1. PHYS 3380 - Astronomy The first exam will be next Monday, September 29 at the regular class time. It is closed book but you may bring in one 8 1/2 X 11 inch “cheat sheet” with writing on both sides. It will cover everything I have covered up to and including last Wednesday’s class (9/17/14).

2. PHYS 3380 - Astronomy Center of Mass Note: the previous calculations assumed that the mass of the orbiting body was much smaller that the central body - center of orbit at center of central body Newton showed that two objects attracted to each other by gravity actually orbit about their center of mass - the point at which the objects would balance if they were connected. This idea is used to find planets orbiting other stars - massive planets cause star to move against background stars Center of Mass - Binary Star Animation

3. PHYS 3380 - Astronomy Einstein 1905 - “The Year of Physics” Submitted doctoral thesis "A New Determination of Molecular Dimensions” Published five pioneering papers in "Annalen der Physik" - revolutionized physics: "On A Heuristic Point of View Concerning the Production and Transformation of Light" - electromagnetic radiation must consist of quantums or photons - explained the photoelectric effect - became the foundation of quantum theory - what he received the Nobel Prize for in 1921 "On the Electrodynamics of Moving Bodies" - special relativity - new interpretation of the conception of space and time - observer can never detect their uniform motion except relative to other objects - coordinate systems - speed of light constant - independent of motion relative to light source "Does the Inertia of a Body Depend upon its Energy Content?" - the equivalence of mass and energy - E = mc 2

4. PHYS 3380 - Astronomy General Relativity 1916 published "The Foundation of the General Theory of Relativity” - generalized special theory of relativity - observer cannot distinguish between inertial forces due to acceleration and uniform gravitational forces - gravity is curvature of space-time - curvature dependent on mass - acceleration of mass dependent on space-time curvature Numerous implications on astronomy and astrophysics - orbital motion - black holes - big bang - formation and structure of galaxies

5. PHYS 3380 - Astronomy Proof of General Relativity Einstein convinced that light deflection by the gravitational field of the sun could be observed during a total solar eclipse - photograph section of sky where eclipse would occur - during eclipse, photograph same section and measure difference in positions - predicted deflection of 1.75 arcseconds for starlight grazing Sun’s surface Theory predicted the deflection of light in a gravitational field Several failed observations of total solar eclipses before proof in 1919 - observed eclipse in island of Principe in the Gulf of Guinea in western Africa and Sobral, Brazil - found shift in stars outward from Sun - 1.61±0.30 arcseconds at Principe - 1.98±0.12 arcseconds at Sobral

6. PHYS 3380 - Astronomy

7. Einstein’s eclipse.

8. PHYS 3380 - Astronomy Proof of General Relativity Precession of long axis of Mercury’s orbit Newton’s formulation predicted precession of 5557.62 arcseconds per century - actually 43.11 arcseconds more - about 29 km past position predicted by Newton per orbit - accumulative - 12,000 km per century - easily measured Einstein predicted 43.03 arcseconds per century difference Effect since observed on Venus, Earth, and asteroid Icarus

9. PHYS 3380 - Astronomy Light

10. PHYS 3380 - Astronomy Light What is light? - A vibration in an electromagnetic field through which energy is transported. Light as a wave Light as a particle E = hf photon f = c The dual nature of light or wave-particle duality:

11. PHYS 3380 - Astronomy Properties of Waves Period: time to complete one cycle of vibration - from crest to crest or trough to trough Frequency (f): number of crests passing a fixed point per second Frequency= 1/period Amplitude (a): maximum displacement from equilibrium Wave length (l): distance between successive crests Speed (of a wave) (s)= wave length x frequency s= l x f

12. PHYS 3380 - Astronomy Light is a vibration in an electromagnetic field through which energy is transported - a transverse wave - vibration or oscillation is perpendicular to direction of propagation of wave (vs. longitudinal wave - vibration or oscillation is in the same direction as propagation of wave) So electrons can be manipulated by light. Electrons wiggle up and down as light passes by. Light as a Wave

13. PHYS 3380 - Astronomy Visible light ranges through 7 major colors from long wavelengths (low frequency - red) to short wavelengths (high frequency - violet) - Red, orange, yellow, green, blue, indigo, violet (Roy G Biv) For a wave, its speed: s = x f But the speed of light is a constant, c. For light: x f = c The higher f is, the smaller is, and vice versa. Our eyes recognize f (or ) as color.

14. PHYS 3380 - Astronomy Light as a Particle (Photon) Light propagates as quanta of energy called photons Photons move with speed of light have no mass are electrically neutral Energy of a photon or electromagnetic wave: E = hf = h c/ where h = Planck’s constant f = frequency of a light wave - number of crests passing a fixed point in 1 second c = velocity of light = wavelength of a light wave Higher frequency/shorter wavelength - higher energy

15. PHYS 3380 - Astronomy The Electromagnetic Spectrum Most wavelengths of light can not be seen by the human eye. The visible part of the electromagnetic spectrum lies between ultraviolet and infrared light (between about 400 and 700 nm). The higher the frequency (shorter the wavelength), the higher the photon energy. Radio waves are at the long wavelength end of the spectrum and gamma rays are at the short wavelength end of the spectrum.

16. PHYS 3380 - Astronomy Light as Information Bearer Spectrum of a distant object - a spectrum is the amount of energy or intensity at different wavelengths. By studying the spectrum of an object, we can learn its: 1Composition 2Temperature 3Velocity We can separate light into its different wavelengths (spectrum).

17. PHYS 3380 - Astronomy Electron Energy Levels Electrons can not have just any energy while orbiting the nucleus. Only certain energy values are allowed. Electrons may only gain or lose certain specific amounts of energy. Each element (atom and ion) has its own distinctive set or pattern of energy levels - holds the key to studying of distant objects in the universe. This diagram depicts the energy levels of Hydrogen. 1 eV (electron volt) = 1.6 X 10 -19 J Electron jumps to higher energy levels can only occur with addition of the particular amounts of energy representing differences between possible energy levels. Energy levels are quantized - study of electron energy levels called quantum mechanics. Atom gains this energy either from KE of another atom colliding with it or from absorption of energy carried by light - falls to lower energy level by emitting light or transfer of energy by collision.

18. PHYS 3380 - Astronomy Absorption and Emission. When electrons jump from a low energy shell to a high energy shell, they absorb energy. When electrons jump from a high energy shell to a low energy shell, they emit energy. This energy is either absorbed or emitted at very specific wavelengths, which are different for each atom. When the electron is in a high energy shell, the atom is in an excited state. When the electron is in the lowest energy shell, the atom is in the ground state.

19. PHYS 3380 - Astronomy The Hydrogen Atom. The hydrogen atom is the simplest of atoms. Its nucleus contains only one proton which is orbited by only one electron. In going from one allowed orbit to another, the electron absorbs or emits light (photons) at very specific wavelengths. Note - wavelength is often written as and the unit used is an angstrom (A) = 10 -8 m

20. PHYS 3380 - Astronomy Interaction of Light with Matter So each electron is only allowed to have certain energies in an atom. Electrons can absorb light and gain energy or emit light when they lose energy. It is easiest to think of light as a photon when discussing its interaction with matter. Only photons whose energies (colors) match the “jump” in electron energy levels can be emitted or absorbed. Hydrogen So visible emission spectrum is created when a gas is heated and collisions in gas continually bump electrons to higher energy levels - emit photons of specific wavelength as they fall back to lower levels. Absorption spectrum is produced when white light is passed through cloud of cool gas. Photons of specific wavelengths absorbed as electrons jump to higher energy levels. Emission Spectrum Absorption Spectrum

21. PHYS 3380 - Astronomy Emission Spectra The atoms of each element have their own distinctive set of electron energy levels. Each element emits its own pattern of colors, like fingerprints. If it is a hot gas, we see only these colors, called an emission line spectrum. Orion Nebula in Ultraviolet

22. PHYS 3380 - Astronomy Absorption Spectra If light shines through a cool gas, each element will absorb those photons whose colors match their electron energy levels. The resulting absorption line spectrum has all colors minus those that were absorbed. We can determine which elements are present in an object by identifying emission and absorption lines. Hydrogen

23. PHYS 3380 - Astronomy Temperature and Thermal Energy Why does water burn your skin so much quicker than air? Why is falling into a 32º F lake more dangerous than standing outside naked on a 32º F? Temperature - measure of the average kinetic energy of the particles in a substance - particles in box on right have higher temperature - higher velocity = more KE = higher temperature Both boxes have same temperature - particles have same average velocity/KE - box on right has more thermal energy - energy contained in a substance - more particles

24. PHYS 3380 - Astronomy This diagram compares three common temperature scales. The Fahrenheit scale is used in the United States, but nearly all other countries use the Celsius scale. Scientists prefer the Kelvin scale because O K represents absolute zero, the coldest possible temperature.

25. PHYS 3380 - Astronomy Thermal/Blackbody Radiation Photons are produced whenever charged particles are accelerated - A moving charge gives rise to a magnetic field, and if the motion is changing (accelerated), then the magnetic field varies and in turn produces an electric field - electromagnetic radiation - photons In an opaque object or dense gas cloud, photons can’t easily escape - they “bounce around” in the object. This randomizes their radiative energies and resulting photon energies depend only on the body’s temperature - produces a continuous spectrum called a thermal radiation or blackbody spectrum. Blackbody - a hypothetical body that completely absorbs all wavelengths of thermal radiation incident on it - does not reflect light - appears black if temperature low enough so as not to be self-luminous. - all blackbodies heated to a given temperature emit thermal radiation with the same spectrum - required by thermal equilibrium - distribution of blackbody radiation as a function of wavelength - the Planck law, cannot be predicted using classical physics. - the first motivating force behind the development of quantum mechanics

26. PHYS 3380 - Astronomy Key Features of a Blackbody Spectrum - a dense object produces light at all possible wavelengths if the object is above absolute zero. - since everything in the universe is above 0 K, all dense objects (solids, liquids, thick gases) will produce a thermal spectrum. - the shape of a continuous spectrum depends on only the temperature of the object not its chemical composition. - as the temperature of an object increases, more light is produced at all wavelengths -as the temperature of an object increases, the peak of thermal spectrum curve shifts to shorter wavelengths (higher frequencies) --cool things appear red or orange, hotter things appear yellow or white, and very hot things blue or purple.

27. PHYS 3380 - Astronomy Temperature (K) of Black Body Wavelength ( max) at Which Most Radiation is Emitted Type of Radiation 30.1 cmRadiowaves 3000.001 cm"Far" Infrared 3,0001000 nm"Near" Infrared 4,000750 nmRed Light 6,000500 nmYellow Light 8,000375 nmViolet Light 10,000300 nm"Near" Ultraviolet 30,000100 nm"Far" Ultraviolet 300,00010 nm"Soft" X-Rays 1.5 million20 nm"hard" x-rays 3 billion0.001 nmGamma rays

28. PHYS 3380 - Astronomy Hotter objects emit more total radiation per unit surface area. E =  T 4 (  = 5.67 x 10 -8 watts/m 2 K 4 ) - Stefan-Boltzmann Law Hotter objects emit photons with a smaller wavelength (higher average energy.) max (nm) = 2.8983 x 10 6 nm-°K/T 10 6 / T(K) [nm] - Wien’s Law

29. PHYS 3380 - Astronomy Derivation of the Stefan-Boltzmann Law

30. PHYS 3380 - Astronomy Solid Angle The solid angle  subtended by a surface S - the surface area  of a unit sphere covered by the surface's projection onto the sphere. This can be written as where n is a unit vector from the origin, da is the differential area of a surface patch, and r is the distance from the origin to the patch. Written in spherical coordinates with  the colatitude (polar angle) and  for the longitude (azimuth), this becomes Solid angle is measured in steradians, and the solid angle corresponding to all of space being subtended is 4  steradians.

31. PHYS 3380 - Astronomy Consider the solid angle subtended by one face of a cube of side length centered at the origin. Since the cube is symmetrical and has six sides, one side obviously subtends 4  /6 steradians.

32. PHYS 3380 - Astronomy

34. So, the luminosity of a star depends on temperature and size (surface area). Amount radiated from every square meter equals σT 4 (Stefan-Boltzmann Law) - the hotter the star, the more energy radiated per square meter Total amount radiated (luminosity): L = 4  R 2 σT 4 R is star's radius, surface area = 4  R 2 Stellar luminosities generally given in number of solar luminosities: L S =3.826 X 10 26 W If we measure L and T, we can estimate R T can be determined using Wein’s Law

35. PHYS 3380 - Astronomy The Stefan-Boltzmann Law can be used to estimate the Earth’s temperature:

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38. Wein’s Law can be derived by taking the derivative of the Planck brightness with respect to wavelength and determining where this function is zero, i.e. set dB /d =0 and solve for. (I’ll leave that exercise for you, if you want to try it.) The Stefan-Boltzmann Law, Wein’s Law, and Newton’s Universal Law of Gravitation together allow us to determine a star's type, its mass, its temperature, its rate of energy production, its diameter, its life expectancy and its future fate.