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Characteristics of Stars Classifying Stars • • • • • Color Temperature Size Composition/Mass Brightness Size Comparison Size Classification Approximate Size (km) Examples Neutron Star 16 km Less than the distance to school pulsar White Dwarf Less than the distance across Asia Sirius B Van Maanen’s star Medium star 109 X size of Earth or 1,392,000 km Sun Giant star 100 X size of Sun Aldebaran Supergiant 1000 X size of Sun Rigel, Betelgeuse, Arcturus Color and Temperature Star Color Surface Temp (oC) Blue or Blue-White 35,000 oC White 10,000 oC Yellow 6,000 oC Red-Orange 5,000 oC Red 3,000 oC Chemical Composition • Most stars are made of – – – • 73% Hydrogen 25% Helium 2% Other elements Astronomers use spectrographs to determine elements found in stars • Spectrograph is a device that breaks light into colors and produces an image of the resulting spectrum Brightness of Stars • The brightness of a star depends upon both its size and temperature • How bright a star looks from Earth depends on both its distance from Earth and how bright the star truly is Brightness of Stars • Apparent magnitude- The brightness of a star as it appears on Earth • Absolute magnitude- The amount of light a star actually gives off (uses formulas) Brightness • In the magnitude scale, lower numbers are associated with brighter stars. • Star A has an apparent magnitude = 5.4 and star B has an apparent magnitude = 2.4. Which star is brighter? • We can't actually move stars around, but we can calculate how bright a star would be if placed at the agreed-upon fixed distance of ten parsecs (about 32.6 light years) • How is brightness measured? It is the amount of energy from an object in space which reaches a square centimeter of a detector each second Why do stars twinkle? • The scientific name for the twinkling of stars is stellar scintillation • Stars twinkle when we see them from the Earth's surface because we are viewing them through thick layers of turbulent (moving) air in the Earth's atmosphere. Measuring Distances to Stars • Astronomers use a unit called the light year to measure distances between the stars • Light travels at a speed of 300,000 km/s • Light year- distance that light travels in one year =10 trillion km • Light year=unit of distance Parallax • Astronomers use parallax to measure distances to nearby stars • Parallax= the apparent change in position of an object when you look at it from different places Hertzsprung-Russell Diagram • Shows the relationship between a star’s temperature and absolute brightness Hertzsprung-Russell Diagram