky Way Galaxy Quasars • Discovered in 1963 • Quasar: quasi-stellar radio source; a very luminous object that produces energy at a high rate. • Not related to star but related to galaxies. • Located in the center of galaxies and are VERY bright • Believed that black holes are present in those galaxies • The arrow in this image points out the record-breaking redshift 5.0 quasar discovered by the Sloan Digital Sky Survey. That faint red dot of light represents an object that is actually a hundred times as luminous as a typical galaxy. Sky Survey astronomers identified this object as a possible high-redshift quasar on the basis of its exceptionally red color compared to ordinary stars and galaxies. Followup spectroscopy with the ARC 3.5-meter telescope confirmed that this unassuming speck was indeed the most distant quasar known to date.