1Exam #1 on Part 1 next monday (March 3rd)HW #2 due Wednesday (Feb 27th)Quiz #2 todayExam #1 is in class next monday25 multiple-choice questions50 minutesSimilar to questions asked in classWe will have a 1-hour review session, time and dateTBD2Knowledge of interior based on models which fit observables:•Mass•Radius•Luminosity•Surface Temperature•Image details: granules, spicules, corona, chromosphere3Structure of the sun•Outermost region is the corona•Followed by the chromosphere•Then the photosphere•Directly beneath the photosphereis the convective zone•The below the this is theradiative zone and then the core.Solar Flares• Solar flares are hugeeruptions of hot gasand radiation in thephotosphere• Can damage satellites,spacecraft, andhumans in space• The study of coronalmass ejections andsolar flares is called“space weather”4A Coronal Mass Ejection5The Aurora• When CME materialreaches the Earth, itinteracts with theEarth’s magnetic fieldand collides withionospheric particles• The collision excitesionospheric oxygen,which causes it toemit a photon• We see these emittedphotons as the aurora,or Northern Lights6The Solar Cycle• The number of sunspots seenincreases and decreasesperiodically.• Every 11 years or so, thesunspot number peaks. This iscalled Solar Maximum• Around 5.5 years after SolarMaximum, the sunspot numberis at its lowest level. This iscalled Solar Minimum• Solar activity (CMEs, flares,etc.) peaks with the sunspotnumberDifferential Rotation• Different parts of the sun rotate at different speeds– Equator rotates faster than the poles– Solar magnetic fields get twisted as time goes on7Which of the following do NOT follow an 11 yearcycle? a) The number of sunspots on the Sun. b) The typical latitude of sunspots on the Sun. c) The rate of solar flares. d) Incidence of strong aurora on the Earth. e) None of the above.TriangulationWe can use triangulation to calculate how far away objects are!8Finding the distance to the Moon byTriangulation• The Moon is a relatively closeobject, and measuring thenecessary angles is not toodifficult.• Other astronomical objects ofinterest are much farther away,and measuring the necessaryangles in degrees is impractical• Degrees have been sub-dividedinto arc-minutes and arc-seconds– 1 degree = 60 arc-minutes– 1 arc-minute = 60 arc secondsA more modern way of finding the distanceto the Moon• Apollo astronauts left facetedmirrors behind when theyreturned to Earth• Scientists can bounce laser beamsoff these mirrors, and measure thetime it takes the laser pulse totravel to the Moon and back.• We know the speed of light, c, socalculating the distance is easy!9Measuring the Distance toAstronomical Objects usingparallaxJust a little Trigonometry…1 parsec = 206265 AU10Moving Stars• The positions of stars are not fixedrelative to Earth– They move around the center of thegalaxy, just as Earth does.– This motion of stars through the sky(independent of the Earth’s rotationor orbit) is called proper motion– Over time, the constellations willchange shape!• The speed of a star’s motion towardor away from the Sun is called itsradial velocityLight and Distance• Brighter objects are notnecessarily the closerobjects– Comet Halley, to theupper left, is within ourSolar System– The background starsare just as bright, buttens, hundreds orthousands of light yearsmore distant• The total amount ofenergy a star emits tospace is its luminosity,measured in Watts.• The amount of lightreaching us from a staris its brightness11The Inverse-Square Law• A star emits light in all directions,like a light bulb. We see the photonsthat are heading in our direction• As you move away from the star,fewer and fewer photons are headingdirectly for us, so the star seems todim – its brightness decreases.• The brightness decreases with thesquare of the distance from the star– If you move twice as far from thestar, the brightness goes down by afactor of 22, or 4!• Luminosity stays the same – the totalnumber of photons leaving a spheresurrounding the star is constant.You see this every day!• More distant streetlights appeardimmer than ones closer to us.• It works the same with stars!• If we know the total energy output ofa star (luminosity), and we can countthe number of photons we receivefrom that star (brightness), we cancalculate its distance• Some types of stars have a knownluminosity, and we can use thisstandard candle to calculate thedistance to the neighborhoods thesestars live in.BLd!4=12The Magnitude System• We can quantify the brightness of astar by assigning it an apparentmagnitude– Brighter stars have lowermagnitudes, possibly negativenumbers– Dimmer stars have higherpositive numbers• Differences in magnitudescorrespond to ratios in brightness– Ex: One star of interest has amagnitude of 6 (dim), andanother star has a magnitude of1 (easily seen). The magnitudedifference of 5 means that thebrighter star is 100 timesbrighter than the dimmer star…The Magnitude System! m1" m1= "2.5logflux1flux2# $ % & ' (13Absolute Magnitude• It is easier to compare twostars’ luminosities if they areat the same distance from theSun• We can calculate how brightthe stars would appear if theywere all the same distancefrom us, say, 10 parsecs• The magnitude of a star“moved” to 10 parsecs fromus is its absolute magnitude.Absolute Magnitude! m " M = 5log d( )"