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The Ultimate Tool of Astronomy: Telescopes Announcements q Homework # 4 is due on Thursday October 27th q Grades for Quiz # 3 are available in the Gradebook. Solutions are available on the course website, under `Quizzes’ q Quiz # 4 will take place on Tuesday, November 1st. Remember to bring a pencil Today’s Goals q Complete discussion of Doppler Effect, and how it is used in Astronomy q Discuss some of the Exam # 1 questions q Introduce and discuss Telescopes Doppler Shift The first crest travels out in circle from the original position of the plane Shorter wavelength (bluer) At a later time, a second crest is emitted from the planes new position, but the old crest keeps moving out in a circle from the planes original position The same thing happens again at a later time Longer wavelength (redder) What we actually see when we do spectroscopy Emission spectrum of hot gas as seen in lab Emission spectrum of hot gas as seen in rapidly moving object Is this object moving towards or away from us? What we do with the Doppler Shift We derive the (approaching or receding) velocity of the source (star, galaxy, etc.). q The Doppler shift causes the rest wavelength, λr , emitted by an object to become bluer (blue-shift) or redder (red-shift): λo- λr q To derive a velocity, we divide by a time T= 1/νr: v = (λo- λr)/T = (λo- λr) νr = c (λo- λr)/ λr v/c = (λo- λr)/ λr This is the formula used to measured the recession of galaxies in the Universe! The Doppler shift n An object shining light with restframe λr=656.3 nm is moving at V=5,000,000 m/s away from you. What is the color, λo, of the light that you see? n n v/c = (λo- λr)/λr v/c =5x106/3x108 = 1.67x10-2 = (λo- λr)/λr λo = λrx(1+1.67x10-2) = 667.3 nm (redder!) Survey Question Two identical stars are observed from the Earth. Star A’s emission lines (that are at visible wavelengths in the rest frame) are observed to be at ultraviolet wavelengths. The same emission lines for Star B are observed to be at X-ray wavelengths. From these observations you conclude that: 1) both stars are moving away from the Earth 2) Star A is moving towards the Earth faster than Star B 3) Star B is moving towards the Earth faster than Star A 4) Star B is moving away from the Earth while Star A is moving towards the Earth. Exam 1: Q. # 4 Friends tell you that they saw the constellation Orion high in the sky at 4 a.m. this morning. You are not interested in getting out of bed so early. How many months will you have to wait until Orion is in the same place of the sky at midnight? A. One month B. Two months C. Three months D. Four months E. Ten months Exam 1: Q. # 4 Friends tell you that they saw the constellation Orion high in the sky at 4 a.m. this morning. You are not interested in getting out of bed so early. How many months will you have to wait until Orion is in the same place of the sky at midnight? A. One month B. Two months C. Three months D. Four months E. Ten months Exam 1: Q. # 11 Newton concluded that some force had to act on the Moon because A. a force is needed to keep the Moon in motion B. a force is needed to pull the Moon outward C. the Moon moved at a constant velocity D. a force is needed to pull the Moon away from a straight-line motion E. all of the above Exam 1: Q. # 11 Newton concluded that some force had to act on the Moon because A. a force is needed to keep the Moon in motion B. a force is needed to pull the Moon outward C. the Moon moved at a constant velocity D. a force is needed to pull the Moon away from a straight-line motion E. all of the above Exam 1: Q. # 15 A cart of mass M is moving at constant speed v. Suddenly a weight of mass m drops on the cart. The momentum of the cart: A. increases B. decreases C. stays the same Exam 1: Q. # 15 A cart of mass M is moving at constant speed v. Suddenly a weight of mass m drops on the cart. The momentum of the cart: A. increases B. decreases C. stays the same Exam 1: Q. # 26 Due to tides raised on the Earth by the Moon, the Moon is gradually moving FURTHER away from the Earth. The average distance between the Earth and the Sun remains constant with time. How will solar eclipses be different in a distant future as a result of the increasing distance between the Earth and the Moon? A. Total solar eclipses will occur only at Full Moon B. Total solar eclipses will last considerably longer C. Total solar eclipses will no longer occur D. Total solar eclipses will only be visible from the Earth’s Southern Hemisphere E. Total solar eclipses will occur at every New Moon Exam 1: Q. # 26 Due to tides raised on the Earth by the Moon, the Moon is gradually moving FURTHER away from the Earth. The average distance between the Earth and the Sun remains constant with time. How will solar eclipses be different in a distant future as a result of the increasing distance between the Earth and the Moon? A. Total solar eclipses will occur only at Full Moon B. Total solar eclipses will last considerably longer C. Total solar eclipses will no longer occur D. Total solar eclipses will only be visible from the Earth’s Southern Hemisphere E. Total solar eclipses will occur at every New Moon The Ultimate Tools of Astronomy: Telescopes! Assigned Reading q For the topic of Telescopes: Units 26.1-3, 27.1-2, 28, 29.1-2, 30.1, 30.4 Telescopes The twin 10-m Keck telescopes (Hawaii) are currently the largest telescopes in existence. The largest combined telescopes (4 x 8 m) are the ESO VLT. Plans for larger telescopes (up to 42-m) are currently being considered (ESO, USA) UMass LMT The 50-m Large Millimeter Telescope The largest millimeter-wavelength telescope in the world U Mass and Mexico Why do you need large telescopes? Nasa is building JWST, a 6.5-m telescope, which will be launched in space ~ 2018 Telescopes are `light buckets’. The bigger, the more light they collect, and the more distant the objects they can observe. Why do telescopes need to be big? n n The main feature of a telescope is its capacity to collect as much light as possible • Like an antenna: the stronger the signal the clearer the transmission. • Well, guess what: an antenna *is* a telescope (a radio telescope, that is) The larger the light collector, I.e. the primary mirror or lens, the more powerful the telescope: Collecting Area ~ 4 π D2 CAA/CAB = (DA/DB)2 • A telescope twice as large collects four times as much light Reflector Focusing Light Refractor Detecting Light (Instruments) n Cameras: n Spectrographs: Detecting Light (Instruments) n Cameras: • To obtain images at desired wavelength or wavelengths (color images) • This is what yields the `pretty pictures’ n Spectrographs: • To study the intensity of the various wavelengths (colors): spectra • This yields the physical nature (star, galaxy, balck hole), chemical composition, physical properties (temperature, density), dynamics (motions, mass), distance of the sources The Telescope’s Sharpness To study galaxy formation both space-based sensitivity and angular resolution required!! Note how many more details and faint objects can be observed with the Hubble Space Telescope Subaru + SUPREME HST + ACS Sharpness (Angular Resolution) • The bigger the telescope, the smaller the detail it can discriminate • α = 0.02 λ(nm) / D(cm) • This also depends on the type of light the telescope detects (the wavelength λ) • On the ground, the limitation is due to the `blurring’ of our own atmosphere (called `seeing’). Telescopes and Earth’s Atmosphere Most wavelengths cannot penetrate the Earth's atmosphere! Telescopes and Earth’s Atmosphere n n In addition to blur light (reduce sharpness), the Earth’s atmosphere prevents some wavelengths from getting to the Earth’s surface: • Ultraviolet, X-ray, infrared: telescopes working at these wavelengths need to be placed in SPACE • We are grateful to the atmosphere for stopping UV and X-ray, otherwise we would be fried to a crisp! Wavelengths that make to the ground: • Optical and radio • We are glad radio wavelengths can travel freely, otherwise we would not have telecommunications! (cellphones operate at ~ 33 cm) Different telescopes for different types of light LMT Herschel Spitzer Hubble Different types of telescopes n n n To detect different types (wavelengths) of light, eg. X-ray, UV, optical, infrared, radio, different technologies are required For example, special mirrors are necessary for X-ray telescopes or else the radiation would pass through them. Hence, it is necessary to specialize telescopes to the wavelength of light one wishes to study. What is up there now… HUBBLE: optical telescope CHANDRA: X-ray telescope SPITZER and HERSCHEL: Infrared Telescopes Why different wavelengths are required n Regardless of the technology, different wavelengths carries different information: • Shorter wavelengths (X-ray, UV) carry information on very energetic phenomena (e.g. black holes, star formation) • Optical wavelengths carry information on the structures of galaxies and their motions (the assembly of the bodies of galaxies, their size) • Longer wavelengths (IR, radio) carry information on the chemical composition, physical state (gas and dust, presence, chemical elements; temperature) Optical Sky Near-infrared sky Boldt et al. Radio Sky Soft X-ray Sky