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MIT AST 100 - Lecture Notes

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Astronomy 100 Tuesday, Thursday 2:30 - 3:45 pm Tom Burbine [email protected] www.xanga.com/astronomy100Thing some people are having trouble withHW (due today)Homework Assignment (Due March 31)OWL assignmentClassification of StarsSlide 7Inverse Square LawPRS questionSlide 10Slide 11Luminosity-Distance FormulaSlide 13Measuring Distance to StarsSlide 15ParallaxSlide 17parsecRememberWhat formula do we useCalculationsSlide 22Slide 23NoteSlide 25Slide 26Surface TemperatureSlide 28Who were these people?Fleming’s classificationAnnie Jump Cannon (1863-1941)More informationOBAFGKMSlide 34Cecilia Payne-Gaposchkin (1900-1979)Slide 36Slide 37Slide 38Slide 39Slide 40Hertzsprung-Russell DiagramSlide 42Slide 43Slide 44Slide 45ClassificationsSlide 47RadiusSlide 49PRS QuestionSlide 51Main Sequence StarsSlide 53What does this tell usSlide 55Main Sequence LifetimesSlide 57AgesQuestionsAstronomy 100Tuesday, Thursday 2:30 - 3:45 pmTom [email protected]/astronomy100Thing some people are having trouble with•The Sun is believed to be ~5 billion years old (like 4.6 billion years)•This age is from radioactive dating of elements in meteorites•The Sun is believed to be able to burn Helium for another ~5 billion years•Its billions not millions!!!!!!!!!!!!!!HW (due today)•I want you to draw me a Hertzsprung-Russell Diagram•Label the axes•Label the regions with different types of stars•O, B A, F, G, K, M•Tell me the phrase people use to remember the orderHomework Assignment(Due March 31)•Make up a test question•Multiple Choice•A-E possible answers•1 point for handing it in•1 point for me using it on test•The question needs to be on material that will be on the 3rd exam•15 people got extra HW credit for me using their question (or inspiring a question)OWL assignment•There will be an OWL assignment due on Thursday March 31 at 11:59 pm.•There are 15 questions and a perfect score will give you 2 homework points.Classification of Stars•Stars are classified according to luminosity and surface temperature•Luminosity is the amount of power it radiates into space•Surface temperature is the temperature of the surfaceInverse Square Law•The apparent brightness varies inversely by the square of the distance (1/d2)•If the Earth was moved to 10 Astronomical Units away, the Sun would be 1/100 times dimmer•If the Earth was moved to 100 Astronomical Units away, the Sun would be 1/10000 times dimmerPRS question If the Earth was moved to 1 x 108 Astronomical Units away, the Sun would be …A) 1 x 10-12 times dimmerB) 1 x 10-14 times dimmerC) 1 x 10-16 times dimmerD) 1 x 10-18 times dimmerE) 1 x 10-20 times dimmerPRS question If the Earth was moved to 1 x 108 Astronomical Units away, the Sun would be …A) 1 x 10-12 times dimmerB) 1 x 10-14 times dimmerC) 1 x 10-16 times dimmerD) 1 x 10-18 times dimmerE) 1 x 10-20 times dimmerLuminosity-Distance Formula•Apparent brightness = Luminosity 4 x (distance)2Usually use units of Solar LuminosityLSun = 3.8 x 1026 WattsMeasuring Distance to Stars•Measuring distances to stars is much harder to measure than brightness•But to determine the Luminosity of the star, we need to know the distance to itParallax•Sine angle = length of opposite side length of hypotenuse•Sine p = 1 AU/(distance)•Distance = 1 AU/sin pparsec•Definition:•1 parsec is the distance to an object with a parallax angle of 1 arc second•1 pc = 1 AU/sin(1 arc second)•1 pc = 1 AU/4.84814 x 10-6 = 206,265 AU•Equals 3.26 light yearsRemember•1 degree = 60 arc minutes (symbol ´)•1 arc minute = 60 arc seconds (symbol ´´)What formula do we use•d (in parsecs) = 1 _ p (in arc seconds)•For small angles, the change in angle is approximately equal to the change in sine of the angleCalculations•For example, a star with a parallax angle of ½ arc seconds is 2 parsecs away•For example, a star with a parallax angle of 1/20 arc seconds is 20 parsecs awayPRS questiona star with a parallax angle of .01 arc seconds isA) 10 parsecs awayB) 100 parsecs awayC) 1,000 parsecs awayD) 10,000 parsecs awayE) 100,000 parsecs awayPRS questiona star with a parallax angle of .01 arc seconds isA) 10 parsecs awayB) 100 parsecs awayC) 1,000 parsecs awayD) 10,000 parsecs awayE) 100,000 parsecs awayNote•We can only measure parallax for stars within a few hundred light years from EarthLuminosity-Distance Formula•Luminosity =Apparent brightness x 4 x (distance)2Surface Temperature•Determine surface temperature by determining the wavelength where a star emits the maximum amount of radiation•Surface temperature does not vary according to distance so easier to measure1913Who were these people?•These were the women (called computers) who recorded, classified, and catalogued stellar spectra•Willamina Fleming (1857-1911) classified stellar spectra according to the strength of their hydrogen lines•Classified over 10,000 starsFleming’s classification•A - strongest hydrogen emission lines•B - slighter weaker emission lines•C, D, E, … L, M, N•O - weakest hydrogen lines emission linesAnnie Jump Cannon (1863-1941)•Cannon reordered the classification sequence by temperature and tossed out most of the classes•She devised OBAFGKMMore information•Each spectral type had 10 subclasses•e.g., A0, A1, A2, … A9 in the order from the hottest to the coolest•Cannon classified over 400,000 starsOBAFGKM•Oh Be A Fine Girl/Gal Kiss Me•Play songCecilia Payne-Gaposchkin (1900-1979)•Payne argued that the great variation in stellar absorption lines was due to differing amounts of ionization (due to differing temperatures), not different abundances of elementsCecilia Payne-Gaposchkin (1900-1979)•She proposed that most stars were made up of Hydrogen and Helium•Her 1925 PhD Harvard thesis on these topics was voted best Astronomy thesis of the 20th centuryIt takes progressively more energy to remove successive electrons from an atom. That is, it is much harder to ionize electrons of He II than He I.Hertzsprung-Russell Diagram•Both plotted spectral type (temperature) versus stellar luminosity•Saw trends in the plots•Did not plot randomlyRemember•Temperature on x-axis (vertical) does from higher to lower temperature•O – hottest•M - coldest•Most stars fall along the main sequence•Stars at the top above the main sequence are called Supergiants•Stars between the Supergiants and main


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MIT AST 100 - Lecture Notes

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