2Light and Other Forms of Radiation The Electromagnetic SpectrumIn astronomy, we cannot perform experiments with our objects (stars, galaxies, …).The only way to investigate them is by analyzing the light (and other radiation) which we observe from them.
3Light as a Wavec = 300,000 km/s = 3*108 m/sLight waves are characterized by a wavelength and a frequency f.f and are related throughf = c/
4Wavelengths and Colors Different colors of visible light correspond to different wavelengths.
5Dark Side of the Moon“There is no dark side really. It’s all dark.” -- Pink Floyd
6More accurate, from Richard Berg Dark Side of the MoonWhat is wrong with this picture?Front: Not all primary colors (eg, pink, magenta), also refraction angles inconsistentBack: Spectrum is Convergent – I think done for art’s sakeFront cover Back coverMore accurate, from Richard Berg
7Light as a WaveWavelengths of light are measured in units of nanometers (nm) or angstrom (Å):1 nm = 10-9 m1 Å = m = 0.1 nmVisible light has wavelengths between 4000 Å and 7000 Å (= 400 – 700 nm).
8The Electromagnetic Spectrum WavelengthFrequencyHigh flying air planes or satellitesNeed satellites to observe
9Light as Particles E = h*f Light can also appear as particles, called photons (explains, e.g., photoelectric effect).A photon has a specific energy E, proportional to the frequency f:E = h*fh = 6.626x10-34 J*sis the Planck constant.The energy of a photon does not depend on the intensity of the light!!!
10Why is energy per photon so important? Real life example: Ultra-Violet light hitting your skin (important in Laramie!)Threshold for chemical damage set by energy (wavelength) of photonsBelow threshold (long wavelengths) energy too weak to cause chemical changesAbove threshold (short wavelength) energy photons can break apart DNA moleculesNumber of molecules damaged = number of photons above thresholdVery unlikely two photons can hit exactly together to cause damage
11Temperature and HeatThermal energy is “kinetic energy” of moving atoms and moleculesHot material energy has more energy available which can be used forChemical reactionsNuclear reactions (at very high temperature)Escape of gasses from planetary atmospheresCreation of lightCollision bumps electron up to higher energy orbitIt emits extra energy as light when it drops back down to lower energy orbit(Reverse can happen in absorption of light)
12Temperature ScalesWant temperature scale with energy proportional to TCelsius scale is “arbitrary” (Fahrenheit even more so)0o C = freezing point of water100o C = boiling point of waterBy experiment, available energy = 0 at “Absolute Zero” = – 273oC (-459.7oF)Define “Kelvin” scale with same step size as Celsius, but 0K = - 273oC = Absolute ZeroUse Kelvin Scale for most astronomy workAvailable energy is proportional to T, making equations simple (really! OK, simpler)273K = freezing point of water373K = boiling point of water300K approximately room temperature
13Planck “Black Body Radiation” Hot objects glow (emit light) as seen in PREDATOR, SSC Video, etc.Heat (and collisions) in material causes electrons to jump to high energy orbits, and as electrons drop back down, some of energy is emitted as light.Reason for name “Black Body Radiation”In a “solid” body the close packing of the atoms means than the electron orbits are complicated, and virtually all energy orbits are allowed. So all wavelengths of light can be emitted or absorbed. A black material is one which readily absorbs all wavelengths of light. These turn out to be the same materials which also readily emit all wavelengths when hot.The hotter the material the more energy it emits as lightAs you heat up a filament or branding iron, it glows brighter and brighterThe hotter the material the more readily it emits high energy (blue) photonsAs you heat up a filament or branding iron, it first glows dull red, then bright red, then orange, then if you continue, yellow, and eventually blue
14Planck and other Formulae Planck formula gives intensity of light at each wavelengthIt is complicated. We’ll use two simpler formulae which can be derived from it.Wien’s law tells us what wavelength has maximum intensityStefan-Boltzmann law tells us total radiated energy per unit areaFrom our text: Horizons, by Seeds
15From our text: Horizons, by Seeds Example of Wien’s lawWhat is wavelength at which you glow?Room T = 300 K soThis wavelength is about 20 times longer than what your eye can see. Thermal camera operates at 7-14 μm.What is temperature of the sun – which has maximum intensity at roughly 0.5 m?From our text: Horizons, by Seeds
16Kirchoff’s laws Hot solids emit continuous spectra Hot gasses try to do this, but can only emit discrete wavelengthsCold gasses try to absorb these same discrete wavelengths
17Atoms – Electron Configuration Molecules: Multiple atoms sharing/exchanging electrons (H2O, CH4)Ions: Single atoms where one or more electrons have escaped (H+)Binding energy: Energy needed to let electron escapePermitted “orbits” or energy levelsFrom quantum mechanics, only certain “orbits” are allowedGround State: Atom with electron in lowest energy orbitExcited State: Atom with at least one atom in a higher energy orbitTransition: As electron jumps from one energy level orbit to another, atom must release/absorb energy different, usually as light.Because only certain orbits are allowed, only certain energy jumps are allowed, and atoms can absorb or emit only certain energies (wavelengths) of light.In complicated molecules or “solids” many transitions are allowedCan use energy levels to “fingerprint” elements and estimate temperatures.From our text: Horizons, by Seeds
18From our text: Horizons, by Seeds Hydrogen LinesEnergy absorbed/emitted depends on upper and lower levelsHigher energy levels are close togetherAbove a certain energy, electron can escape (ionization)Series of lines named for bottom levelTo get absorption, lower level must be occupiedDepends upon temperature of atomsTo get emission, upper level must be occupiedCan get down-ward cascade through many levelsn=3n=2n=1From our text: Horizons, by Seeds
19Astronomical Telescopes Often very large to gather large amounts of light.In order to observe forms of radiation other than visible light, very different telescope designs are needed.The northern Gemini Telescope on Hawaii
20Refracting / Reflecting Telescopes Refracting Telescope: Lens focuses light onto the focal planeFocal lengthReflecting Telescope: Concave Mirror focuses light onto the focal planeFocal lengthAlmost all modern telescopes are reflecting telescopes.
21Secondary OpticsIn reflecting telescopes: Secondary mirror, to re- direct light path towards back or side of incoming light path.Eyepiece: To view and enlarge the small image produced in the focal plane of the primary optics.
22Disadvantages of Refracting Telescopes Chromatic aberration: Different wavelengths are focused at different focal lengths (prism effect).Can be corrected, but not eliminated by second lens out of different material.Difficult and expensive to produce: All surfaces must be perfectly shaped; glass must be flawless; lens can only be supported at the edges.
23The Powers of a Telescope: Size does matter! 1. Light-gathering power: Depends on the surface area A of the primary lens / mirror, proportional to diameter squared:DA = (D/2)2
24The Powers of a Telescope (II) 2. Resolving power: Wave nature of light => The telescope aperture produces fringe rings that set a limit to the resolution of the telescope.Astronomers can’t eliminate these diffraction fringes, but the larger a telescope is in diameter, the smaller the diffraction fringes are. Thus the larger the telescope, the better its resolving power.min = 1.22 (/D)minFor optical wavelengths, this givesmin = 11.6 arcsec / D[cm]
25SeeingWeather conditions and turbulence in the atmosphere set further limits to the quality of astronomical imagesBad seeingGood seeing
26The Powers of a Telescope (III) 3. Magnifying Power = ability of the telescope to make the image appear bigger.A larger magnification does not improve the resolving power of the telescope!
27The Best Location for a Telescope Far away from civilization – to avoid light pollution
28The Best Location for a Telescope (II) Paranal Observatory (ESO), ChileOn high mountain-tops – to avoid atmospheric turbulence ( seeing) and other weather effects
29Traditional Telescopes (I) Secondary mirrorTraditional primary mirror: sturdy, heavy to avoid distortions.
30Traditional Telescopes (II) The 4-m Mayall Telescope at Kitt Peak National Observatory (Arizona)
31Advances in Modern Telescope Design Lighter mirrors with lighter support structures, to be controlled dynamically by computersFloppy mirrorSegmented mirror
32Adaptive OpticsComputer-controlled mirror support adjusts the mirror surface (many times per second) to compensate for distortions by atmospheric turbulence
33Examples of Modern Telescope Design The Very Large Telescope (VLT)8.1-m mirror of the Gemini Telescopes
34Recall: Resolving power of a telescope depends on diameter D. InterferometryRecall: Resolving power of a telescope depends on diameter D. Combine the signals from several smaller telescopes to simulate one big mirror Interferometry
35CCD Imaging CCD = Charge-coupled device CCD = Charge-coupled deviceMore sensitive than photographic platesData can be read directly into computer memory, allowing easy electronic manipulationsFalse-color image to visualize brightness contours
36The SpectrographUsing a prism (or a grating), light can be split up into different wavelengths (colors!) to produce a spectrum.Spectral lines in a spectrum tell us about the chemical composition and other properties of the observed object
37Radio AstronomyRecall: Radio waves of ~ 1 cm – 1 m also penetrate the Earth’s atmosphere and can be observed from the ground.
38Radio TelescopesLarge dish focuses the energy of radio waves onto a small receiver (antenna)Amplified signals are stored in computers and converted into images, spectra, etc.
39 Use interferometry to improve resolution! Radio InterferometryJust as for optical telescopes, the resolving power of a radio telescope depends on the diameter of the objective lens or mirror min = /D.For radio telescopes, this is a big problem: Radio waves are much longer than visible lightThe Very Large Array (VLA): 27 dishes are combined to simulate a large dish of 36 km in diameter. Use interferometry to improve resolution!
40The Largest Radio Telescopes The 100-m Green Bank Telescope in Green Bank, West Virginia.The 300-m telescope in Arecibo, Puerto Rico
41Science of Radio Astronomy Radio astronomy reveals several features, not visible at other wavelengths:Neutral hydrogen clouds (which don’t emit any visible light), containing ~ 90 % of all the atoms in the universe.Molecules (often located in dense clouds, where visible light is completely absorbed).Radio waves penetrate gas and dust clouds, so we can observe regions from which visible light is heavily absorbed.
42NASA infrared telescope on Mauna Kea, Hawaii Infrared AstronomyMost infrared radiation is absorbed in the lower atmosphere.However, from high mountain tops or high- flying aircraft, some infrared radiation can still be observed.NASA infrared telescope on Mauna Kea, Hawaii
43Spitzer Space Telescope Infrared TelescopesSpitzer Space TelescopeWIRO 2.3m
44Ultraviolet Astronomy Ultraviolet radiation with < 290 nm is completely absorbed in the ozone layer of the atmosphere.Ultraviolet astronomy has to be done from satellites.Several successful ultraviolet astronomy satellites: IUE, EUVE, FUSEUltraviolet radiation traces hot (tens of thousands of degrees), moderately ionized gas in the universe.
45NASA’s Great Observatories in Space (I) The Hubble Space TelescopeLaunched in 1990; maintained and upgraded by several space shuttle service missions throughout the 1990s and early 2000’sAvoids turbulence in Earth’s atmosphereExtends imaging and spectroscopy to (invisible) infrared and ultraviolet
46Hubble Space Telescope Images Mars with its polar ice capA dust-filled galaxyNebula around an aging star
47NASA’s Great Observatories in Space (II) The Compton Gamma-Ray ObservatoryOperated from to 2000Observation of high-energy gamma-ray emission, tracing the most violent processes in the universe.
48NASA’s Great Observatories in Space (III) The Chandra X-ray TelescopeLaunched in 1999 into a highly eccentric orbit that takes it 1/3 of the way to the moon!X-rays trace hot (million degrees), highly ionized gas in the universe.Two colliding galaxies, triggering a burst of star formationVery hot gas in a cluster of galaxiesSaturn
50The Highest Tech Mirrors Ever! Chandra is the first X-ray telescope to have image as sharp as optical telescopes.
51NASA’s Great Observatories in Space (IV) The Spitzer Space TelescopeLaunched in 2003Infrared light traces warm dust in the universe.The detector needs to be cooled to -273 oC (-459 oF).
52Spitzer Space Telescope Images A CometWarm dust in a young spiral galaxyNewborn stars that would be hidden from our view in visible light
53Spitzer Space Telescope Discovered by a Wyoming grad student and professor. The “Cowboy Cluster” – an overlooked Globular Cluster.
54Kepler’s Supernova with all three of NASA’s Great Observatories Just 400 years ago: (Oct. 9, 1604)Then a bright, naked eye object (no telescopes)It’s still blowing up – now 14 light years wide and expanding at 4 million mph.There’s material there at MANY temperatures, so many wavelengths are needed to understand it.
55A Multiwavelength Look at Cygnus A A merger-product, and powerful radio galaxy.