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					            Telescope parameters
     • Light-gathering power (ability to see faint
       objects)
     • Resolving power (ability to see fine details)
     • Magnification (least important)




Slide 1
                 Other factors:
     • Optical quality
     • Atmospheric conditions
     • Light pollution




Slide 2
          Seeing
          Weather
          conditions
          and
          turbulence in
          the
          atmosphere
          set further
          limits to the
          quality of
          astronomical
          images.




                          Bad seeing   Good seeing
Slide 3
          The Best Location for a
          Telescope




Slide 4   Far away from civilization – to avoid light pollution
          The Best Location for a
          Telescope (2)




                            Paranal Observatory (ESO), Chile




           On high mountain-tops – to avoid atmospheric
Slide 5        turbulence and other weather effects
                            Well-known astronomer at Cerro-Tololo, Chile

                           Now Physics Professor at TAMU


          Nick Suntzeff




                                                Cerro-Tololo observatory

           Supernova in Centaurus A
Slide 6
          Traditional Telescopes (1)




                                       Secondary mirror


                         Traditional primary mirror: sturdy,
Slide 7                  heavy to avoid distortions.
          Advances in Modern Telescope Design
           Modern computer technology has made
           possible significant advances in telescope
           design:
           1. Lighter mirrors with lighter support structures,
           to be controlled dynamically by computers


                Floppy mirror            Segmented mirror




           2. Simpler, stronger mountings (“Alt-azimuth mountings”)
Slide 8
           to be controlled by computers
          Adaptive Optics
          Computer-controlled mirror support adjusts the mirror
          surface (many times per second) to compensate for
          distortions by atmospheric turbulence




Slide 9
Slide 10
           Examples of Modern Telescope
           Design (1)

                                          Design of the Large
                                          Binocular Telescope
                                                 (LBT)




            The Keck I telescope mirror


Slide 11
           Interferometry
           Recall: Resolving power of a telescope depends on
           diameter D:
                               amin = 1.22 l/D.

           This holds true even
           if not the entire
           surface is filled out.

           • Combine the signals
           from several smaller
           telescopes to simulate
           one big mirror 

           Interferometry
Slide 12
           Examples of Modern Telescope
           Design (2)




                                            The Very Large Telescope (VLT)



Slide 13
           8.1-m mirror of the Gemini Telescopes
           Giant Magellan Telescope




Slide 14
           CCD Imaging
                          CCD = Charge-coupled device
           • More sensitive than
           photographic plates
           • Data can be read
           directly into computer
           memory, allowing easy
           electronic manipulations

            Negative image to
            enhance contrasts



                                    False-color image to visualize
                                         brightness contours
Slide 15
           The Spectrograph
                              Using 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

Slide 16
           Exploring other wavelengths
     •     Radio
     •     Infrared
     •     UV
     •     X-ray
     •     Gamma-ray




Slide 17
           Radio Astronomy
             Recall: Radio waves of l ~ 1 cm – 1 m also
             penetrate the Earth’s atmosphere and can be
                      observed from the ground.




Slide 18
           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.


Slide 19
           Radio Telescopes
           Large dish focuses
           the energy of radio
           waves onto a small
           receiver (antenna)




                                 Amplified signals are
                                 stored in computers
                                 and converted into
Slide 20
                                 images, spectra, etc.
           The Largest Radio Telescopes
                             The 300-m telescope in
                             Arecibo, Puerto Rico




                      The 100-m Green Bank Telescope in
Slide 21
                      Green Bank, WVa.
           Radio Interferometry
           Just as for optical telescopes, the resolving power of
           a radio telescope is amin = 1.22 l/D.
           For radio telescopes, this is a big problem: Radio
           waves are much longer than visible light




Slide 22    Use   interferometry to improve resolution!
            Radio Interferometry (2)
           The Very
           Large Array
           (VLA): 27
           dishes are
           combined to
           simulate a
           large dish of
           36 km in
           diameter.




            Even larger arrays consist of dishes spread out over the
            entire U.S. (VLBA = Very Long Baseline Array) or even the
            whole Earth (VLBI = Very Long Baseline Interferometry)!
Slide 23
           Very Long Baseline Interferometry




Slide 24
           Radio observations with Very Long Baseline
           Interferometry (VLBI) are thousands of times more
           precise than optical observations (good enough to
           easily pinpoint a source the size of a pea in New
           York when sitting in Paris)




Slide 25
Slide 26
                           Discovery of the Infrared




  Frederick William Herschel 1738-1822

           He directed sunlight through a glass prism to create a spectrum (the rainbow created when light is divided into its colors) and
           then measured the temperature of each color. Herschel used three thermometers with blackened bulbs (to better absorb heat)
           and, for each color of the spectrum, placed one bulb in a visible color while the other two were placed beyond the spectrum as
           control samples. As he measured the individual temperatures of the violet, blue, green, yellow, orange, and red light, he
           noticed that all of the colors had temperatures higher than the controls. Moreover, he found that the temperatures of the colors
           increased from the violet to the red part of the spectrum. After noticing this pattern Herschel decided to measure the
           temperature just beyond the red portion of the spectrum in a region where no sunlight was visible. To his surprise, he found
           that this region had the highest temperature of all.
Slide 27
           Cups with cold and hot water




                                          chameleon

Slide 28
           Infrared Astronomy (l ~ 1-300 m)
           Most infrared radiation is absorbed in the lower
           atmosphere.
           However,
           from high
           mountain
           tops or high-
           flying air
           planes,
           some
           infrared
           radiation
           can still be
           observed.



Slide 29               NASA infrared telescope on Mauna Kea, Hawaii
           IRAS image of the Milky Way




Slide 30
           NASA’s Space Infrared Telescope Facility
              (Now Spitzer Space Telescope)




Slide 31
           Space Astronomy




Slide 32
           The Hubble Space Telescope
                    • Launched in 1990;     • Avoids
                    maintained and          turbulence in
                    upgraded by several     the Earth’s
                    space shuttle service   atmosphere
                    missions throughout
                    the 1990s and early     • Extends
                    2000’s                  imaging and
                                            spectroscopy
                                            to (invisible)
                                            infrared and
                                            ultraviolet




Slide 33
Slide 34
           Hubble Deep Field
                               10 day exposure photo!

                               Over 1500 galaxies
                               in a spot 1/30 the
                               diameter of the Moon

                               1011 galaxies in the
                               observable universe


                               Farthest and oldest
                               objects are 12-13 billion
                               ly away!

                               Space observations
                               as a time machine


Slide 35
           Ultraviolet Astronomy
           • Ultraviolet radiation with l < 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.
Slide 36
           X-Ray Astronomy
           • X-rays are completely absorbed in the atmosphere.

             • X-ray astronomy has to be done from satellites.



                                              X-rays trace hot
                                              (million degrees),
                                              highly ionized gas
                                              in the Universe.



                                               NASA’s
                                               Chandra X-ray
Slide 37
                                               Observatory
           Gamma-Ray Astronomy
           Gamma-rays: most energetic electromagnetic radiation;
            traces the most violent processes in the Universe




                                                    The Compton
                                                    Gamma-Ray
Slide 38
                                                    Observatory
                      Stars as black-body emitters




           It takes 10,000 years for a photon emitted in the core
           to reach the surface!
Slide 39
           Black Body Radiation (1)
           The spectrum of a star’s light
           is approximately a thermal
           spectrum called a black body
           spectrum.
           A perfect black body emitter
           would not reflect any radiation.
           Thus the name “black body”.
           The spectrum of a black body
           emitter is described by a
           universal formula first
           suggested by Planck. It
           depends only on surface
           temperature.
Slide 40
           Two Laws of Black Body Radiation

            1. The peak of the black body spectrum shifts
            towards shorter wavelengths when the
            temperature increases.
              Wien’s displacement law:

                       lmax ≈ 3x106 nm / T(K)
            (where T(K) is the temperature in Kelvin).




Slide 41
           Color and Temperature
               Stars appear in         Orion
               different colors,
                                       Betelgeuse
            from blue (like Rigel)
            via green / yellow (like
                   our sun)
           to red (like Betelgeuse).


            These colors tell               Rigel

           us about the star’s
              temperature!
Slide 42
           Two Laws of Black Body Radiation
            2. The hotter an object is, the more luminous it is.
                 The Stefan-Boltzmann law:
              Radiation Flux, or power emitted by unit area of
              a black-body emitter, is proportional to the
              fourth power of its surface temperature:

                                       J
                            Flux  sT   2
                                               4

                                      m s
                   s = Stefan-Boltzmann constant
                                                J
                         s  5.67  10  8
                                             m2 s K 4

           Luminosity, or total power:              L = A*s*T4
                       where A = surface area
Slide 43
                                 3  10 6 nm
           Wien’s law:        l               Note units!!
                                    T (K)




                                                 J
           The Stefan-Boltzmann law   Flux  sT 4

                                                m2 s
Slide 44
  Example of black-body emitter:
             our sun
           Yellow light: l ~ 520 nm
           Maximum of the black-body spectrum:
                                                            3  10 6 nm
                                            Wien’s law   l
                                                               T (K)

Surface temperature T =3x106 nm/520 nm 5800 K
                                                                 J
                           The Stefan-Boltzmann law   Flux  sT  4
                                                                  2
                                                                m s

           Radius = 7x105 km
           Total radiated power (luminosity) L = sT4 4R2 = 4x1026 W
Slide 45
           Comparing radiation fluxes and luminosities
           from two sources A and B:


                                        4
                          Fa T
                                      a
                                        4
                          Fb T         b


                                   2            4
                       La  Ra         Ta 
                                     
                       Lb  Rb 
                           
                                       T 
                                        b


Slide 46
           The Spectra of Stars
                                                  Inner, dense layers of a
                                                 star produce a continuous
                                                   (blackbody) spectrum.




           Cooler surface layers absorb light at specific frequencies.

                                   => Spectra of stars are absorption spectra.
Slide 47
            Bad news: stars are too far away to scoop their matter for testing

            Good news: they consist of the same atoms as
            the stuff on the Earth

           Fraunhofer in early 1800’s measures solar spectrum
           and identifies it with the spectrum of hydrogen in the lab



           English astronomer Lockyer, in the late-1800's, discovered an unknown
           element in the Sun, i.e. a set of spectral lines which did not correspond to
           elements in the lab. He named this element helium (Latin for Sun element).




Slide 48
           What is spectrum?




Slide 49
           Light and Matter
           Spectra of stars are
           more complicated than
           pure blackbody spectra.

            characteristiclines,
           called absorption lines.

           To understand
           those lines, we
           need to
           understand atomic
           structure and the
           interactions
           between light and
           atoms.
Slide 50
           Atom is mostly empty space!
                           Size of proton or neutron: ~10-15 m

                                      Size of an electron cloud:
                                      ~10-10 m (1 Angstrom)

                                      Proton mass: 1.7x10-27 kg
                                      Electron mass: 9x10-31 kg




Slide 51
           Thomson’s atom
             1899




Slide 52
           Rutherford atom
              1911




Slide 53
Slide 54
           “Planetary” model of atom




                                 Proton mass: 1.7x10-27 kg
                                 Electron mass: 9x10-31 kg




Slide 55
           Nuclear Density

              If you could fill a teaspoon
              just with material as dense as
              the matter in an atomic
              nucleus, it would weigh
              ~ 2 billion tons!!
              Neutron stars have such
              density
Slide 56
           Different Kinds of Atoms
           • The kind of atom
             depends on the
             number of protons
                                    Different
             in the nucleus.        numbers of
                                    neutrons ↔
           • Most abundant:         different
             Hydrogen (H),          isotopes
             with one proton
             (+ 1 electron).
           • Next: Helium (He),
             with 2 protons (and
             2 neutrons + 2 el.).

                   Helium 4
Slide 57
Slide 58
      The atom contains a nucleus surrounded by a cloud of negatively charged
      electrons. The nucleus is composed of neutral neutrons and positively
      charged protons. The opposite charge of the electron and proton binds the
      atom together with electromagnetic forces.




Slide 59
                                     b
                                     e
                                     t
                                     w
     Matter is effected by forces or interactions (the terms are interchangeable)
                                     e
                                     e
     There are four fundamental forces in the Universe:
                                     n
         gravitation (between particles with mass)
         electromagnetic (between particles with charge)
                                     p
         strong nuclear force (between quarks)
                                     a
         weak nuclear force (that changes quark types)
                                     r
                                    t
                                    i
                                    c
                                    l
                                    e
                                    s

                                    w
                                    i
                                    t
                                    h

                                    m
Slide 60                            a
               Catastrophe with atoms
     Accelerating electron produces EM
     radiation (light), loses energy and spirals
     into nucleus, i.e. atom should not work




Slide 61
           Ultraviolet catastrophe with black-body radiation




Slide 62
                          Bohr’s atom
           There is a stable orbit (ground state) on which electrons do not
           radiate.

           Changes of energy, such as the transition of an electron from one
           orbit to another around the nucleus of an atom, is done in discrete
           quanta. Quanta are not divisible. There is no ``in between''.

           The quantization, or ``jumpiness'' of action as depicted in quantum
           physics differs sharply from classical physics which represented
           motion as smooth, continuous change.




Slide 63
           Atomic Transitions
           • An electron can
             be kicked into a     Eph = E3 – E1
             higher orbit
             when it absorbs                          Eph = E4 – E1
             a photon with
             exactly the right
             energy.
                                                          Wrong energy
           • The photon is
             absorbed, and
             the electron is in         (Remember that Eph = h*f)
             an excited state.
           • All other photons pass by the atom unabsorbed.
Slide 64
Slide 65
                    Wave-particle duality
       Perhaps one of the key questions when Bohr offered his quantized orbits as
       an explanation to the UV catastrophe and spectral lines is, why does an
       electron follow quantized orbits? The response to this question arrived from
       the Ph.D. thesis of Louis de Broglie in 1923. de Broglie argued that since light
       can display wave and particle properties, then perhaps matter can also be a
       particle and a wave too.


                                             Energy and momentum of a particle
                                             are related to wavelength:

                                                           h           h
                                           p  mv  k        ;        10  34 J  sec
                                                           l          2

                                                                    p2  2k 2
           Wave packet                                          E    
                                                                  2m   2m
                                 l
                                    mv
Slide 66
           Your de Broglie wavelength:

                     1034 J  s
              l    2               10 31 m
                 mv 10 kg  10m / s

           de Broglie wavelength for the electron in an atom:

                    1034 J  s
           l     29              1010 m
              mv 10 kg  105 m / s

            Note the velocity dependence!



Slide 67
                      Why the orbits are quantized


     The electron matter wave is both finite
     and unbounded. But only certain
     wavelengths will `fit' into an orbit. If the
     wavelength is longer or shorter, then
     the ends do not connect.

     Thus, de Broglie explains the Bohr
     atom in that on certain orbits can exist
     to match the natural wavelength of the
     electron. If an electron is in some
     sense a wave, then in order to fit into
     an orbit around a nucleus, the size of
     the orbit must correspond to a whole
     number of wavelengths.




Slide 68
       If an electron is a wave around the atom, instead of a particle in orbit
       `where' is the electron at any particular moment?

       The answer is that the electron can be anywhere around the atom. But
       'where' is not evenly distributed. The electron as a wave has a maximum
       chance of being observed where the wave has the highest amplitude. Thus,
       the electron has the highest probability to exist at a certain orbit.




                                           Werner Heisenberg

                                                                Erwin Shrödinger
                                                     1920s
Slide 69
             Heisenberg’s Uncertainty
                    Principle
                                          x  p  

                                         E  t  


      It is often stated that of all the theories proposed in this
      century, the silliest is quantum theory. Some say the only
      thing that quantum theory has going for it, in fact, is that it is
      unquestionably correct. - R. Feynman
Slide 70
           Interference and diffraction of electron waves




Slide 71
           1859: Kirchhoff explains spectra of stars




Slide 72                                               p. 99
Slide 73
           Kirchhoff’s Laws of Radiation (1)
           1. A solid, liquid, or dense gas excited to emit
              light will radiate at all wavelengths and thus
              produce a continuous spectrum.




Slide 74
           Kirchhoff’s Laws of Radiation (2)
           2. 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




                 Transition back to lower states
                emits light at specific frequencies
Slide 75
           Kirchhoff’s Laws of Radiation (3)
           3. If light comprising a continuous spectrum
              passes through a cool, low-density gas,
              the result will be an absorption spectrum.

                                            Light excites electrons in
                                          atoms to higher energy states




               Frequencies corresponding to the
               transition energies are absorbed
Slide 76         from the continuous spectrum.
           Analyzing Absorption Spectra
           • Each element produces a specific set of
             absorption (and emission) lines.
           • Comparing the relative strengths of these sets of
             lines, we can study the composition of gases.

                                                       By far the
                                                       most
                                                       abundant
                                                       elements
                                                       in the
                                                       Universe




Slide 77
           Lines of Hydrogen
           Most prominent lines
           in many astronomical
           objects: Balmer
           lines of hydrogen




Slide 78
           The Balmer Lines
           n=1
                                               Transitions
                                               from 2nd to
                                               higher levels
                                               of hydrogen

                     Ha      Hb                         Hg

                                     The only hydrogen
                                     lines in the visible
                                     wavelength range.


            2nd to 3rd level = Ha (Balmer alpha line)
            2nd to 4th level = Hb (Balmer beta line)
                    …
Slide 79
           Observations of the H-Alpha Line
           Emission nebula, dominated
           by the red Ha line.




Slide 80
           Absorption Spectrum Dominated
           by Balmer Lines



                         Modern spectra are usually
                            recorded digitally and
                       represented as plots of intensity
                               vs. wavelength




Slide 81
           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



Slide 82
           Measuring the Temperatures of Stars




               Comparing line strengths, we can
              measure a star’s surface temperature!
Slide 83
           Spectral Classification of Stars (1)
            Different types of stars show different
            characteristic sets of absorption lines.




                                                       Temperature
Slide 84
           Spectral Classification of Stars (2)




            Mnemonics to   Oh         Oh      Only
            remember the   Be         Boy,    Bad
            spectral
            sequence:      A          An      Astronomers
                           Fine       F       Forget
                           Girl/Guy   Grade   Generally
                           Kiss       Kills   Known
                           Me         Me      Mnemonics
Slide 85
           Stellar Spectra


                             O




                                 Surface temperature
                             B
                             A
                             F
                             G
                             K
                             M




Slide 86

				
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