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Stars A Star is…. • A ball of matter that is pulled together by gravity, and that gives off energy as a result of NUCLEAR FUSION. – Nuclear fusion- when two atoms bond together to make one heavier atom. The process releases large amounts of energy. 1. Composition of Stars • The two most abundant elements in stars are HYDROGEN and HELIUM. – Scientists determine the composition by studying the LIGHT that comes from a star. How Light shows composition • When elements in a star burn, they give off certain colors of light. The color of light corresponds to a specific wavelength of the light. How Light Shows Composition • Scientists look at the light from a star through a SPECTROSCOPE. A spectroscope breaks light apart and shows the wavelengths of the light. Example: Color Spectrum of calcium: The bright lines on the spectrum tell you the wavelength of the light given off. How Light Shows Composition • Each element gives off a unique pattern, or color spectrum. If scientists find that pattern in a stars light, then they know that element is present in the star. – Example: When you burn sodium, it produces a yellow flame. – If a star’s spectrum has two lines in the same place, that star contains sodium. How Light Shows Composition • Astronomers match up the lines on the spectrum coming from a star with the dark lines different elements produce when they burn to determine the composition of the star. 2. Temperature • Scientists estimate the temperature of stars by the COLOR of the star. Hottest: Blue White Yellow Orange Coldest: Red 3. Magnitude • The Brightness of a star depends on three things: – Size – Temperature – Distance from us • Apparent Magnitude: How bright a star is when viewed from Earth. A very large, hot star could look dim just because it is so far away. (how bright we see it) • Absolute Magnitude: the amount of light that is actually given off by a star. (how bright it actually is) 4. Measuring the Distance to Stars • We measure the distance between objects in space using PARALLAX. – Parallax is the apparent change in position of a star in the sky when viewed from two different positions in earth’s revolution. – The closer a star is, the larger its parallax, or apparent movement. The farther away a star is, the smaller its parallax. Location 1 Star X Star A Star B Location 2 • When we observe Star X from Location 1, it appears to be beside Star B. • When we observe Star X from Location 2, it appears to be beside Star A. • Did Star X really move? 5. Measuring Distance in Space • Light-Year- The DISTANCE that light can travel in one year. 5,865,696,000,000 miles • Astronomical Unit (AU)- the distance between the sun and the earth (93 million miles) 6. Constellations • Constellations are patterns of stars in the sky. • Circumpolar Constellations: Constellations that appear to revolve around the North Star. • Major constellations: Big Dipper Orion 7. Important Stars • Polaris: The North Star: The star directly above the earth’s axis of rotation, or the north pole. It appears to stay in the same place all year, and other constellations revolve around it. • Alpha Centauri: Closest star to us (other than the sun)