Life Beyond the Earth
Life Beyond the Earth ASTR 3420
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This 7 page Class Notes was uploaded by Willie Cummings on Monday September 21, 2015. The Class Notes belongs to ASTR 3420 at University of Virginia taught by Staff in Fall. Since its upload, it has received 38 views. For similar materials see /class/209720/astr-3420-university-of-virginia in Astronomy at University of Virginia.
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Date Created: 09/21/15
Review of the Exam This will involve concepts discussed in the class notes including my own additions as well as the book itself this will in general be slightly more involved than the notes I handed out in class Basic concepts involve 1 familiarity with SI units of energy Joule power Watt mass kilogram 2 familiarity with scienti c notation exponential notation 3 all the topics which we have discussed up to now a the Galaxy its basic properties a galactic year etc b the Drake Equation this includes the early searches for extraterrestrial intelligence as well as the various terms associated with it c Stars i ii The main sequence and the mass luminosity relation of stars The sun s structure and evolution This involves for example the nuclear reaction that occurs in the sun while it is on the main sequence as well as its eventual evolution iii The properties of degenerate matter iv Concepts of brightness luminosity and distance Brightness is an informal de nition of powerunit area v The energy frequency and energy wavelength relations of photons light particles Remember Planck s constant vi Why certain stars are favorable or unfavorable for the development of intelligent life d Planets i How stars and their star systems I may have a question on the test regarding a numerical answer on how quickly they form ii The previously accepted theory of planetary formation iii Problems with the current theories of planetary formation before the discovery of planets Justi cation for the use of Radio Frequencies in SETI Our local environment is largely transparent to radio signals in the range of frequencies above that of optical equencies especially in the 110 GHZ range The plot below shows the level of noise in Kelvins in the environment Also shown is the noise level given the elevation of the telescope relative to the horizon as well as relative to the galactic equator quick way to nd the galactic equator look along the Milky Way Which is why radio telescopes and antennae are used in SETI 1 nine x Terrestrial microwavave WlndOW 1 UC39 Henthermal background degrees Kelvin K 21 2 cosmic 39 background I I l l l I I l l l 1 1 18 13931 1008 v SHE Furthermore radio telescopes can do the following 0 Measure the radiation from a speci c direction of sky 0 Measure how the intensity varies with direction thus making a map of an extended part of the sky Measure intensity as a function of equency getting a spectrum Measure the polarization of light waves magnetic elds in the source or intervening medium or arti ciality 2 of source Observations with Radio Telescopes A typical radio telescope is a paraboloid dish similar to paraboloid lenses in r35 quot13222 5398139 ll 35115 39He etiedmimiun U K Em good few the shape Julia 3935 2 parabolaLi optical telescopes The collecting area of your dish receiver is proportional to D2 Typical radio telescopes for astronomical use are these types of paraboloid dishes 100 meters in diameter Thus the angular resolution of these telescopes at 1 GHz or 03 in wavelength radio 6 0003 radians 2103 The size of the moon is approximately 30 30 arcminutes so compared to our eyes the single resolution of a big telescope is not that good These radio telescopes would need to be a paraboloid to an accuracy such that any bumps ltlt 1 Otherwise the angular resolution is reduced even further due to the fact that all the radiation is not centered on the focus Since the angular resolution is so bad radio telescopes need to raster across the sky in order to get an image Polarization and Some Features of Lign Waves e e t i e t i c 3x108 ms The frequency v and wavelength A are related by c xiv Light travels at speeds less than c through any materials This velocity v cn where n is the index ofre action e particlese 39 39 39 39 c Electric ti wmtemm held i a39w Divem ion quot41 ieiihi quot39 helectricor Lfit trace out a line 7 lmearly polarzzed Lfit trace out a circle 7 drcularly p0 arized An L c L arr 39 39 mi 2 vvnndnm polarizations Other Issues that are Important for Radio Frequencies Radio waves are relatively low frequency to what we observe Furthermore although space is largely a vacuum it consists of ee charged particles esp electrons magnetic elds and some turbulence Although these effects are not that important for optical SETI they do play an important role in the radio 0 Space consists of ee charged particles electrons play the most important role What other material consists of large quantities of free as in not bound to atoms a METAL Space like a metal will distort those waves traveling through it this is especially important in waves of a few GHz or smaller Space also consists of charged particles embedded in a magnetic field Waves undergo Faraday rotation the polarization of the light waves will change direction as they pass through charged particles in a magnetic eld Scintillation like the twinkling of stars in our atmosphere space also has largescale turbulence These are changes in density on the scale of a few AU in size resulting in twinkling of a few minutes to hours Arrays To get resolutions on the order of 1 or much better which is possible one can use arrays of telescopes and interfere combine the signals a y a H H AntennaB AmennaA Atomic Atomic clock clock clock 7 t 95 5 my Tape recorer Tape recorder E Tape recorder A 03 m radio one would need a single dish of s1z A 8 km To get comparable resolutions of the human eye approximately 01 at 1 GHZ e D 39 61 9 7rl8C3600 However one can get the same resolution if one has a baseline separation between individual radio telescopes of this distance The total collecting area of this telescope becomes however only the total collecting area of the individual telescopes Furthermore the distance to each telescope must be known to an accuracy ltltA otherwise the focus in getting a signal is degraded This is done in one of two wa s 0 Through laser electrical or other direct physical connections between the radio dishes in the array The VLA Very Large Array operated by NRAO out ofNew Mexico uses this Calibration by atomic clocks and tape recorders of the signal received from each source This is how VLBI Very long baseline interferometry operates 7 uses the entire earth 12000 km diameter as baseline getting resolutions of 10396 arcseconds in the radio SETI is not limited by angular resolution 7 just getting a signal would be good enough for now 7 but rather by total collecting area Closeup View ofthe Ver LaIge Array VLA in New Mexico Map ofthe locations of telescopes of the VLBA Ver long baseline array an example of VLBI