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Astronomical Telescopes Often very large to gather large amounts of light. Chapter 5 Astronomical Tools In order to observe forms of radiation other than visible light, very different telescope designs are needed. The northern Gemini Telescope on Hawaii Light as a Wave (I) Light and Other Forms of Radiation λ The Electromagnetic Spectrum In 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. c = 300,000 km/s = 3*108 m/s • • Light waves are characterized by a wavelength λ and a frequency f. f and λ are related through f = c/λ Wavelengths and Colors Light as a Wave (II) • Wavelengths of light are measured in units of nanometers (nm) or angstrom (Å): 1 nm = 10-9 m 1 Å = 10-10 m = 0.1 nm Different colors of visible light correspond to different wavelengths. Visible light has wavelengths between 4000 Å and 7000 Å (= 400 – 700 nm). 1 Light as Particles • 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: The Electromagnetic Spectrum Wavelength E = h*f Frequency h = 6.626x10-34 J*s is the Planck constant. The energy of a photon does not depend on the intensity of the light!!! Refracting / Reflecting Telescopes Focal length Focal length Need satellites to observe High flying air planes or satellites The Focal Length Refracting Telescope: Lens focuses light onto the focal plane Reflecting Telescope: Concave Mirror focuses light onto the focal plane Almost all modern telescopes are reflecting telescopes. Secondary Optics In reflecting telescopes: Secondary mirror, to redirect 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. Focal length = distance from the center of the lens to the plane onto which parallel light is focused. Disadvantages 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. 2 The 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: D The 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. αmin = 1.22 (λ/D) A=π (D/2)2 For optical wavelengths, this gives αmin αmin = 11.6 arcsec / D[cm] The Powers of a Telescope (III) Seeing 3. Magnifying Power = ability of the telescope to make the image appear bigger. Weather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images A larger magnification does not improve the resolving power of the telescope! Bad seeing Good seeing The Best Location for a Telescope The Best Location for a Telescope (II) Paranal Observatory (ESO), Chile Far away from civilization – to avoid light pollution On high mountain-tops – to avoid atmospheric turbulence (→ seeing) and other weather effects 3 Traditional Telescopes (I) Secondary mirror Traditional primary mirror: sturdy, heavy to avoid distortions. Advances in Modern Telescope Design (I) Modern computer technology has made possible significant advances in telescope design: 1. Simpler, stronger mountings (“alt-azimuth mountings”) to be controlled by computers Traditional Telescopes (II) The 4-m Mayall Telescope at Kitt Peak National Observatory (Arizona) Advances in Modern Telescope Design (II) 2. Lighter mirrors with lighter support structures, to be controlled dynamically by computers Floppy mirror Segmented mirror Adaptive Optics Computer-controlled mirror support adjusts the mirror surface (many times per second) to compensate for distortions by atmospheric turbulence Examples of Modern Telescope Design (I) Design of the Large Binocular Telescope (LBT) 4 Examples of Modern Telescope Design (II) Interferometry Recall: Resolving power of a telescope depends on diameter D. → Combine the signals from several smaller telescopes to simulate one big mirror → Interferometry The Very Large Telescope (VLT) 8.1-m mirror of the Gemini Telescopes CCD Imaging Negative Images CCD = Charge-coupled device The galaxy NGC 891 as it would look to our eyes (i.e., in real colors and brightness) • More sensitive than photographic plates • Data can be read directly into computer memory, allowing easy electronic manipulations Negative images (sky = white; stars = black) are used to enhance contrasts. False-color image to visualize brightness contours The Spectrograph Using a prism (or a grating), light can be split up into different wavelengths (colors!) to produce a spectrum. Radio Astronomy Recall: Radio waves of λ ~ 1 cm – 1 m also penetrate the Earth’s atmosphere and can be observed from the ground. Spectral lines in a spectrum tell us about the chemical composition and other properties of the observed object 5 Radio Telescopes Radio Maps In radio maps, the intensity of the radiation is color-coded: Large dish focuses the energy of radio waves onto a small receiver (antenna) Red = high intensity; Violet = low intensity Amplified signals are stored in computers and converted into images, spectra, etc. Radio Interferometry Just as for optical telescopes, the resolving power of a radio telescope depends on the diameter of the objective lens or mirror αmin = 1.22 λ/D. Analogy: Seat prices in a baseball stadium: Red = expensive; blue = cheap. The Largest Radio Telescopes For radio telescopes, this is a big problem: Radio waves are much longer than visible light → Use interferometry to improve resolution! The Very Large Array (VLA): 27 dishes are combined to simulate a large dish of 36 km in diameter. Science 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. The 100-m Green Bank Telescope in Green Bank, West Virginia. The 300-m telescope in Arecibo, Puerto Rico Infrared Astronomy Most infrared radiation is absorbed in the lower atmosphere. However, from high mountain tops or highflying aircraft, some infrared radiation can still be observed. NASA infrared telescope on Mauna Kea, Hawaii 6 Ultraviolet Astronomy NASA’s Great Observatories in Space (I) • Ultraviolet radiation with λ < 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: IRAS, IUE, EUVE, FUSE • Ultraviolet radiation traces hot (tens of thousands of degrees), moderately ionized gas in the universe. Hubble Space Telescope Images The Hubble Space Telescope • Launched in 1990; maintained and upgraded by several space shuttle service missions throughout the 1990s and early 2000’s • Avoids turbulence in Earth’s atmosphere • Extends imaging and spectroscopy to (invisible) infrared and ultraviolet NASA’s Great Observatories in Space (II) The Compton Gamma-Ray Observatory Operated from 1991 to 2000 Mars with its polar ice cap A dust-filled galaxy Nebula around an aging star NASA’s Great Observatories in Space (III) NASA’s Great Observatories in Space (IV) The Spitzer Space Telescope The Chandra X-ray Telescope Launched 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 formation Observation of high-energy gamma-ray emission, tracing the most violent processes in the universe. Very hot gas in a cluster of galaxies Launched in 2003 Infrared light traces warm dust in the universe. The detector needs to be cooled to -273 oC (-459 oF). Saturn 7 Spitzer Space Telescope Images The Future of Space-Based Optical/Infrared Astronomy: A Comet Warm dust in a young spiral galaxy Newborn stars that would be hidden from our view in visible light The James Webb Space Telescope 8 The Amazing Power of Starlight Chapter 6 Just by analyzing the light received from a star, astronomers can retrieve information about a star’s Starlight and Atoms 1. Total energy output 2. Surface temperature 3. Radius 4. Chemical composition 5. Velocity relative to Earth 6. Rotation period Light and Matter Spectra of stars are more complicated than pure black body spectra. → characteristic lines, called absorption lines. To understand those lines, we need to understand atomic structure and the interactions between light and atoms. Atomic Structure • An atom consists of an atomic nucleus (protons and neutrons) and a cloud of electrons surrounding it. • Almost all of the mass is contained in the nucleus, while almost all of the space is occupied by the electron cloud. Different Kinds of Atoms If you could fill a teaspoon just with material as dense as the matter in an atomic nucleus, it would weigh ~ 2 billion tons!! • The kind of atom depends on the number of protons in the nucleus. • Most abundant: Hydrogen (H), with one proton (+ 1 electron). Different numbers of neutrons ↔ different isotopes • Next: Helium (He), with 2 protons (and 2 neutrons + 2 el.). Helium 4 1 Electron Orbits • Electron orbits in the electron cloud are restricted to very specific radii and energies. • These characteristic electron energies are different for each individual element. r3, E3 Atomic Transitions • An electron can be kicked into a higher orbit when it absorbs a photon with exactly the right energy. Eph = E3 – E1 • The photon is absorbed, and the electron is in an excited state. r2, E2 r1, E1 Eph = E4 – E1 Wrong energy (Remember that Eph = h*f) • All other photons pass by the atom unabsorbed. Color and Temperature Stars appear in different colors, from blue (like Rigel) Orion Betelgeuse via green / yellow (like our sun) The light from a star is usually concentrated in a rather narrow range of wavelengths. The spectrum of a star’s light is approximately a thermal spectrum called black body spectrum. to red (like Betelgeuse). These colors tell us about the star’s temperature. Black Body Radiation (I) Rigel Two Laws of Black Body Radiation A perfect black body emitter would not reflect any radiation. Thus the name ‘black body’. The Color Index (I) B band 1. The hotter an object is, the more luminous it is. 2. The peak of the black body spectrum shifts towards shorter wavelengths when the temperature increases. → Wien’s displacement law: λmax ≈ 3,000,000 nm / TK (where TK is the temperature in Kelvin). V band The color of a star is measured by comparing its brightness in two different wavelength bands: The blue (B) band and the visual (V) band. We define B-band and V-band magnitudes just as we did before for total magnitudes (remember: a larger number indicates a fainter star). 2 The Color Index (II) Kirchhoff’s Laws of Radiation (I) We define the Color Index 1. A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths and thus produce a continuous spectrum. B–V (i.e., B magnitude – V magnitude) The bluer a star appears, the smaller the color index B – V. The hotter a star is, the smaller its color index B – V. Kirchhoff’s Laws of Radiation (II) 2. If light comprising a continuous spectrum passes through a cool, low-density gas, the result will be an absorption spectrum. Kirchhoff’s Laws of Radiation (III) 3. A low-density gas excited to emit light will do so at specific wavelengths and thus produce an emission spectrum. Light excites electrons in atoms to higher energy states Frequencies corresponding to the transition energies are absorbed from the continuous spectrum. The Spectra of Stars Inner, dense layers of a star produce a continuous (black body) spectrum. Light excites electrons in atoms to higher energy states Transition back to lower states emits light at specific frequencies Lines of Hydrogen Most prominent lines in many astronomical objects: Balmer lines of hydrogen Cooler surface layers absorb light at specific frequencies. => Spectra of stars are absorption spectra. 3 The Balmer Lines 4 n = 3 n= 2 Hα Transitions from 2nd to higher levels of hydrogen Hβ Absorption spectrum dominated by Balmer lines n=5 n= n=1 Hγ The only hydrogen lines in the visible wavelength range. Modern spectra are usually recorded digitally and represented as plots of intensity vs. wavelength 2nd to 3rd level = Hα (Balmer alpha line) 2nd to 4th level = Hβ (Balmer beta line) … Emission nebula, dominated by the red Hα line. The Balmer Thermometer Balmer line strength is sensitive to temperature: Most hydrogen atoms are ionized => weak Balmer lines Almost all hydrogen atoms in the ground state (electrons in the n = 1 orbit) => few transitions from n = 2 => weak Balmer lines Spectral Classification of Stars (I) Different types of stars show different characteristic sets of absorption lines. Temperature Measuring the Temperatures of Stars Comparing line strengths, we can measure a star’s surface temperature! 4 Spectral Classification of Stars (II) Mnemonics to remember the spectral sequence: Oh Oh Only Be Boy, Bad A An Astronomers Fine F Forget Girl/Guy Grade Generally Kiss Kills Known Me Me Mnemonics The Composition of Stars Stellar spectra A F G K M Surface temperature O B From the relative strength of absorption lines (carefully accounting for their temperature dependence), one can infer the composition of stars. The Doppler Effect The light of a moving source is blue/red shifted by ∆λ/λ0 = vr/c Blue Shift (to higher frequencies) vr λ0 = actual wavelength emitted by the source Red Shift (to lower frequencies) ∆λ = Wavelength change due to Doppler effect vr = radial velocity 5 Example (I): Earth’s orbital motion around the sun causes a radial velocity towards (or away from) any star. Example (II): Take λ0 of the Hα (Balmer alpha) line: λ0 = 656 nm Assume, we observe a star’s spectrum with the Hα line at λ = 658 nm. Then, ∆λ = 2 nm. We find ∆λ/λ0 = 0.003 = 3*10-3 Thus, vr/c = 0.003, or vr = 0.003*300,000 km/s = 900 km/s. The line is red shifted, so the star is receding from us with a radial velocity of 900 km/s. 6