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Stars Luminous gaseous celestial body – spherical in shape held by its own gravity How do we study stars? • Light!! Stellar Radiation • H fusion occurs in star’s interior converting mass to E (mass deficit). • T must be ~ 107 K, for nuclei to overcome Coulomb force & fuse. • Interior of the star is so hot it is plasma. BE of He higher than BE 4H. He - 4. H isotopes. Excess E is carried away by g photons & neutrinos n. Some E gets absorbed in star heats interior more & exerts outward pressure. Stellar Equilibrium- outward P from radiation balances gravity inward in stable stars. Stable Stars maintain size. The sun is stable Ex 1. The sun is losing mass at 4.26 x 109 kg/s. At what rate does the sun emit energy? • • • • Assuming the mass is converted to E. E = mc2. (4.26 x 109 kg/s)(3 x 108m/s)2. 3.83 x 10 26 J each second. Star Power Luminosity (L) = total power output of a star W or J/s. As we just calculated the sun converts mass to Energy Sun L = 3.9 x 1026 W. Luminosity (W) depends on: - Surface Area - Temperature - Which equation relates power to A & T? Stars are regarded as black bodies - L = sAT4. - L = s4pr2T4. L – Watts J/s A surface A m2 T Kelvin s = 5.67 x 10-8W/ m2 K4. Apparent Brightness (b): how bright stars appear. What we see from Earth depends on L & distance from Earth Def. Apparent brightness • radiation from star that is incident on the Earth per m2. Calculation of Apparent Brightness (b): L b 2 4pd L = luminosity in W d = distance to Earth m b = apparent brightness W/m2. Intensity Ex 2: The apparent brightness of a star is 6.4 x 108 W/m2. If its distance to Earth is 50 LY, find its luminosity. L b 2 4pd • d = (9.46 x 10 15 m/LY)(50 LY) = 4.73 x 1017 m • b4pd2 = L • (6.4 x 108 W/m2) (4p)(4.73 x 1017 m)2. • 1.8 x 10 45 W Finding Star Temperature Remember Black Bodies? Wein’s Displacement Law relates peak l & surface temp for black body. 3 2.9 x10 l mK T T in Kelvin l in meters Star’s spectra similar to black body. as T inc. • Tot intensity increase for all l. • Peak changes to shorter l higher f. Ex 3: A star has a surface temp of 17 000 K and L = 6.1 x 10 29 W. a. What is the peak l? b. Find its radius. 3 2.9 x10 7 l mK 1.7 x10 m T Use Stephen Boltzmann to find R. L AsT 4pr sT 4 2 4 L 9 r 3 . 2 x 10 m 4 s 4pT Solar Spectrum • Some radiation l absorbed by outer layers. • Can identify elements in outer layers. • If H is present, H will absorb l = to dif between Bohr orbit levels. Form black lines. Motion & Speed of Stars • Doppler Effect/Red or Blue shift gives info. • Absorption lines shift toward longer or shorter l, depending on motion. Red Shift Spectrum – stars moving away from us show dark line shift. • Find v, direction by shift of line spectra. Blue Shift – moving toward us Amount of Shift relates to speed of motion List 3 observations we can make using light to get information about stars. State what we can learn from each type of observation. Use Spectrum to find: • Chemical composition surface • (absorption spectrum) • Motion toward or away from Earth • Red/blue shift • Surface temp • Peak l (color) Ex 4: Our sun has T = 6000 K and L = 3.9 x 1026 W. If star Z has T = 4000 K, &L = 5.2 x 10 28 W would expect: It to be larger or smaller to our sun? Calculate its radius in terms of our sun’s radius. • Larger • 26 x Rsun. Early Star Classification • Spectral Class • Color Temperature Composition. Sun Stellar spectra • http://www.youtube.com/watch?v=jjmjEDY qbCk • From 4:48 Star Types Types of Stars • Single – not bound to another. Sun. • Binary – 2 stars appear close. Most bound together by grav. • Cepheid – varies in brightness on regular cycle of days – changing size. • Red Giant – Old star. H burning is over. Low surface T. High L, lg area. • Supergiant – very heavy star fuses elements beyond carbon. • White dwarf – solar mass but planetary size no more fusion. Binary Stars – • Optical binary – appear together but not physically near each other. Visual binaries orbit together around center of mass. Can be distinguished visually. Mass can be determined from period of revolution & separation. Eclipsing Binary – Cannot see separate stars but 1 passes in front of the other so observed brightness varies with regular period. Animation of eclipsing binary • http://www.youtube.com/watch?v=zoekfYo mfjI Why is there a larger dip in intensity for 1 position? Brighter/hotter star blocked bigger dip in light curve. Spectroscopic Binary too close to distinguish eclipse but can see doppler shift Red & Blue Shifted w/motion Binary Star Types 4 min. • http://www.youtube.com/watch?v=1kFFwH kxBiI Star Classification Spectral Classes. • Stars characterized by temperature, absorption lines & color. OBAFGKM • Oh be a fine girl – kiss me. • Then subdivided in 10 smaller groups 0-9. • Sun – G2. H-R diagram graphs temp against luminosity – Not Linear • Be able to identify general regions of star types on the H-R diagram • 90% Stars on Main sequence. MS High Mass H-R Diagram Cool, Super-Large Fast Burners Cool, Large Small, Hot MS Low Mass Long Lives http://www.youtube.com/watch?v=yX0HWr9xQ6M HR Diagram start at 1:24 Black body radiation 12 min • https://www.youtube.com/watch?v=TiOpU AI_9mk&autoplay=1&app=desktop