ASTR 1020General Astronomy: Stars and GalaxiesREVIEWNewton’s Laws• Law #1: A body at rest or in motion remains that way unless acted upon by an outside force.• Law #2: a=F/m• Law #3: For every action there is an equal and opposite reaction.Newton’s Law of Universal Gravitation22112rmGmF =22121rmGmF =(Law #3 Satisfied)G = 6.7x10-8dyne.cm2/g2in cgs unitsValue of G is measured. Nobody knows why it is the value it is.221reeFe=Electrical Force Equation SimilarExampleCalculate the period of a planet that is the same distance from the starSirius that the Earth is from the Sun.r=1.5x1013cmG=6.7x10-8M=4x1033gsxxxxxxxxxxxP7612132639338313105.2102510153.6105.13.6105.21043.6104107.6)105.1(28.6 ======−About 9 MonthsEscape VelocityRGMmmv =221Fall from Large HeightSame Energy Needed to Reverse and Fly AwayRGMve2=REscape velocity is the speedat which object must be thrownupward to escape and never comedown.Surface Gravity2RGMmF =2RGMmma =Force of Gravity at Surface of Body isSoor2RGMa =The acceleration of a bodynear the surface.Independent of mass.Remember Galileo and the Leaning Tower of PisaFormation of the Solar SystemUniverse is about 12 Billion Years old (3x1017s)Milky Way 10 Billion Years oldSun and Planets – 4.6 Billion Years oldSun has done about 25 orbits of Milky WayStars are being formed all the time -- some young, some oldStars are formed from clouds of gasA Star Is Held Up ByThermal Pressure From BelowIndividual Atoms Don’tOrbit Entire Inside ofStar Like ThisThey Jostle Each OtherBut Effect Is The SameOuter MassGravityTemperature Scales• Fahrenheit –– 0=salt water freeze 100=human body• Celsius– 0=pure water freeze 100=water boil (sea level)– C=(F-32)x5/9•Kelvin– 0=absolute zero 100 degrees between freeze and boil– K=C+273– -273C = 0K = Absolute ZeroAt Absolute Zero Atoms Stop MovingThe Nuclear Corecore15x106KEnvelope1 Million KPhotosphere 5000K At SurfaceCosmic Composition• H hydrogen 89% by number• He helium 11%• O oxygen 0.1%• C carbon 0.06%• N nitrogen 0.015%Pretty much the composition of the entire universe.Sun and Jupiter have this compositionEarth does not.Proton-Proton ChainBottom Line: H+H+H+H → He1H1 +1H1→1H2 + e+ + ν1H2 +1H1→2He3 + γ2He3 +2He3→2He4 + 1H1+ 1H15x106< T < 2x107KCNO Cycle6C12 +1H1→7N13 + γ7N13→6C13 + e++ ν6C13 +1H1→7N14 + γ7N14 +1H1→8O15 + γ8O15→7N15 + e++ ν7N15 +1H1→6C12 + 2He4Net: 1H1+1H1+ 1H1+1H1→2He4+ 2e+ + 4γ +2νhydrogen -> helium + energy2x107< T < 108KTriple-α Reaction2He4 +2He4⇔4Be8 + γ4Be8 +2He4→6C12 + γT < 108KNet: 2He4+2He4+ 2He4+2He4→6C12+ 2γhelium -> carbon + energyMust be very dense for this to workBe8decays back into helium very quicklyunless struck by another He4Too low densityin Big BangSolar SchematicSunspot CycleDuring mid 1600’s sunspots became non-existentMaunder MinimumPhotosphere 5500KChromosphere 104KTransition Region 105KCorona 2x106KSolar Wind 5x105KStar Names• Arabic Names– Antares, Capella, Mira, etc.• Constellationsα Orionis, β Cygni, … then 49 Ori, 50 Ori, etc.• Catalogues HD80591, SAO 733421, etc• RA and Dec – just position in the skyProper MotionAll stars moveNearby stars move fasterAppear to move against fixed fieldCan Take Many YearsUse Old Photographic Plates19002003ParallaxI year cycleThe Parsec1AU1 parsec1 arcsecond360 degrees in circle60 arcminutes per degree60 arcseconds per arcminute200,000AU = 1 parsec = 3x1018cmparsec ---- parallax secondThe Magnitude SystemPtolemy Broke Stars into 5 magnitude groupsm=1 the brightest, m=5 the faintestIn 1700’s it was found this was a logarithmic scale, as that is howthe naked eye responds. Also, faintest were about 100x fainter thanbrightest. Break the factor of 100 into 5 equal factors:51.210015==magStart with Vega m=1Polaris 2.51x fainter m=22.5x fainter than Polaris m=32.5x fainter than that m=4etcAbsolute Magnitude5log510+−=dmMWe see a star of magnitude m=10 at 100 pc.What would be its magnitude (M) if it were at 10 pc instead of 100pc? The magnitude a star would have were it at 10pcAt 10 times closer the star would be 100x brighter = 5 magnitudesM = 10-5 = 55510105100log510=+−=+−=MLight Wavesλlambda is lower case Greek Lstands for lengthEach photon is a sine wave moving at the speed of lightWavelength is usually measure in Angstroms1Å = 10-8cm =10-10mabout the diameter of an atom.Electric and MagneticFields Sloshing BackAnd ForthParts of EM SpectrumRadio λ > 1mm (107A)Infrared 1mm> λ > 10000ΑVisible 10,000Α > λ > 3500ΑUltraviolet 3500Α > λ > 100ΑX-ray 100A > λ > 0.1AGamma-ray 0.1A > λSpeed of LightSpeed of Light c = 3x1010cm/sThat’s a very odd statement1 car at 130mph2 cars at 65mphCover same distance in same amount of timeThe Relative speeds are the sameFrequencyλλλλMoves λ during each cycleFrequency is the number of cycles per second, νGreek “nu”Moves distance λ for each of ν cycles each secondc=λνEnergy of a Photonνεh=h = 6.63x10-27erg sPlanck’s Constantergsxxxx121427104106106.6−−==εenergy of yellow photonSunlight is 107ergs/cm2/sOutside we have 1019photons/cm2/s hit usSpectroscopySpectrum is plot of number of photons as a function of wavelengthTells us huge amounts about nature of object emitting light.Thermal Radiation11252−=kThcehcIλλPlanck’s LawTemperature Determines Where Spectrum PeaksPosition of Peak Determines ColorWien’s LawTxpeak7103=λAs T rises, λ dropsBluer with temperature Å(T in Kelvin)300K 100,000A Earth5500 5500 Sun10630 X-ray sourceStefan-Boltzman Law4ATLσ=σ = 5.67x10-5A is area in cm2T in KelvinsExample: The SunL = 5.7x10-5x 4 x 3.14 x (7x1010cm)2x (5500K)4= 4 x 1033ergs4x1033ergs = 4x1026Watts = 100 billion billion MegaWatts!!Emission LinesElectron DropsPhoton EscapesCan Only Happen BetweenCertain Pre-determined orbitalsEach Element Has Different OrbitalsSo Each Element Has Different LinesSpectrum of HydrogenEnergy Levels of HAbsorption LinesLight moving through coldgas can have photons removed.Creates dark wavelengthscalled absorption linesStellar ClassificationFull range of surface temperatures from 2000 to 40,000KSpectral Classification is Based on Surface TemperatureO B A F G K MOh Be A Fine GirlGuyKiss MeEach Letter has ten subdivisions from 0 to 90 is hottest, 9 is coolestCoolestHottest{}Stellar Classification (2)Sun G2α Cen G2 + K5Sirius A1Antares M1Rigel B8O5 40,000KB5 15,500A5 8500F5 6580G5 5520K5 4130M5 2800Letters are odd due to confusion in sorting out