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Measuring Starlight Objectives • Describe how astronomers determine the compositions and temperature of stars. • Explain why stars appear to move in the sky. • Describe one way astronomers measure the distances to stars. • Explain the difference between absolute magnitude and apparent magnitude. Stars • A Star is- a large gaseous body which generates energy through nuclear fusion in its core • Stars are made up of different elements, primarily Hydrogen and Helium • Stars are essentially all made of the same elements!!! • Nuclear fusion: the combination of 2 or more atomic nuclei into a single, heavier atomic nuclei • The Sun fuses 2 Hydrogen nuclei together to form Helium • See Periodic Table behind you The Amazing Power of Starlight Light from Stars- Also Known As “Spectra” allows astronomers to determine the star’s: a. Chemical Composition b. Temperature c. Luminosity…a fancy word for Brightness d. Velocity e. Rotation rate f. Radius (Size) Analyzing Starlight • Because each element absorbs different wavelengths of light, we can determine the elements which make up a star by studying its spectrum. • A spectrum is a display of colors and lines produced when light passes through a prism. • Starlight passing through a spectrograph produces such a spectrum -------> • ***The spectral difference is due to the difference in temperature of the star • The dark lines in the spectrum reveal the star’s composition and temperature. • Every element has a different spectra • Here are the spectra for some common elements: • Notice how each element has unique dark bands? These are called absorption bands. Too Much Information?? • What you need to know: • We can determine the chemical composition and temperature of a star by analyzing its spectra. • We divide the stars up into seven major spectral or temperature classes O, B, A , F, G , K, M (Oh Be A Fine Girl (Guy) Kiss Me O – Hottest Stars M – Coolest Stars The Spectral Sequence Spectral Class O Temperature Color Spectral Lines Example 30,000 to 50,000 K BlueViolet Ionized Helium Minataka B 11,000 to 30,000 K 7,500 to 11,000 K 5,900 to 7,500 K 5,200 to 5,900 K BlueWhite White Neutral HELIUM, Hydrogen Hydrogen (Strong) Rigel, Spica YellowWhite Yellow Ionized Metals Procyon Ionized CALCIUM, Ionized and Neutral Metals The Sun, Capella Orange Neutral Metals RedOrange Neutral Metals, Molecular Bands Arcturus, Aldebaran Betelgeuse, Antares A F G K M 3,900 to 5,200 K 2,500 to 3,900 K Sirius, Vega Color and Temperature Stars appear in different colors, from blue (like Rigel) Orion Betelgeuse to red (like Betelgeuse). These colors tell us about the star’s temperature. Rigel The Temperatures of Stars • The spectral difference is due to variations in temperature of different stars. • The various temperatures also lead to a difference in colors that we see: • - Hotter stars appear Blue • - Cooler Stars appear Red • Blue stars have average surface temperatures of 35,000˚C. • Red stars have average surface temperatures of 3,000˚C. • Yellow stars, such as the sun, have surface temperatures of about 5,500˚C. Stellar Brightness • Two factors determine the luminosity (brightness) of a star: – Temperature – Size • Apparent magnitude: brightness of a star as seen from the Earth. • Absolute magnitude: brightness a star would have at a distance of 32.6 light-years from Earth Stellar Brightness • The brighter a star is, the lower the number of its absolute magnitude, as you can see from the diagram below. Compositions of Stars • Scientists have learned that all stars are made up of the same elements which compose Earth. • The most common elements in stars are hydrogen and helium, in that order. • Small quantities of carbon, oxygen, and nitrogen are also found in stars, but stars are primarily composed of…. • HYDROGEN and HELIUM The Sizes and Masses of Stars • Stars vary in size and mass, and a star’s brightness is related to its size • Big stars appear brighter than smaller stars of the same temperature • Our sun is considered a medium-sized star • Most stars visible from Earth are medium-sized stars. • Many stars also have about the same mass as the sun, however some stars may be more or less massive. Stellar Motion • Apparent Motion of Stars: – motion visible to the unaided eye. Apparent motion is caused by the movement of Earth. • The rotation of Earth causes the apparent motion of stars seem as though the stars are moving counter-clockwise around the North Star. Stellar Motion • Actual Motion of Stars: • Stars rotate on an axis. • Some stars may revolve around another star. • Stars either move away from or toward our solar system • Remember Red Shift and Blue Shift Doppler Effect • Doppler effect: an observed change in the frequency of a wave • Red and Blue Shifting occurs due to the Doppler Effect…in other words: • The Doppler Effect causes a change in the frequency of the light wave, which is why we observe a red shift for those stars moving away from Earth and blue shift for those moving toward Earth Stellar Motion Question Why does Polaris (the North Star) appear to remain stationary in the night sky? Polaris is almost exactly above the pole of Earth’s rotational axis, so it moves only slightly around the pole during one rotation of Earth. Relative Motion • Circumpolar Stars: stars which are always visible in the night sky. These stars never pass below the horizon. • In the Northern Hemisphere, the movement of these stars makes them appear to circle the North Star.