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29.2 Measuring the Stars Constellations • 88 constellations • Many civilizations looked at the brightest stars and named them after animals, mythological characters, or everyday objects Classifying Constellations • Constellations are classified as summer, fall, winter and spring constellations • People in years past used the constellations to know when to prepare for planting, harvest and ritual celebrations Greek Letters- Bayer Letters (Johann Bayer) • The Alphabet of Greek letters is generally assigned to stars in a constellation in order of brightness. • The alpha star is the brightest (alpha, beta, gamma, delta, epsilon) Sagittarius Leo Leo- Regulus as Alpha Star Star Clusters • When a group of stars are gravitationally bound to each other it is called a cluster Binary Stars • When two stars are gravitationally bound to each other, they orbit a common center of mass • Often appear bound to each other, even with a telescope Properties of a Star • Mass, diameter and luminosity (energy output of the star per second) are the basic properties of a star • Magnitude: how bright the star appears; +1 was the brightest star, +2 was the next brightest star, +3 was the third brightest star • Absolute magnitude: takes into account distance and measures how bright a star would appear if they all were at the same distance; the brightest star has a -40 value and the dimmest star has a value around +40 Temperature of a Star • Classified using letters: O, B, A, F, G, K, and M and further subdivided into numbers. Example: A4 or A5 • O stars are the brightest whereas M stars are the coolest • The sun is a G2 star with a surface temperature of 5800 K Composition of Stars • All stars are approximately 73% Hydrogen, approximately 25% Helium and the remaining 2% varies between stars; however, these numbers change as the star ages