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Measuring Stars Visible Spectrum & Wavelengths Spectral Types • Continuous spectrum: Produced when white light passes through a prism • Emission spectra: Produced from a heated gas giving off light • Absorption spectra: Produced when white light passes through a cool gas Spectra & Star Composition • Can tell a star’s composition by looking at its spectra (usually an absorption spectra) • We can identify the spectra of various elements • Comparing a stars absorption spectra to the elements can indicate composition Spectra & Star Composition Spectral Lines of a Star Dopplar/Wavelength Shifts In space there is no up down left or right, only towards or away • Motion towards & away causes the light given off to be squeezed or stretched • Blue-shift: wavelengths from objects moving toward another get squeezed • Red-shift: wavelengths from objects moving away get stretched Wavelength Shifts Groups of Stars & Judging Distance in Space Constellations • Stars named after animals, mythical characters, & everyday objects 1. 88 constellations named by ancient people 2. Circumpolar stars: can be seen all year long depending on the hemisphere you live • Stars may not actually be close to each other in space Constellations Ursa Major (Big Dipper) Cygnus The Swan Star Clusters & Binaries Pleiades Cluster Arches Cluster Binary System • Clusters: Stars actually close to each other & held close by their gravity – Open clusters: stars NOT densely packed – Globular Cluster: stars ARE densely packed • Binaries: Stars that come in pairs orbiting a center of mass (over ½ of all stars) Judging Distances: Parallax • Parallax: apparent change in a star’s position compared to background stars • More apparent change = closer the star is Units of Distance • Astronomical Unit (AU): average distance from Sun to Earth (93 million miles) • Light Year (ly): distance light travels in one year (~9.5 trillion km, or 5.9 trillion mi) • Parsec (pc): it is equal to 3.26 ly Basic Properties of Stars • Magnitude: general brightness of a star – Ranked using + & - numbers – A +1 is 100x’s brighter than a +6 – Brightest stars have negative magnitude numbers • Apparent Magnitude: brightness as it appears from Earth • Absolute Magnitude: brightness of a star from 10 parsecs • Luminosity: amount of energy output from the surface (very similar to brightness) Magnitude Comparison