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LT 5: I can describe how astronomers determine the composition and temperature of stars. star a large celestial body that is composed of gas and that emits light. Nuclear fusion is the combination of light atomic nuclei to form heavier atomic nuclei Astronomers learn about stars by analyzing the light that the stars emit. Starlight passing through a spectrograph produces a display of colors and lines called a spectrum. Spectra Sun’s Spectra All stars have dark-line spectra Because different elements absorb different wavelengths of light, scientists can determine the elements that make up a star by studying its spectrum LT 6: I can explain why stars appear to move in the sky. Apparent Motion – The motion visible to the unaided eye; caused by the movement of Earth – Stars move counterclockwise around the North Star – Revolution around the Sun causes the stars to appear to shift slightly to the West each night. Actual Motion – Rotate on an axis – May revolve around another star – May move away from or toward our solar system Doppler Effect An observed change in the frequency of a wave when the source or observer is moving BLUE shift: stars moving toward Earth RED shift: stars moving away from Earth Distances to Stars Distances are measured in light-years (distance light travels in one year). Parallax: an apparent shift in the position of an object when viewed from different locations LT 7: I can describe one way astronomers determine the composition and temperature of stars. COMPOSITION – Stars are made up of the same elements that compose Earth – Most common element is HYDROGEN – Second most common element is HELIUM – Small quantities of carbon, oxygen, and nitrogen Temperature Most stars are from 2,800 ºC to 24,000 ºC Blue stars average 35,000 ºC Yellow stars average 5,500 ºC Red stars average 3,000 ºC LT 8: I can explain the difference between absolute and apparent magnitude. Apparent Magnitude Absolute Magnitude – The brightness of a star – The brightness that a as seen from the Earth – Depends on how much light star emits and how far it is from Earth star would have at a distance of 32.6 lightyears from Earth – The brighter the star, the lower the absolute magnitude Hertzspring-Russell Diagram