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Download Time From the Perspective of a Particle Physicist
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Test 1 Results • Course grade is based on number of points including review questions, in-class exercises, tests, extra credit 260+ A 225-259 B 180-224 C 150-179 D • Lowest test score (out of 4) will be dropped • Test 1 results average = 69 #exams approximate grade 78-100 A 14 66-77 B 12 51-65 C 10 41-50 D 8 0-40 F 0 PHYS 162 1 The Nature of Stars • Measure properties of Stars Distance Mass Absolute Brightness Surface Temperature Radius • Find that some are related Large Mass Large Brightness • Determine model of stellar formation and life cycle PHYS 162 2 Luminosity of Stars • Luminosity=Absolute Brightness=how much light/energy a star produces • Scale relative to Sun. So Lsirius = 23LS means Sirius radiates 23 times more energy than the Sun • Stars range from .0001xLS to 1,000,000xLS Another scale: “magnitude” often used. A log scale to the power of ~2.5. YOU DON’T NEED TO KNOW. The lower the Mag the brighter the object PHYS 162 3 Binary Stars Stellar Masses Stellar Sizes (sometimes) • Get size by length of time it takes to eclipse other star plus amount of light reduction related to relative sizes of stars • Measure orbital information period and separation distance. Get Mass though Kepler/Newtonian-like methods PHYS 162 4 Mass vs Luminosity always on these plots it is the Absolute Luminosity of the star High mass High brightness PHYS 162 5 HST image. “add” together images taken with different color filters surface temperature of star PHYS 162 6 HertzprungRussell Diagram Plot Luminosity versus surface temperature PHYS 162 7 HertzprungRussell Diagram size is along diagonal Stars with larger sizes are brighter then a smaller star with the same surface temperature PHYS 162 8 Temperature vs Luminosity vs Radius of Stars Energy emitted by surface of star due to EM radiation is Energy/area = sT4. Examples • Two stars. Same temperature and radius same Luminosity • Two stars. Same temperature. Radius(B) = 2xRadius(A). So surface area(B)= 4xsurface area(A) Luminosity(B)= 4xLum(A) Radius = 1 radius = 2 PHYS 162 9 Temperature vs Luminosity vs Radius of Stars Energy emitted by surface of star due to EM radiation is Energy/area = sT4. Examples • Two stars. Same radius. Temperature(B) = 2xTemp(A). (Energy/Area)B = 24(Energy/Area)A or (Energy/Area)B = 16x(Energy/Area)B Luminosity(B) = 16xLum(A) Temp = 6000 Temp = 12,000 PHYS 162 10 Hertzprung-Russell Diagram • Most stars are on a “line” called the MAIN SEQUENCE with hot surface temp large radius medium temp medium radius cool surface temp small radius • Stars with cool surface temperature but very large radius: RED GIANTS • Stars with hot surface temperature but very small radius: WHITE DWARVES PHYS 162 11 PHYS 162 12 Spectroscopic Parallax • If we use well-understood close stars to determine the overall brightness scale of a specific class of star, then measuring the spectrum can be used to give the distance for stars > 500 LY away 1. Determine Surface Temperature + spectral class of star 2. Determine where on HR diagram should go 3. Read off absolute luminosity from HR diagram 4. Measure apparent luminosity and calculate distance • works best if many close-by stars PHYS 162 13 Activity 1: Orion over time • Plot stars in Orion as of today • But stars are moving through Milky Way Galaxy • In 1 Million years, relative angular position will change, Orion will look differently • Second part of activity is plotting new positions in 1 My • Some stars may not be there in 1 million years (Rigel and Betelguese may supernova) PHYS 162 14