Observing the Universe



Electromagnetic radiation: electric and magnetic fields traveling               through space.


Travels at c = 3 x 105 km/s (in vacuum)


Light a type of e-m radiation



Acts like a wave, and a particle



Wavelength: distance between two successive wavecrests. Any           value greater than 0.



Frequency (f): number of wavecrests that pass per second.



For electromagnetic radiation:

c = 3x105 km/sec


Short wavelength - high frequency



Long wavelength - low frequency



Visible light:



300-800 nm (nanometer)

1 nm = 10-9 m



Blue light 400 nm



Red light 700 nm

Spectrum: arrangement of electromagnetic radiation by wavelength





Regions of the spectrum:



Name            Wavelength



High energy

      Gamma rays       10-12 m (10-3nm)

      X-rays                 10-2nm - 10nm

Ultraviolet            10nm - 300nm

Visible                 300nm - 800, 1000 nm

Infrared                 1000 nm - 106nm (0.1 cm)

Radio                 longer than 0.1 cm (includes microwave)



Photon: particle of light

      Amount of energy depends on wavelength


      Shorter wavelengths - higher energy (Blue light, x-rays)


      Longer wavelengths - lower energy (Red light, radio waves)



Refraction: the bending of electromagnetic radiation as it passes from one transparent medium to another


Prism: different wavelengths refracted different amounts


Light spread out by wavelength (color), producing spectrum


Types of Spectra (Kirchhoff's Laws)

continuous spectrum - hot, dense body


absorption spectrum - continuous spectrum behind cool gas cloud


emission spectrum - hot, low density gas cloud


Elements identified by patterns of lines



Atoms


Nucleus: 0.0000016 nm


Nucleus: protons (+)

               neutrons (0)


Electron Cloud: 0.1-0.5 nm
            electrons (-)


Bohr Model: electrons exist only in certain orbits


Orbits correspond to electron energy (energy levels)


Energy state of electron is raised if add energy.


Must add energy equal to energy difference of two energy levels


Only photons of particular energies (wavelengths) can be absorbed by atom


Each element has its own set of energy levels, absorbs only particular photons


Atoms return to lowest energy state by emitting photons corresponding to energy difference of two levels



Doppler effect: change in wavelength of emitted light because           of motion towards or away from earth.


          Motion towards earth, lines shift to shorter wavelength,                     'Blueshift'


          Motion away from earth, lines shifted to longer wavelength,     'Redshift'



          Measurement of radial velocities only



Telescopes


Refracting telescopes - lenses




Telescopes have 2 lenses


          Objective lens

          Eyepiece



Problems with refracting telescopes


Reflecting telescopes - use mirrors - glass coated with aluminum


          Primary mirror (objective mirror): focuses image at prime                     focus.


          Various arrangements of secondary mirrors


Modern telescopes


          a. Segmented mirrors, e.g. Keck telescope


          b. Spin-cast mirrors - honeycombed glass


          c. Floppy mirrors - thin - (computer controlled)


          d. Adaptive optics: nullify effects of atmosphere on light                               passing through it.


Seeing: blurring of light as it passes through the atmosphere


          Telescopes in space avoid problems with seeing


Adaptive optics measures bright star to determine distortion, adjusts a deformable mirror to correct for the distortion.



Telescope Parameters


Light gathering power - depends on area of objective lens or primary mirror.

Light gathering power proportional to diameter squared.


Resolving power (R.P.) - angular distance between 2 stars can just separate in telescope.



Radio Telescopes


          reflecting 'dishes' (antenna)


          Resolving power (R.P.)

Diam. = 25 meter


R.P. = 104 arcsec = 2.8 deg.


Want a large diameter!!


          E.g. 3 arcsec resolution at 21 cm requires 18 km telescope!!!


Radio Interferometer


          > 2 radio telescopes linked together


          R.P. equivalent to a telescope with diameter equal to           distance between telescopes


          Very Large Array (VLA)


                    27 telescopes


                    Simulates radio telescope >30 km in diameter


Very Long Baseline Interferometer (VLBI)


VLB Array - 10 telescopes Virgin Islands to Hawaii


Space Astronomy - Why Bother?


      1. Detect wavelengths that don't penetrate to earth's surface.


      2. Get above turbulence in earth's atmosphere


Infrared (IR) Astronomy


      Dry mountain tops


      Balloons, SOFIA - 747 - 2005


      IRAS - Infrared Astronomy Satellite


          Telescope cooled to near 0 deg.


      ISO - Infrared Space Observatory (European)


      SIRTF (Space Infrared Telescope Facility) 2003


Ultraviolet Astronomy (UV)


      < 300 nm (nm = 10-9 m) absorbed by ozone

      Must observe from space


      Rockets


      Satellites


X-ray Astronomy


      Violent events


      10 nm - 0.01 nm


      balloons, rockets, satellites

          CHANDRA - currently in orbit


Gamma-Ray Astronomy


      Satellites


            'bursters'


            active galactic nuclei


Cosmic Rays


      energetic particles - protons, electrons, positrons, other             atomic nuclei


      detected from satellites


      'showers' detected on ground


      source not well-understood


Neutrinos


      particle with tiny mass, no electric charge, travels at speeds                   very close to c


      generated in nuclear reactions in stars, at origin of universe


      Difficult to detect - don't interact with ordinary matter


      Detectors:

            Large tanks of chlorine or very pure water