We emit infrared light (heat) and reflect visible lightL =4piR^2sigmaT^4Smallest unit of light called a photon – has specific wavelength and specific energyWavelength – distance from peak to peak, called gammaLight of different colors corresponds to photons of different wavelengthAll light travels at the “speed of light”, regardless of its wavelengthIn a vacuum, that speed is 3 x 108 meters/secondUse the letter “c” to represent the speed of light in equationsFrequency – the number of peaks that go by a point per secondf = c / λLight actually behaves as both a particle and a wave at the same time  called the wave-particle duality of lightEnergy of a photon E is:Energy= E =h c/λ= h fH = planck’s constantUnits in joules (J)Long wavelength photons have small energies shorter wavelength photons have more energy and X-ray’s and Gamma ray’s have the most energyWhite light is really a mixture of all colors (wavelengths) of visible lightMatter can:emit light -- the element of a light bulbabsorb light -- a black cloth reflectscatter light – a white clothPlanck curve  describes how a body emits light according to its temperatureCold bodies emit light at infrared or radio wavelengths, warmer bodies emit more light at all wavelengthsWien’s Displacement Law –describes the wavelength of maximum emission of the Planck Curve as a function of temperature.λmax = b / Tλmax is the wavelength of the peak and b = 2.9 x 10-3 meters-KEmission = σ T4σ = 5.67 x 10-8 watts/m2/K4Watts/m2 = Joules/sec/m2Stephen-Boltzman LawLuminosity = surface area x σ T4L = 4π R^2σ T4 for spherical bodies, like planetsWatts = joules/secApparent brightness = b = L/ (4 π d2)Matter can absorb light, however no type of material uniform absorbs light of all wavelengths so the interaction typically is a combination of reflection, absorption, and transmissionIn many situations, the absorption spectrum is the complement of the reflection spectrumWe see the light that is reflected so the color of the wavelengths of the reflected light is the color that we assign to the objectTransmission, which is the lack of absorption, occurs when the matter does not interact with light at a particular wavelength. In general, any material is only transparent over a range of wavelengths.There are three broad types of spectra:continuous spectraGenerally thermal emission from a solid body. It is characterized by emission over a wide range of wavelength without detailed structure.absorption line spectraAbsorption features arise when a continuous spectrum is seen through material that absorbs light at discrete wavelengths.The simplest example is a source seen through a cloud of gas.emission line spectraEmission spectra arise when you view a gas without a continuous spectrum in the background.Spectral lines arise because the energy levels of atoms are quantized – that means that the atom can only have discrete energy levels like the steps of a stairs.An atom can change its energy level by emitting or absorbing a photon (think of light as a particle).Since the energy difference between any two levels is very specific, the photon has a specific energy – wavelength!If the atom absorbs a photon from a continuous spectrum, an absorption line is create. If it emits a photon without a background source... it creates an emission line spectrum.What is an atom doing when it changes energy level?One of the electrons in the cloud surrounding the nucleus is receiving or giving up energyBecause spectral lines are well-understood and unique signatures of elements, they can be used to study things like:the abundance of elements in stars and nebulae the temperature of astronomical objectsAnd, perhaps surprisingly, spectral lines can be used to measure the velocities of motion of astronomical objects.The Doppler Shift is a change in the wavelength of light that occurs when the light is emitted from a body that is moving toward or away from an observer.The doppler shift also occurs for soundWhen an object is moving toward you, the wave is bunched-up – shifted to shorter wavelengths... and we call that Blue-shifted. The light appears “bluer” than expected.When an object is moving away from you, the wave is stretched-out – shifted to longer wavelength... we call that Red-shifted. The light appear “redder” than expected.λ(measured)–λ(lab) / λ(lab)= velocity / cLight can interact with matter in 4 ways:EmissionTransmissionAbsorptionReflection/scatteringSun is 300,000 times bigger (in mass) than the EarthLuminosity: 3.8x10^26Visible surface of the sun is called the photosphereEmits light with the spectrum of 5800 KSurface is not uniformly bright (not all at same temp)Granules – little cells of rising and falling blobs of gas  they rise (get hotter) and fall (cool down) like a boiling pot of waterSunspots = regions of strong magnetic activity that causes it to be cooler and appear blackNumber and location of sunspots is constantly changing11-year sunspot cycle of activityQuiet sun at minima, active sun at maximaNot sure what causes the cycleSolar flares and prominences – releases of energy associated with sunspots and magnetic field activityLargest flares blow solar gas out into space at high speeds, which are called coronal mass ejection eventsMaterial travels out into space as a wind of materialSun can throw off huge amounts of material, and its nothing compared to sun’s massIf a coronal mass ejection is headed for earth, it could be seriousSolar wind material interacts with the Earth’s magnetic field disrupting communications and creating danger for airplanes flying near polesAbove the photosphere is the thin gas of the chromosphere, which is 5000 to 20,000 KAbove that is the corona where the temp is as much as 1 million degreesThe corona is very low density, very hot plasma that extends millions of kilometers from sun’s surfaceConvective zone – extends from just below the surface in roughly 30% of the radiusConvection is strong in this regionRadiative zone – extends from about .25 to .7 of the sun’s radiusNo convectionEnergy streams through as radiation2-7 million kelvinsCore – inner 25% of the sun’s radiusMost energy of the sun’s energy is generated in the coreTemp and density inside the sun are determined by hydrostatic equilibrium and the need to transport the energy from the core out into space to keep the energy balanceEnergy generated in the core equals energy given off at the