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Chapter 2: The Sky The daily motion of the sky is caused by the rotation of our planet. Stars Constellations- groups of stars Stars we know in the Western culture originated in Mesopotamia over 5000 years ago. Other constellations were added by the Babylonian, Egyptian, & Greek astronomers 48 constellations are still in use of all the ancient one named Stars Each culture grouped and named constellations differently. According to ancient astronomers, a constellation was a loose grouping of stars. – Many fainter stars were not included – Regions of the southern sky not visible to the astronomers in the northern latitudes were not identified with constellations – If any constellation boundaries existed, they were only approximate. Alpheratz was part of Pegasus or part of Andromeda What’s in a name: Arabic for “horse’s shoulder” Claim to fame: Brightest star in Andromeda. At one tome it was part of Pegasus. Type of star: White Main Sequence Star How far away: 72 light years away How bright: About as bright as the Sun Where to view: In the constellation Andromeda, the princess When to view: August through December Stars 1928, the International Astronomical Union established 88 official constellations with clearly defined boundaries. As a result, a constellation now represents an area of the sky and any star within that region belongs to only one constellation. Asterisms A named group of stars not identified as a constellation The Big Dipper, which is part of Ursa Major (the Great Bear) The Great Square, which is three stars from Pegasus and one from Andromeda Constellations & Asterisms Most are made of stars that are not physically associated with one another Some may be many time farther away than others and moving through space in different directions Only commonality among stars is they lie in approximately the same direction from Earth The Names of the Stars Names come from Greek (mythology) names translated into Latin, which is the language of science But most names come from ancient Arabic Sirius (the Scorched One), Capella (the Little She Goat), Aldebaran (the Follower of the Pleiades) Naming the Stars Johann Bayer, a lawyer by trade, published an atlas of the sky called Uranometria, in which he assigned Greek letter to the brighter stars of each constellation in approximate order of brightness Brightest star is alpha, next brightest is beta Greek letter followed by the possessive form of the constellation name This method only approximately identifies a stars brightness Greek Alphabet a b g d e z h Q Alpha Beta Gamma Delta Epsilon Zeta Eta Theta i k l m n x o p Lota Kappa Lambda Mu Nu Xi Omicron Pi r s t u f c Y W Rho Sigma Tau Upsilon Phi Chi Psi Omega Brightness of Stars Brightness is measure by using a magnitude scale First appeared in writing 140 AD by Ptolemy, but was probably used by Hipparchus (160127 BC) Stars are divided into six classes: brightest stars are first-class stars, fainter stars are second-class and so on until the sixth class of stars is reached Brightness of Stars As telescopes were developed, fainter stars could be seen so the magnitude scale was extended to include fainter stars. The Hubble Space Telescope is currently (copyright of book 2005) the most sensitive astronomical telescope and can detect stars fainter that +28 magnitude. The brightest stars are brighter than first class stars so the scale has been extended to include these stars also. Example: Vega (alpha Lyrae) is almost zero magnitude at 0.04, Sirius (the brightest star in the sky) has a magnitude of -1.42, the sun has a magnitude of -26.5 and the moon -12.5 Apparent Visual Magnitude The numbers associated with the star brightness is apparent visual magnitude (mv) Describes how stars look to human eyes Apparent visual magnitude ignores the effect of distance and tells us only how bright the star looks as seen from Earth Magnitude & Intensity Magnitude depends on the physiology of the eye and psychology of perception. Intensity is more accurate and precise Intensity is a measurement of the light energy from a star that hits one square meter in one second Magnitude & Intensity The human eye senses brightness by comparing the ratios of the objects intensities. Constant intensity ratio= 2.5 See table 2-1, page 14 IA/IB=(2.512) (mB-mA)